thenewbonsai

Course 05 · Lesson 5 of 6 · Feeding plans that follow the tree

When fertilizer is the wrong first response

Pale leaves and weak growth have many causes, and fertilizer fixes only one of them. Learn to check the roots, water, light, salts, and pH first, to read the feeding record, and to tell when a shortage really is the best explanation.

The question

Why might feeding fail to fix pale leaves or weak growth?

By the end of this lesson, you will be able to:

  • Explain why leaf color alone cannot establish a specific nutrient deficiency.
  • Investigate roots, water, light, salt buildup, and pH before adding fertilizer to a struggling tree.
  • Choose the next check or action for poor growth, including when to get help with a diagnosis.

About 18 minutes to read and explore

Late July in the small backyard. Like every case in this course, it is an instructional example, set in one temperate Northern Hemisphere city at about 40° N (USDA zone 7a). On the bench in the western third, the household's trident maple (Acer buergerianum) stands in the 18 cm (7 in) black nursery pot of bark-based mix it arrived in, the spring before last. Its leaves have brown, crisp edges, and a white crust has formed on the rim and on the surface of the mix. Every afternoon its leaves hang limp, though the pot is heavy and a chopstick in the core comes out damp and cool.

It has not gone short of feed. Since mid-June both adults have given it the 8-4-6 liquid the household bought in April (like every product in this course, fictional, with a label made for the course), and each has kept a note of their own feeds. Out on the lawn, the household's ficus (Ficus microcarpa), fed the 14-4-14 powder since late May to suit its rain-barrel water, has come through a rainy week standing in a saucer full of rain. Its older leaves are yellowing and dropping, and its mix smells faintly sour.

One of the two adults is already reaching for the feed for both, the 8-4-6 for the trident and the 14-4-14 for the ficus: yellow and brown leaves look to them like a tree asking for more. Will feeding fix either? This lesson shows why fertilizer is so often the wrong first response to pale leaves and weak growth, which checks come before any feed, and how to tell when a shortage really is the best explanation.

Illustration of typical cases, not a record of particular trees. Four trident maple leaves yellowed nearly alike by four causes: from left, a nitrogen shortage, roots short of air in a waterlogged pot, too little light, and the start of autumn color. The leaf alone cannot say which; where it grew, the checks, and the history narrow it down.

One color, many causes

Leaves are green because of the chlorophyll in them, and anything that cuts how much a leaf makes or keeps turns it pale or yellow: . A shortage of a mineral nutrient is one cause, because nitrogen and magnesium are part of chlorophyll and iron is needed to build it (Course 01, Lesson 6). It is far from the only one. Roots short of water or air, roots lost to a dry spell or to root work, too little light, cold roots, salts in the root zone, an alkaline root zone that locks iron away, insects feeding on the leaves, root and leaf diseases, and the ordinary turn of the season all pale or yellow a tree, and from a step away many of them look alike.

That is why leaf color alone never establishes a specific . The four leaves above yellowed nearly alike for four different reasons, and a symptom says only that something is wrong, not what (Course 01, Lesson 14). Two things narrow the list. One is where the change shows: Lesson 1 explained why a shortage of a nutrient the tree can move tends to show first on the older leaves, and a shortage of one that stays put tends to show first on the newest. The other is the pattern: pale all over, yellow between green veins, or brown at the edges. On junipers and other conifers, an iron shortage shows as an overall yellowing of the needles, never as green veins. Both are clues; the checks below the surface, the light, and the tree's records decide.

Feeding answers only one of those causes, and it makes several of the others worse. Between a feed and new growth, four things must happen: the nutrients reach the pot, dissolve and reach the roots, are absorbed, and are used, with sugar from the leaves, to grow, and fertilizer widens only the first (Course 01, Lesson 10). So the question for a struggling tree is not which feed to give but which step is failing. This lesson checks them in the order that is quickest and most often finds the cause: the roots and the water, then the light, then salts and pH, and only then a shortage.

Predict first

The small backyard in late July: the trident shows a white crust, brown leaf edges, and an afternoon droop in a moist pot after weeks of feeding; the ficus on the lawn has stood in a saucer of rain for a week, its older leaves yellowing and its mix smelling faintly sour. Will a feed fix either tree?

Start with the roots and the water

Nearly every nutrient a tree uses passes through its , and those roots need water films to draw from, air in the pores, warmth, and sugar from the leaves to power the uptake (Course 01, Lesson 10). Take any one away, and roots sitting in a well-fed mix take up little of it. Roots in a root zone that stays run short of oxygen, and their tips die back; roots in one that dried out lose their root hairs and finest roots; roots cut back at a repot have fewer tips to absorb with, which is why feeding waits for new growth after a dry spell or root work (Lesson 4). In each case the leaves can pale, yellow, or drop while the pot holds plenty of feed, and adding more only puts salts around roots that are already struggling, salts that concentrate as the mix dries (Course 03, Lesson 4).

So the first checks are Course 03's. Confirm the moisture below the surface with the chopstick in the core, the lift, and the drainage holes; ask what has changed; look underneath for water standing in a tray, a saucer, or a cover pot; smell the mix (Course 03, Lesson 4); and look at the roots you can see at the holes, where firm, pale tips are a good sign and soft, dark roots whose outer layer slides off are a warning (Course 01, Lesson 10).

The household's ficus fails at this first step. A week of rain has kept its saucer full, so the bottom of its root ball has sat in water for days, and the faint sour smell says part of the root zone has gone without air. Its yellowing older leaves most likely come from its roots, not from a lack of feed: it has had the 14-4-14 at its label line since late May. The first move is Empty the standing water and let it drain, adding nothing and leaving the saucer off while rain is forecast. Its feed waits as well: a waterlogged root zone gets no fertilizer until new growth shows the roots are working again, with new leaves that hold their color and, where you can see them, white root tips at the holes. If it keeps yellowing once the pot drains and dries back normally, the roots need a diagnosis (Course 07, Tree Health and Diagnosis).

Light and the season

A tree makes its sugar in light, and in too little light it cannot build healthy leaves, however it is fed. New growth comes in pale, thin, and stretched, with long gaps between small leaves: (Course 02, Lesson 4). Shaded inner leaves yellow and drop. Indoors in winter, a tree a few steps from its window can do all of this with a feeding record that is exactly right. Fertilizer cannot stand in for light (Course 01, Lesson 6): in dim light, extra feed adds little to growth, and what the tree cannot use stays in the pot as salts.

The answer is more light first, raised in steps so that leaves grown in shade do not (Course 02, Lesson 3), and then a look at the next new growth before any change to the feeding. New leaves that come out greener and closer together in the brighter spot show that light was the limit.

Some yellowing needs nothing at all: autumn color, a conifer's shed of older inner foliage, and a juniper's winter bronze are the season (Lesson 1), and each is Nothing to fix: normal for the season.

Course 01 left one case open: a tree in strong light whose new leaves still come out pale (Course 01, Lesson 6). With light ruled out, the pattern leads. Newest leaves yellow between green veins point to the water and its pH (below). Pale all over, older leaves first, after weeks with little or no feed, points to the feeding record and a shortage. If neither fits, look at the roots again, and if the tree keeps getting worse, get help with a diagnosis (Course 07).

Practice check · not scored

A friend's young Chinese elm in July, bought in spring and kept on a patio table: over the past month its leaves have paled and it has put out little new growth. Nothing looks obviously wrong from above, and the friend wants to do something about it today. Which two of these come first? Select 2.

Salts that build up

Most fertilizers are , or release them as they break down, and every feed adds some to the root zone (Course 01, Lesson 10). At the label's rate and interval, a growing tree takes up much of what it is given, and each carries some of the rest out of the holes. Salts build up when feed goes in faster than the tree uses it or water carries it out, and they concentrate as the mix dries. Very salty water in the root zone makes it harder for roots to draw water in, so a tree can droop in a moist pot, and strong solutions damage root tips, and leaf edges then brown and crisp. That damage is .

Three signs point to it, most strongly together: a white crust on the rim and the surface, leaf edges browning, and a droop in the afternoon over a root zone that checks Moist. The household's trident has all three, and it spends its afternoons in the shade of the neighbor's maple, so its droop is not the sun (Course 03, Lesson 4).

Illustration of typical signs, not a record of a particular tree. A white crust on the rim and on the surface of the mix, with leaf edges browning: the look of salts building up in the root zone. Hard tap water can leave a similar crust on its own; with brown edges, or a droop in a moist pot, after frequent feeding, it points to salts.

Why the salts built up

Course 03 asked for a look at the feeding whenever salts are suspected, and for the reason they built up before feeding starts again (Course 03, Lesson 5). Read the against the label, and the watering against the pot. These are the usual reasons:

  • More often or stronger than the label. A feed given more often than the label's interval, mixed stronger than its rate, or taken from the wrong line, such as a garden-bed line used on a pot, adds salts faster than the tree uses them (Lesson 2).
  • Two feeds at once. Granules already in the mix with a liquid feed on top, cakes with a liquid, or two people feeding the same tree, each unaware of the other's feeds (Lesson 3).
  • Water that never runs through. Small sips, or a pot standing in its own runoff, carry nothing out, so the salts stay (Course 03, Lesson 3).
  • Feed on a dry root zone. In a dry mix, the feed dissolves into the little water there and concentrates around the roots, which is why a pot that checks Drying or drier is watered first and fed once it drains to Moist (Lesson 4).
  • Granules in heat. release faster than their label's rating in a warm pot, and a black nursery pot in summer sun runs through them early (Lesson 3).
  • A softener. adds sodium with every watering, which builds up and browns leaf edges much as fertilizer salts do (Course 03, Lesson 5).

The household's trident has two of these at once. Each adult's own notes show the 8-4-6 about once a week since mid-June, apart from the two weeks in July when they were away, on a label that says every 14 days: twice its frequency from either of them alone, and both of them have been feeding it (Lesson 6 adds it up).

A hard-water crust or fertilizer salts?

A crust on its own does not say which. The city's tap water is moderately and leaves one wherever water evaporates, most of all on terracotta; on a tree growing well, a crust mostly shows minerals building up, one to watch rather than treat (Course 03, Lesson 5). The tree's history tells them apart: how hard the water is, and how much, how often, and how strongly the tree has been fed. A crust on a tree fed at its label's rate and interval, watered thoroughly so that water runs through, and growing well is mostly the water's. A crust that comes with brown leaf edges, or with a droop in a moist pot, after frequent or heavy feeding points to salts in the root zone.

A conductivity meter can add evidence, used the way Course 04 set out: shows how much is dissolved, not which ions, and readings compare only when they are taken the same way (Course 04, Lesson 5). For feeding, two kinds of reading help. One of the mixed can beside one of the plain tap water shows how much the feed adds, which checks the mixing, not the tree. A series from the root zone, each a taken the same way at the same point between feeds (described below), shows whether salts are building or falling after a rinse. A single reading by any method is not a verdict, and neither a high one nor a low one is a reason to feed.

Practice check · not scored

The west balcony in early August: a white crust has thickened on the trident's terracotta rim since spring. Since late June it has had the 8-4-6 at its outdoor line every 14 days, always on a moist root zone, with thorough waterings from the kitchen tap. Its leaves are green and firm. What does the crust most likely show?

Rinse, then resume

When the signs and the history point to salts, the response is Course 03's: pause feeding, and rinse the salts out with plenty of plain water. Water thoroughly until water runs from the holes, wait a few minutes, and water again, with rainwater or ordinary unsoftened tap water, never softened water (Course 03, Lesson 5). That is , and it is a response to evidence, such as these signs after frequent feeding, readings taken the same way that keep rising, or sips that never drain, not a routine: every thorough watering already carries a little out, and a deliberate rinse washes out nutrients along with the salts. The household rinses the trident that evening on its bench, with the hose, and pauses feeding. The leaves that are already brown will stay brown; the next ones show whether it worked.

Feeding resumes once new growth looks normal: new leaves of the usual size and color with clean edges, and no droop over a moist root zone. Resume at or below the label rate, never with a catch-up dose, and only with the cause corrected, because a rinse removes the salts and not the habit that put them there. For the household's trident, correcting the cause means one person feeding, written in one record both adults can see: once its new growth looks normal, a first feed at half strength, then the 8-4-6 at its outdoor line every 14 days, the plan Lesson 6 writes. If the browning or the droop continues after the rinse, look beyond the salts (Course 07).

pH and iron

The household's azalea has a different look. It stands on the front step in a small glazed pot and is watered from the hose, with the city's moderately hard, unsoftened water, and since late May it has had the household's 8-4-6 at the outdoor line every 14 days, apart from the two weeks in July when the household was away. In the May flush its newest leaves came out while the older leaves stayed dark green, and by late July nothing has been done about it.

Illustration of typical signs, not a record of a particular tree. On an acid-loving azalea watered with hard water, the newest leaves come out yellow between green veins while the older leaves stay green, a well-known clue to iron made unavailable by a rising pH. It is a clue, not a diagnosis: roots short of air can pale new leaves too.

That pattern is the one Course 03, Lesson 5 warned about for acid-loving trees on hard water. The city's hard water carries , which neutralizes acid, so each watering nudges the root zone's up, and over months a small pot watered often drifts toward alkaline. In an alkaline root zone, iron, manganese, and zinc form solids that roots cannot take up: still in the pot, but no longer (Course 01, Lesson 10). Iron stays where it was put in the tree, so the newest leaves, which depend on what arrives now, show it first (Lesson 1). An azalea, which needs an acidic root zone, shows it soonest; most of the trees in this course grow well on the same water.

It is still a clue. Roots short of air, a cold, wet spell, and other shortages can pale new leaves too, so the root zone comes first here, as for any tree. The azalea's root zone checks Moist and drains freely, its light on the step, morning sun and then bright shade from midday, suits an azalea, and its feeding record shows the 8-4-6 at the label's rate and interval, which leaves the water and the pH. More of that feed, which the pattern tempts, would add iron in a form that is less reliable as the pH rises, along with salts.

Getting a reading you can use

A pH reading can test what the leaves only suggest, if it is taken in a way you can use (Course 04, Lesson 5). Two routes work for a pot. A laboratory media test reads a sample of the mix by a stated method. At home, a pour-through test samples the water held in the root zone without disturbing the roots: half an hour to two hours after a thorough watering, raise the pot a little above a clean saucer, pour just enough distilled water evenly over the surface to push out a small cupful, the same amount each time, and read what drains with a pH pen calibrated that day, a meter that reads a collected sample, not a probe pushed into the mix. Home test strips give only a rough reading. Whichever route you use, use it the same way each time, write down how you took it, and treat one result as a starting point, not a verdict.

The household borrows a calibrated meter and takes a pour-through sample from the azalea. In this course's words, it reads Slightly alkaline: far from the acidic root zone an azalea needs. One reading is still a starting point, but this one agrees with the leaves, the hard water, and months of hose water, and more rainwater was a safe step anyway (Course 03, Lesson 5), so the household acts on it and plans a second reading, taken the same way, later in the season.

A pour-through test, an illustration: (1) water the pot thoroughly and let it drain for half an hour to two hours; (2) raise it a little above a clean saucer and pour just enough distilled water evenly over the surface to push out a small sample; (3) read what drains out with a calibrated meter. Readings compare only with others taken the same way.
Go deeper · optional, not assessedWhat a pour-through reading can and cannot say

A pour-through sample is the water held in the root zone, pushed out by water poured on top. North Carolina State University's procedure for nursery pots (NC State's pour-through procedure) waits until a thoroughly watered pot has drained but still holds its water, pours just enough over the surface to push out a small sample, and samples the same pots each time to follow a trend. Nurseries pour their own irrigation water and subtract its reading. A version from the University of Georgia, which UMass Extension reprints, pours distilled water, which adds nothing of its own and is the simpler choice at home (UMass Extension: Interpretation of soluble salt and pH measurements). Either works if you keep to it.

The method sets the number. A pour-through, a diluted media extract, and a saturated media extract dilute the root-zone water by different amounts, so the same pot gives three different readings, each with its own interpretation; a laboratory says which method it used. Timing matters too: a sample taken the day after a feed reads saltier than one taken just before the next. So a pour-through answers questions about change in one pot: are salts building, did the rinse bring them down, has the pH drifted since spring? It cannot say which ions make up the salts, whether any one nutrient is short, or what the leaves contain.

Iron that stays available

The response has two parts, and neither is more of the usual feed. The first changes what raises the pH: from now on the azalea is watered mostly from the rain barrel, which is soft, slightly acidic, and adds no alkalinity (Course 03, Lesson 5), with some hose water mixed in. Plenty of rainwater slows the drift, and the hose water still brings a little of the calcium and magnesium that rainwater lacks and the acid-loving feed does not list (Lesson 2). The second supplies iron in a made for alkaline root zones, such as EDDHA, rather than the common EDTA and DTPA, which are less reliable there (Lesson 2; Colorado State University Extension: Iron chlorosis of woody plants).

So read the chelate type on the label once you know the pH. The 8-4-6's iron is EDTA, which is less reliable in a root zone that reads Slightly alkaline, and that pH will come down only slowly; the granules' iron comes from iron sulfate, which is not chelated and does not stay available in an alkaline root zone. Neither is a dependable answer here, and both add salts the azalea does not need. The acid-loving feed's iron is EDDHA, made to stay available as the pH rises. At their label rates, its can carries about the same amount of iron as the 8-4-6's; what differs is what holds the iron. Its nitrogen, all ammonium and urea, also tends to make the root zone more acidic over time (Lesson 2). The household switches the azalea to it, 1 level scoop (1 g) per 1 L of water every 14 days while it grows, and keeps it for the azalea alone: its label says not to use it on plants that prefer neutral or alkaline conditions.

Neither part is a quick cure. The leaves already yellow may stay pale; the answer shows in the next new leaves and in that second reading. A chelate treats the symptom, not the cause. Colorado State's guide, written for garden soils, is plain that iron chlorosis has no easy, lasting correction; in a pot the lasting fix is slower than a chelate: the water and, at a suitable repotting, fresh mix (Course 11, Repotting and Root Development), because a mix resists changes to its pH, its , and one that has drifted comes back only slowly.

When a shortage is the best explanation

Sometimes a feed is the answer. A shortage becomes the best explanation when the evidence lines up. The pattern fits one: older leaves paling first and then the whole tree, veins included, or growth falling behind while every leaf stays green. The history makes one likely: a season or more without feed in a mostly mineral mix, which holds little and loses nitrate with every watering, or a nursery's granules long spent. And the other causes have been checked and ruled out: roots firm with white tips, a root zone that dries and rewets between waterings, nothing standing under the pot, good light, no insects under the leaves or along the stems, no sign of salts, and no reason to suspect the pH. When every check is clear and growth stays weak or pale, Course 01 calls a shortage “a reasonable suspect”, though a missed pest or disease is still possible (Course 01, Lesson 10).

Color is the weakest part of that evidence. A mild shortage can slow growth before any leaf changes color, so green leaves do not prove a tree has enough (Course 01, Lesson 6). Lesson 1 calls this a , and it is inferred the same way: from growth judged against what this kind of tree usually does at this point in the season, its (Course 01, Lesson 14), and against its own growth last year; from the feeding and substrate history; from the other limits ruled out first; and then from a label-rate feeding judged by the new growth that follows.

The response is the label plan at its rate and interval, applied to a moist root zone and recorded (Lesson 4). Expect the answer in the new growth over the following weeks: shoots that extend more strongly, and new leaves or needles that come out a fuller green. Leaves that were already pale may stay pale or drop, so they do not show the result. A fair trial uses the right line on the label, measured, at its interval, for long enough to see new growth, with nothing else changed at the same time.

Worked example: the gift juniper's first feeding

The west balcony in May. The gift juniper, which a club member confirmed in March as Japanese garden juniper (Juniperus procumbens ‘Nana’), has lived on the slatted shelf since its first winter, in a black plastic training pot about 22 cm (9 in) across: the old peat-rich supermarket root ball in the middle, inside a ring of coarse mineral mix that its roots have grown into (Course 03, Lesson 2). It has never been fed. It is green, but its new tips this spring are a little shorter than last year's. The owner has just bought the 8-4-6 for the balcony (Lesson 2) and works through the checks before the first can:

  1. Where and how it shows. Not in color: every needle is green. Only in its growth: set beside last year's photos from the same spot and the 's notes on new shoots, this spring's soft new tips are a little shorter at the same stage, on a juniper whose yearly growth is modest anyway (Course 02, Lesson 2). A trident or an elm marks where each flush began with bud scale scars (Lesson 1); a juniper leaves no such marks, so the record is the yardstick.
  2. The root zone and roots. The core and the ring dry and rewet as the log expects, the chopstick in the core still decides each watering, and white root tips show at the drainage holes.
  3. Under the pot. Nothing stands in the tray, which is emptied after every watering.
  4. The light. The shelf in the open middle takes every hour of afternoon sun.
  5. Pests. Nothing under the foliage or along the stems: no fine webbing, no pale speckling, no scale.
  6. The feeding record. Never fed, and nothing in the pot still releases: no granules or cakes have ever shown in the ring or on the surface, and whatever the supermarket's peat core once held has had a year and a half of thorough waterings to wash out.
  7. The water source. The kitchen tap, moderately hard and unsoftened, which brings some calcium and magnesium but little or no nitrogen.

Every check is clear, the history makes a shortage likely, and the only sign is in the growth: a hidden shortage is the best explanation. So the owner starts the 8-4-6 at its outdoor line, 4 mL per 1 L of water, which is 8 mL in the 2 L (about half a gallon) can, every 14 days while it grows, always on a moist root zone, watering first whenever the check says Drying or drier, and records each feed. Before the first can, they photograph the juniper from the same spot, measure three of this spring's tips, and note the date, so that in four to six weeks, around late June, they can compare like with like. Tips that by then reach as far as last year's or further, in the same fresh green, will say supply was the limit; no change at all will say it was not, and the next step is the one below, not a stronger can.

When a fair label-rate trial fails

Course 01, Quiz 2 ended one case with an open question: what if several weeks at the label rate bring no greener, stronger growth? Then supply was probably not the limit, or not the only one, and the answer is to look past supply rather than add more of it:

  • The pH and the water. An alkaline root zone can lock away the iron, manganese, and zinc a feed supplies, and the usual feed's chelate is less reliable there.
  • Salts. A root zone already salty from earlier feeding, softened water, or waterings that never run through can stop the roots using what is added.
  • Pests. Insects and mites feeding on the leaves pale them from outside the root zone; look under the leaves and along the stems (Course 07).
  • The product and how it is used. The right line for this use, measured with a syringe or the product's own cap or scoop rather than a kitchen spoon (Lesson 2), and a feed that supplies what the tree's water does not, such as calcium and magnesium for a tree on rainwater.
  • The light and the roots, again. A spot that has grown shadier since the checks moves the limit to light, and roots that look fine at the holes can still be failing deeper in the pot, which is a question for a diagnosis (Course 07).

Practice check · not scored

Course 01's ficus in inert grit, outdoors in full summer sun: after two unfed summers in a mix of mostly pumice and lava, it has had the 8-4-6 at its outdoor line every 14 days for six weeks, always on a moist root zone. Its new leaves come out no greener and its shoots no longer, though its roots are firm with white tips. What next?

When to get help

Some problems go past what checks at home can settle. Get help with a diagnosis when the tree keeps after you have worked through the checks and fixed what they found; when it shows signs no feeding check explains, such as speckling, stickiness, webbing, spots, soft dark roots, or shoots dying back from their tips; or when the tree is valuable enough that a wrong guess costs more than someone's time. Diagnosis belongs to Course 07; in the meantime, a local bonsai club, an experienced grower, or a good nursery can look at the tree with you.

Bring the evidence: photos from the same spot over the past weeks, close-ups of the tops and undersides of the leaves, the feeding record and the watering log, the water source, and what you changed and when. A record turns “it looks unwell” into a history someone else can read (Course 03, Lesson 7). Laboratories can test leaf tissue and the mix as well; for one bonsai the cost rarely makes sense, but for a valuable tree or a collection it can (Course 07).

The bright window ficus shows the kind of sign that sends you straight to help. In late August its leaves turn sticky, with pale speckling on top and small raised bumps along the veins underneath. Stickiness is often , left by insects feeding on sap, and the bumps match what its owner noted on its stems the day it came home from a greenhouse shop almost two years ago: typical of scale insects, never confirmed (Course 02, Lesson 8). Its feeding record is in order, the houseplant and indoor bonsai line every 14 days since spring, and no feed does anything about insects on the leaves. Keep it apart from other plants, photograph the leaves top and bottom, and get help (Course 07).

Three pairs of yellowing trees

Same yellow, different cause sets two trees side by side that look nearly alike and asks what each one needs first. There are three pairs: Yellowing in a moist pot, the household's trident and ficus from the start of this lesson; Newest leaves pale; and Older foliage yellowing. Each tree keeps its own place and month in its half of the stage; on a phone, the two halves stack, with each tree's words for the week right under them. Try these:

  1. Choose a pair, and open the Evidence cards in any order: Which leaves, Root zone check, Light where it stands, Feeding record, Water and pH, and Season and history. Each card opens for both trees at once, so you can compare their readings side by side; a card you opened earlier folds to a row, and Show opens it again.
  2. Under Your call, choose for each tree Feed as planned, Nothing to fix: normal for the season, or Investigate before feeding. After Investigate before feeding, choose under What first?: Empty the standing water and let it drain, Give it more light, Pause feeding and rinse the salts out with plenty of plain water, Check the water source and pH for iron, or Get help with a diagnosis (Course 07).
  3. Press Watch three weeks to see both choices play out in the next new growth or in the root zone, from Today to Week 3; Pause, Back, Next, Replay, and Skip to the end let you stop on any week. Each call is graded Right call or Not quite, or Lucky call when it was right but no card you opened pointed to it. Read the explanation under each, and change any call that was not quite right before you watch again.
  4. Solve all three pairs to finish. The finish card counts the calls that were right on the first watch, with Lucky calls counted apart, and the cards you opened.

Exercise

Same yellow, different cause

Choose a pair and open the Evidence cards; each one reads both trees at once. Make Your call for each tree (after Investigate before feeding, choose What first?), then press Watch three weeks. Solve all three pairs.

Pairs solved · 0 of 3

Pair 1 of 3 · Instructional examples

Yellowing in a moist pot

Same look: Drooping and yellowing over a moist root zone.

The two look alike from a step away. Read below the surface and the records, side by side.

The household's trident maple Acer buergerianum

What you see Brown, crisp edges on many leaves, a white crust on the rim and the surface, and leaves that hang limp every afternoon.

The household's ficus Ficus microcarpa

What you see Older leaves yellowing and dropping, a faint sour smell from the mix, and a saucer full of rain.

Evidence

0 of 6 opened · each card reads both trees

Your callTrident
Your callFicus

Make a call for each tree first.

Both trees' next three weeks play out above. Then each call is graded here, with what happened and why.

Text description

An exercise with three pairs of instructional-example trees. The two trees in a pair look nearly alike from a step away, and each stands in its own place and month, drawn side by side. The drawings show leaves and needles as they look, a white crust as white flecks on a pot's rim and surface, standing water as a pale blue surface in a saucer, and fallen leaves or needles around a pot.

Six evidence cards open in any order, each for both trees of a pair at once, with a small drawing and each tree's reading: Which leaves, Root zone check, Light where it stands, Feeding record, Water and pH, Season and history. For each tree, choose one call: Feed as planned; Nothing to fix: normal for the season; Investigate before feeding. After Investigate before feeding, a second question, What first?, offers five first steps: Empty the standing water and let it drain; Give it more light; Pause feeding and rinse the salts out with plenty of plain water; Check the water source and pH for iron; Get help with a diagnosis (Course 07). With a call for each tree, Watch three weeks plays both together through four stops, Today, Week 1, Week 2, Week 3. On the first day the call is carried out: a can pours, a saucer is emptied, a tree is moved, a sample is taken, or photos are taken for help. Over the weeks a round magnifier shows each tree's next new growth, and words under each tree name the new growth and what happened that week. Then each call is graded Right call, Not quite, or Lucky call (right, though no card you had opened pointed to it), each with its own explanation. A call that was not quite right can be changed and watched again. Solving all three pairs shows a finish card with the pattern.

  1. Pair 1, Yellowing in a moist pot. Same look: Drooping and yellowing over a moist root zone.
    • The household's trident maple (Acer buergerianum), backyard bench, late July. What you see: Brown, crisp edges on many leaves, a white crust on the rim and the surface, and leaves that hang limp every afternoon. Which leaves: Older leaves first · Brown edges. The lower, inner leaves went first: their edges yellowed, then browned and crisped from the lobe tips. The newest leaves are still green. Root zone check: Chopstick: Damp and cool on the lower half. Lift: Heavy. Root-zone moisture: Moist. Under the pot: the bench slats; nothing stands under the pot. Roots at the holes: Browned tips. A white crust rings the drainage holes. Light where it stands: Part sun or part shade. Morning sun on the bench. From early afternoon the neighbor's maple shades it, so the droop comes in the shade, not the heat. Feeding record: 8-4-6 liquid, outdoor line. Morning round: June 21 · 8 mL in 2 L · At the label rate; June 28 · 8 mL in 2 L · At the label rate; July 19 · 8 mL in 2 L · At the label rate; July 26 · 8 mL in 2 L · At the label rate. Evening round: June 18 · 8 mL in 2 L · At the label rate; June 25 · 8 mL in 2 L · At the label rate; July 16 · 8 mL in 2 L · At the label rate; July 23 · 8 mL in 2 L · At the label rate. Two sets of notes, one from each of them. Each shows a feed about every week since mid-June, apart from the two weeks away in July. The label says every 14 days. Water and pH: The hose: city water, moderately hard and unsoftened. No pH reading taken. Season and history: Midsummer, when a trident's leaves should be green and firm. Bought in leaf two springs ago; no root work and no move this year.
    • The household's ficus (Ficus microcarpa), backyard lawn, late July. What you see: Older leaves yellowing and dropping, a faint sour smell from the mix, and a saucer full of rain. Which leaves: Older leaves first · Yellowing and dropping. The oldest, lowest leaves turn yellow all over and fall at a touch. The newest leaves are still green. Root zone check: Chopstick: Dark, cool, and wet along its whole length. Lift: Heavy, as if just watered. Root-zone moisture: Wet. Under the pot: a saucer full of rain; the pot has stood in it all week. Roots at the holes: Browned tips. The mix smells faintly sour. Light where it stands: Full sun. The open lawn: full sun on most days, though this week was mostly cloud and rain. Feeding record: 14-4-14 powder, outdoor line. May 30 · 2 scoops (2 g) in 2 L · At the label rate; June 13 · 2 scoops (2 g) in 2 L · At the label rate; June 27 · 2 scoops (2 g) in 2 L · At the label rate; July 18 · 2 scoops (2 g) in 2 L · At the label rate. Every 14 days at its outdoor line since late May, apart from the two weeks away in July. Water and pH: The rain barrel: soft water with almost no calcium or magnesium, which is why it gets the 14-4-14. No pH reading taken. Season and history: Late July. It has stood on the lawn since late May. A rainy week has just ended: it rained most days, and nobody emptied the saucer.
  2. Pair 2, Newest leaves pale. Same look: New growth paler than the old.
    • The household's azalea (Rhododendron indicum), front step, late July. What you see: Its newest leaves are yellow between green veins, while the older leaves stay dark green. Which leaves: Newest leaves first · Yellow between green veins. Only the leaves of this year's flush: yellow between a net of green veins, palest at the very tips. The older leaves are dark green all over. Root zone check: Chopstick: Damp and cool on the lower half. Lift: Lighter than after watering. Root-zone moisture: Moist. Under the pot: the stone step; the glazed pot drains freely and nothing stands under it. Roots at the holes: New white root tips. Light where it stands: Part sun or part shade. Morning sun on the front step, then bright shade from midday: the light an azalea likes. Feeding record: 8-4-6 liquid, outdoor line. May 30 · 8 mL in 2 L · At the label rate; June 13 · 8 mL in 2 L · At the label rate; June 27 · 8 mL in 2 L · At the label rate; July 18 · 8 mL in 2 L · At the label rate. The 8-4-6 at its outdoor line every 14 days since late May, apart from the two weeks away in July. Water and pH: The hose: city water, moderately hard and unsoftened. A pour-through sample taken this week, read on a borrowed calibrated meter: Slightly alkaline. Season and history: Late July. The yellowing came with the May flush and has not changed since. It has stood on the front step all summer.
    • The courtyard ficus (Ficus microcarpa), hallway shelf, December. What you see: New leaves pale and thin, with long gaps between them, and lower leaves yellowing and dropping. Which leaves: Newest leaves first · Pale all over · Long shoots with long gaps between leaves. This winter's leaves came out pale, thin, and small, far apart on long, thin stems. A few lower, older leaves yellow and drop. Root zone check: Chopstick: Damp and cool on the lower half. Lift: Heavy. Root-zone moisture: Moist. Under the pot: a cork pad on the shelf; nothing stands under the pot. Roots at the holes: Few new tips. Light where it stands: Shade. A hallway shelf far from the windows, in December's short days: dim even at noon. Feeding record: 8-4-6 liquid, houseplant line. October 6 · 2 mL in 1 L · At the label rate; November 3 · 2 mL in 1 L · At the label rate; December 1 · 2 mL in 1 L · At the label rate. The houseplant and indoor bonsai line, every 28 days while it keeps growing in winter. Water and pH: City water from the courtyard's brass tap, moderately hard. No pH reading taken. Season and history: December. It spent the summer on the courtyard bench and grew well there, then came indoors to this shelf in October.
  3. Pair 3, Older foliage yellowing. Same look: Yellowing in the older foliage.
    • The household's Japanese larch (Larix kaempferi), patio bench, June. What you see: Needles pale all over, palest on the older spurs, and this year's long shoots short. Which leaves: Older leaves first · Pale all over. Every needle is paler than it should be, the older spurs palest, and this year's long shoots are short. No brown tips. Root zone check: Chopstick: Damp and cool on the lower half. Lift: Lighter than after watering. Root-zone moisture: Moist. Under the pot: the patio bench; nothing stands under the pot, and the mix dries and rewets between waterings. Roots at the holes: New white root tips. Light where it stands: Full sun. The patio bench by the sunny south wall: sun from morning to late afternoon. Feeding record: No feed this year. Last year: Spring to June · every week, at full strength; June · rinsed with plain water; feeding paused. This year: No feeds. Nothing since last June, when weekly feeds at full strength built up salts and the pot was rinsed. No one restarted the feeding. Water and pH: The hose: city water, moderately hard and unsoftened. No pH reading taken. Season and history: June, in the spring flush, when a larch's long shoots should be lengthening fast. Since last June's rinse it has had plain water only.
    • The nursery juniper (Juniperus procumbens ‘Nana’), west balcony, October. What you see: Needles yellowing and browning inside the pads, while the tips and the outer foliage stay fresh. Which leaves: Older leaves first · Inner needles yellowing. Only the oldest needles, deep inside the pads, which fall at a touch. The tips and the outer foliage are fresh green. Root zone check: Chopstick: Damp and cool on the lower half. Lift: Lighter than after watering. Root-zone moisture: Moist. Under the pot: a tray on the shelf, emptied after each watering. Roots at the holes: New white root tips. Light where it stands: Part sun or part shade. The balcony shelf: sun from midday on, lower and softer now in October. Feeding record: 8-4-6 liquid, outdoor line. August 22 · 8 mL in 2 L · At the label rate; September 5 · 8 mL in 2 L · At the label rate; September 19 · 4 mL in 2 L · Half the label rate; October 3 · 4 mL in 2 L · Half the label rate. The outdoor line every 14 days since May, held after a July dry spell until new growth showed recovery, resumed in August, and tapered to half strength in late September as growth slowed. Water and pH: The kitchen tap, carried out in the 2 L can: city water, moderately hard. No pH reading taken. Season and history: October, as growth slows. The yellowing began deep inside the pads this month. In July a hot spell dried its pot while the owner was away.
Simplified: three invented pairs drawn as typical signs, three weeks compressed into seconds. Real trees can have more than one problem at once.

Each pair looked alike from a step away; up close the leaves narrowed the list, and the other cards settled it. In Yellowing in a moist pot, the Feeding record and the Root zone check did: the trident's two sets of notes each showed a feed at the label rate about every week, and its new leaves came out clean only after a rinse and a pause, while the ficus's saucer was full and its mix sour, and its new leaves stopped yellowing once the water was emptied. Fed instead, both got worse. In Newest leaves pale, Water and pH and Light where it stands did: the azalea's pour-through sample read Slightly alkaline, and mostly rain-barrel water, with some hose water, and the acid-loving feed's EDDHA iron brought greener new leaves; the courtyard ficus stood on a hallway shelf away from the windows, its feed already at the houseplant line, and only a brighter spot gave it shorter, greener growth.

Older foliage yellowing was the one pair where a feed was right, for one tree only. The larch, unfed since its salts were rinsed out last June, had needles pale all over and palest on the older spurs, and its long shoots were short; with firm roots, full sun, and a root zone cycling normally, a shortage was the best explanation, and restarting the label plan, recorded, gave it green new needles and longer shoots. The nursery juniper's yellowing sat inside the pads in October, its tips fresh and the tree fed on the label since May, apart from a pause after July's dry spell, and already tapering as growth slowed: the normal autumn shed, which feeding did not change. Across the six trees, a feed was the right first move once, and every right move came from the cards, not the color.

Investigate before you feed a struggling tree

The bonsai decision

Find the cause before you feed

Starting point

When a tree's leaves pale or its growth weakens, work through these before any feed, and fix what you find:

  1. Note where and how it shows, and look under the leaves: older leaves first, newest leaves first, all over the tree, or only in its growth; pale all over, yellow between green veins, or brown at the edges; and any speckling, stickiness, webbing, or insects (Lesson 1).
  2. Check the root zone and the roots, what stands under the pot, and what changed: the chopstick in the core, the lift, the drainage holes, the smell, any tray, saucer, or cover pot, and any recent root work or move (Course 03, Lesson 4). If water stands under it or the mix smells sour, empty it, let it drain, and hold the feed until new growth shows the roots working.
  3. Check the light where the tree stands, against what its kind needs (Course 02, Lesson 4).
  4. Read the two records: the feeding record (which product, how much, how often, and who fed it) and the water source (tap, rain, softened, or filtered, with the utility's report if you have one).

Feed only when a shortage is the best explanation, at the label's rate and interval, and judge it by the new growth over the following weeks.

This assumes

  • You can check below the surface with a chopstick and the lift (Course 03, Lesson 2), and you know roughly what the tree is and how it normally looks at this time of year (Course 02, Lesson 2).
  • The change has come on over days or weeks. A sudden droop is a watering question first (Course 03, Lesson 4).
  • You have, or can rebuild, the last few weeks of feeding and watering, from a record or from memory (Lesson 6).
  • The months in this lesson belong to the course's Northern Hemisphere example climate. In the Southern Hemisphere, swap north and south, and shift the months by half a year.

Change course if

  • A crust and brown leaf edges, or a droop in a moist pot, after frequent feeding: pause feeding and rinse the salts out with plenty of plain water (Course 03, Lesson 5). Resume once new growth looks normal, at or below the label rate, with the cause corrected.
  • Newest leaves yellow between green veins on hard water: check the water source and pH for iron. Get a reading taken one consistent way; if it is higher than the tree needs, switch to mostly rainwater and choose a feed whose iron is chelated to stay available there, read on its label, rather than more of the usual feed.
  • Pale, thin new growth with long gaps between the leaves: give it more light first, raised in steps, and judge the next new growth before you change the feeding.
  • The pattern and the history fit a shortage, and every check is clear: start the label plan at its rate and interval (Lesson 4), record it, and judge the new growth over the following weeks. If a fair trial fails, look past supply; never double the dose.
  • The tree keeps declining after the checks, or its leaves turn sticky or speckled: this is not a feeding problem: get help with a diagnosis (Course 07), and bring the photos and the records.

Back to the question

Why might feeding fail to fix pale leaves or weak growth?

Because fertilizer answers only one cause of pale leaves and weak growth: too little of a nutrient in a form the roots can take up. Roots short of air or water, too little light, salts built up from feeding, an alkaline root zone that locks iron away, insects on the leaves, and the turn of the season all pale or yellow a tree, and more feed fixes none of them; where the roots cannot use it or salts are already high, it makes things worse. Leaf color narrows the list but never settles it. So check the root zone, what stands under the pot, and the light, read the feeding record and the water source, and fix what you find. Feed when a shortage is the best explanation, at the label rate, and judge it by the new growth.

Key takeaways

  • Pale or yellow leaves are a symptom with many causes: roots short of air or water, low light, salts, pH, pests, the season, and sometimes a real shortage. Where and how the change shows narrows the list, but it never settles it.
  • Check the roots and the water before the feed. Roots that are short of air, dried out, or recently cut take up little of what is already there, and feeding them only adds salts around them.
  • A crust with brown leaf edges, or a droop in a moist pot, after frequent feeding points to salts: pause feeding, rinse with plenty of plain water, and resume at or below the label rate once new growth looks normal, with the cause corrected.
  • Newest leaves yellow between green veins on hard water are a clue to iron locked away by a rising pH. Get a reading taken one consistent way, switch to mostly rainwater, and use iron chelated to stay available at that pH; more of the usual feed does not help.
  • Feed when a shortage is the best explanation: the pattern and history fit, and roots, water, light, pests, salts, and pH are ruled out. Judge it by the new growth over weeks; if a fair trial fails, look past supply and never double the dose.

Optional · not required for completion

Write one tree's month, then check it in order

Pick one tree, healthy or not, and write down its last month from your records or from memory: each feeding (the product, the amount, and who gave it), each watering, the weather, and anything that changed. Then work through this lesson's checks in order: where any change in color or growth shows, what is under the leaves, the root zone and the roots, what stands under the pot, the light, the feeding record you have just written, and the water source. Which check would you have skipped, and what would that month's record tell someone helping you?

No tree? Find a hedge, a shrub, or a street tree with yellowing leaves. Note where the yellow is (older leaves, newest leaves, or all over) and its pattern, then list two causes it could have and the check that would tell them apart.

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All lessons in Mineral Nutrition and Fertilizing

  1. Section 1

    Nutrients, labels, and sources

    Find out what mineral nutrients do in a tree and why a small pot runs low, read a fertilizer label line by line, and compare how liquid, solid, organic, and mineral feeds release what they carry.

    1. 18 min
    2. 18 min
    3. 18 min
    4. 15 min

  2. Section 2

    Feeding plans that follow the tree

    Build a feeding plan from the label and adjust it to the tree's growth, goal, roots, and weather, investigate before feeding a struggling tree, and write a plan and a record that everyone who helps can follow.

    1. 18 min
    2. 18 min
    3. 18 min
    4. 18 min