Drying, saturation, and root oxygen

The west balcony in July. 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). At 6 pm, after a day of 32 °C (90 °F), the sinking western sun has been reaching under the balcony above since midafternoon, onto the trident maple by the back wall. Its leaves hang limp, and its soft shoot tips bend over. Before watering, the owner checks: the terracotta pot is heavy, and a chopstick left a few minutes in the root zone comes out damp and cool on the lower half.

In June, while the owner was away for a work week, a friend came every second day to check the pots and water any that needed it. On one visit, the nursery juniper looked dull and grayish, though nothing on it drooped. Its pot was light, and the chopstick came out dry and pale along its whole length.

At the bright window, there is last winter to look back on. In the ficus's first months on the sill, its owner took to giving it a little water every morning where it stood, skipping the walk to the sink and the look into its outer pot an hour later. After a few weeks, its lower leaves began to yellow and drop, though the surface was damp every day. The sill, the curtain, and the grow light were as they had been since October.

Three trees and three different signs, and none of them, on its own, says what is happening at the roots. A tree can droop, dull, or yellow because its roots are short of water, because they are short of air, or because a hot afternoon has briefly outrun roots that are fine. The first two have opposite remedies, and the third needs none. This lesson shows how each happens, how to investigate a droop before you water, and which watering habits leave roots short of water or air.

Two problems, one look

Leaves stay firm only while the roots replace the water they lose. When loss outruns supply for long enough, the leaf cells go limp: wilting (Course 01, Lesson 8). A root zone that has dried out has too little water left to take up. One that stays saturated has plenty, but its roots are short of oxygen. Roots respire day and night, and when air cannot get back into the pores, their tips stop growing and die back, so the tree takes up less of the water around them (Course 01, Lesson 7). Either way, the leaves droop, dull, yellow, or brown at the edges.

Of the five ways Course 01 lists for loss to outrun supply, four matter to a watering decision: a dry root zone, roots that cannot take up enough (short of air, reduced by root work, or overheated in a sunny pot), salty root-zone water, and very high demand. The first two call for opposite remedies, water for one and none for the other, and only a check below the surface tells you which you have (Lesson 2).

Junipers, like most conifers, have stiff needles or scale leaves that barely sag. A thirsty juniper turns dull and grayish before parts of it brown, and one whose roots are short of air can stay green for weeks while they fail (Course 02, Lesson 7), so check its root zone rather than wait for its foliage. Thin-leaved broadleaf trees such as the trident maple and the Chinese elm droop clearly, which makes them easier to read, and easier to misread. A ficus's thick, leathery leaves droop less; it more often yellows and drops leaves.

A droop that passes by evening

The trident was showing temporary wilting: a droop on a hot, bright, or windy afternoon that recovers by evening in a moist root zone. In late afternoon, evaporative demand, the air's pull on the water in leaves, is still near its peak just as the sinking western sun reaches under the overhang (Course 02, Lesson 5), and for an hour or two loss outruns uptake. More water cannot speed the roots up. So when a broadleaf droops late on a hot afternoon over a moist root zone, with nothing else changed and no water standing under it, the decision is Not yet: check again this evening. Leaves that have firmed by dusk need nothing more.

Temporary is not the same as harmless. Limp leaves make little sugar, so a tree that droops most afternoons grows less and needs shade through the hottest hours (Course 02, Lesson 5). A droop over a dry root zone is never temporary wilting: water thoroughly now. And a droop in the morning or on a cool day, one that comes with dulling or yellowing, or one still there at dusk calls for Investigate before watering.

When a root zone dries

This lesson uses Lesson 2's six moisture levels: Bone dry, Dry, Drying, Moist, Wet, and Saturated. As the mix dries beyond Drying toward Dry, the last water is held more and more tightly in the thinnest films, and the roots find it harder to take up (Course 01, Lesson 8). A hormone signal narrows the stomata, which saves water but lets in less carbon dioxide, so the leaves make less sugar (Course 01, Lesson 4). That is drought stress, and it begins before any leaf droops: a tree that often runs a little dry makes less to grow with (Course 01, Lesson 6).

If the drying goes on, a broadleaf droops, and the trident maple's leaf edges brown and crisp, a kind of leaf scorch; a juniper turns dull and grayish. Below the surface, root hairs shrivel first, then the young fine roots and their root tips in the driest layers die. Watering brings the water back, but not those roots: they do not revive, and the tree must grow new ones, which takes time and sugar (Course 01, Lesson 2).

At Bone dry, a peat-rich or bark-rich mix shrinks from the pot wall and turns water-repellent for a while (Lesson 3). Water poured on top then runs down the gap or a few easy paths and out of the holes, leaving the core dry: channeling. By then many fine roots have probably died (Course 02, Lesson 6).

Four habits leave roots short of water:

• Waiting for the leaves to droop. By the time a broadleaf droops from drying, drought stress has already cost it growth, and a juniper may never droop.
• Checks too far apart for the weather. In summer heat, a small pot can dry within a day.
• Small sips in heat. They wet only the top few centimeters, and the rest dries.
• Counting on rain. An overhang or a dense canopy keeps most of it off the pot (Lesson 5).

What to do when it is dry

A dry root zone needs urgent care, whatever the time of day and whatever else is going on with the tree (Course 01, Lesson 14):

• Water thoroughly now. Give a thorough watering, in slow, even passes across the whole surface until water runs from the holes. If it runs straight through, water again slowly, aiming at the dry part; if the root ball has shrunk from the wall, or passes do not wet it, use immersion watering (Lesson 3). Once it drains, confirm with the lift test and a chopstick in the core that it wetted through.
• Check more often for a week or two. A tree that lost fine roots draws water more slowly, and a root ball that dried out once can rewet unevenly.
• Expect some lost growth. New shoots may stall while roots regrow, and a juniper can stay green for weeks, then brown in patches (Course 01, Lesson 14).
• Do not feed. Fertilizer cannot replace roots, and salts concentrate in a mix that has dried, adding stress to the roots that are left (Course 01, Lesson 10). When and how to feed again is Course 05, Mineral Nutrition and Fertilizing.

That is what the friend did in June: three slow passes until water ran into the tray under the shelf, then the lift and the chopstick to confirm that the juniper had wetted through, and the tray emptied. Its color came back by the next visit, but a dry spell like that is one reason the August plan does not rely on visits every second day alone (Lesson 6).

When a root zone stays wet

Wet is not the problem. A thorough watering fills most of a pot's pores, and for a short time the root zone is Wet. In a pot that drains, the extra water runs out within the hour and draws air back into the larger pore spaces (Course 01, Lesson 7). That supply of air, aeration, is what roots live on between waterings. A thin, wetter layer always lingers at the very bottom (Course 04, Soil, Substrates, and the Root Environment, explains why), but above it, a free-draining pot holds water in its small pores and air in its large ones.

The harm comes when saturation lasts. Roots and soil microbes use up the dissolved oxygen within hours to a day or so, sooner in warm weather, and the root zone falls into hypoxia. Root tips die back, then the fine roots behind them, and days later the foliage can droop, yellow, or drop in a pot full of water. Some root-rot organisms thrive in waterlogged mix and attack weakened roots, a matter for Course 07, Tree Health and Diagnosis.

So overwatering is a root zone kept wet so constantly, or draining so poorly, that air never gets back in (Course 01, Lesson 7). It is about how often a pot is watered and how well it drains, not how much is poured at once: one thorough watering of a pot that drains cannot overwater it, but a little every day into a pot that never dries back can.

Setups that keep air out

Lasting wetness often comes from the setup or the watering habit, which you can change today. A mix that is too fine or has broken down holds too much water as well; that is Course 04's subject, and Lesson 7 shows how a log reveals it. The common setups:

• Standing water under the pot. A tray, a saucer, or a cover pot that holds drained water keeps the bottom of the root ball saturated, however well the pot drains (Course 02, Lesson 7). Empty it after every watering and every rain; a grid, like the one under the balcony trident's pot, keeps the pot above any water until you do.
• A pot much bigger than its roots. Mix the roots have not reached stays wet long after each watering (Course 02, Lesson 7).
• Watering a pot that is still moist. In cool weather, in shade, while a deciduous tree is bare, or after roots have been reduced, a pot dries slowly, and a schedule tops it up before it has dried back (Lesson 1).
• Small sips into a pot that drains poorly. In a slow-draining pot, or one standing in an outer pot or a saucer, what the tree does not use collects at the bottom, out of sight, while the surface looks freshly watered.

The bright window's ficus spent its first winter in two of these setups at once: standing water under the pot, and watering a pot that was still moist. In winter's low light it grew slowly, and even with the heating on it used far less water than a daily watering gave it. Each morning's water drained into its outer pot, where nobody looked. The light had not changed, and the yellowing began weeks after the habit did, so the habit was the first suspect.

Two weeks of watering, seen from the roots

Four ways to water for two weeks opens the pot that Lesson 1's comparison kept closed. It follows the balcony trident from late May into a June warm spell: cool and cloudy, then warm, then hot, starting the morning after a thorough watering. The trident stands in the rain shadow, and the fortnight's rain falls still, so rainy days slow its drying but add no water.

The cutaway shows moisture and air in the top, middle, and bottom zones. Beside it, Root-zone moisture reads the root zone, Root tips by zone, magnified shows each zone's root tips, and Leaves shows the canopy; on a phone they stack under the drawing, and a Day slider takes the place of the day buttons. Try these:

1. Under Watering pattern, start with Check, then water thoroughly. Under Your call, pick the pattern you think will leave the fewest new root tips, or press Skip. Your call locks when you run, and it is stamped Called it or Surprise once the runs settle it.
2. Press Run the fortnight. Use Pause, Back, Next, the day buttons, or Skip to the end to stop on any day. At each check, watch the chopstick come out and be read beside the pot, and watch how the three zones move between waterings.
3. Read the report card: Days roots short of water, Days the bottom stayed wet, New root tips, Foliage, and Waterings, beside your previous run. Then press the button under it, which names a pattern you have not run yet, and run that one.
4. In A little every morning and A full can every day, where do the white tips end up, and which zone never dries back? In A full can every day, watch the day strip: a marker drops when a zone loses its white tips, and a dotted line joins it to the day the leaves change.
5. In Only when the leaves droop, compare the day the leaves droop with the day the roots are hurt. Once all four patterns have run, the Pattern report sums them up.

Only Check, then water thoroughly kept every zone healthy. It watered eight times, rarely in the cool start and more often in the heat, each time when the chopstick came out dry with only the tip faintly damp. The root zone cycled from Wet through Moist to Drying, white tips grew in all three zones, and the leaves stayed firm. A little every morning kept the surface looking freshly watered, but its splash never got past the top few centimeters. Once the warm days came, the middle and bottom dried out and the roots crowded toward the top, and in the hot days even the top dried by afternoon. The leaves drooped on nine afternoons and scorched at the edges, no water ran through to carry salts out (Lesson 5 returns to that), and by day 14 no white tips were left: the fewest of any pattern.

A full can every day suited the hot days, but in the cool start its bottom zone never dried back. Its root tips stopped growing and browned on day 3, while the leaves only dulled on day 7 and yellowed later: on the day strip, a dotted line joined the two markers. Only when the leaves droop let the root zone dry out again and again, killing fine roots in the top and middle each time, so only the bottom kept white tips, and weaker growth is still to come. In both runs that dried out, the leaves showed it the same day the roots were hurt, but by then the fine roots were already dying. Harm from lasting wetness reached the leaves days later. Either way, the harm built below the surface first, so water by the root zone, not by the clock or the leaves.

The wet-but-wilting investigation

When the leaves and the root zone disagree, the decision is Investigate before watering: a tree that droops, dulls, or yellows in a pot that checks Moist or wetter, a pot that has been standing in water, or a pot that stays Wet far longer than the weather explains. It means finding out why first, never watering by default. Temporary wilting is the one expected exception: it gets Not yet: check again this evening, and the investigation starts only if the leaves have not firmed by dusk. Work in this order, because an early finding can settle it:

1. Confirm the moisture below the surface, with the chopstick in the core, the lift, and the mix at the drainage holes. In a two-zone root ball like the gift juniper's, read the old core (Lesson 2). If the root zone is dry after all, water thoroughly now.
2. Ask what changed in the last days or weeks: root work, a move, a heat wave, strong wind, a new tray or cover pot, a change in who waters, or a pot wall left in the sun, which can injure roots even in a moist pot (Course 02, Lesson 5).
3. Look underneath for water standing in a tray, a saucer, or a cover pot, and check that water can leave the holes.
4. Smell the mix at the surface and at the holes. Earthy is normal; sour or rotten suggests roots that have been short of air.
5. Consider salts if the tree has been fed often or heavily, especially if the rim is crusted white. Soluble salts make water harder for roots to draw in, so a tree can wilt in a moist pot (Course 01, Lesson 10). On moderately hard tap water a crust forms anyway, so a crust without heavy feeding points elsewhere.
6. Look again at dusk. Leaves that firm by evening over a moist root zone point to a passing peak in demand; leaves still limp at dawn point to the roots.

Then act on what you found. A wet root zone gets no water, however the leaves look. Reduced roots get less exposure: bright, but sheltered from wind and hot afternoon sun. A sun-baked pot wall gets shaded, and standing water gets emptied and its cause fixed. Suspected salts are the one exception to adding no water: pause feeding, and run plenty of plain water through the pot to carry the salts out. In a pot that drains, that water runs straight through and air follows it back in (Lesson 5 shows how). A tree still wilted the next morning, or declining while its root zone stays moist, needs a diagnosis from an experienced grower, a local bonsai club, or a good nursery (Course 07).

Worked example: the Chinese elm in a cover pot

The small backyard's Chinese elm (Ulmus parvifolia) lives in a plastic nursery pot about 15 cm (6 in) across, on the bench in the western third of the yard, with morning sun and afternoon shade. In June, the household set it in a glazed cover pot on the patio for a family party, and it stayed there, forgotten, through a week of rain. On the first hot, sunny afternoon after the rain, its soft shoots hang limp. The household works through the sequence:

1. Moisture below the surface. The pot is heavy, as if just watered, and the chopstick comes out dark, cool, and wet along its whole length.
2. What changed. A move into full sun and the heat off the paving and the house wall, and a week of rain into a pot standing inside another.
3. Underneath. Lifted out, the nursery pot drips, and water stands in the cover pot deep enough to have reached the bottom of the root ball.
4. Smell. Faintly sour at the drainage holes: part of the root zone has gone short of air.
5. Salts. No fertilizer this year, and no crust on the rim.
6. Dusk. The decision cannot wait for it, because the roots go on running short of air while the water stands.

So the household tips out the cover pot and leaves it off, adds no water, and moves the nursery pot back to its bench to drain: there it is bright, but shaded through the hot afternoon and sheltered by the fence. Some lower roots may have died, so for the next few weeks the elm is treated as a tree with reduced roots. If its leaves have firmed by the next morning and the root zone dries back toward Moist over a day or two, watering resumes by the check. If it droops at dawn on any of the next few mornings, or is getting worse while the pot stays moist, get help with a diagnosis (Course 07). One of the case files below finds the elm drooping at dawn two days later.

Watering a tree with reduced roots

A tree has reduced roots whenever it has fewer working fine roots than its foliage needs: after root work, like Course 01's trident maple two weeks after its roots were trimmed (Course 01, Lesson 8), or after a dry spell or standing water has killed fine roots. Fewer roots draw water, so the pot dries more slowly than before, yet the leaves can droop on a sunny, windy afternoon while it is still moist. Full aftercare belongs to Course 11, Repotting and Root Development; for watering:

• Check below the surface every day, rather than assuming the pot dries as it used to.
• Water thoroughly only when the root zone is drying, never because the leaves droop. More water cannot raise what the remaining roots take up, and a pot kept wet slows the new roots the tree needs.
• Keep it bright but sheltered from wind and hot afternoon sun, not in a dark room or deep shade, because new roots are built from sugar the leaves make in light (Course 01, Lesson 8). Make sure nothing stands under it.
• Return it to its usual spot in steps as new growth extends and the leaves stay firm.

Six trees that look unwell, case by case

Why is it drooping? puts the investigation to work on six case files, each a tree with a one-line report: Trident, 6 pm, Repotted trident, Window ficus, Nursery juniper, Patio larch, and Backyard elm. Choose a case and read its report under the scene. Open the evidence cards in any order: Chopstick in the core, Lift, Drainage holes, Look under the pot, Smell the mix, Recent history, Weather now, and Look again at dusk, which moves the scene ahead to evening. Under What do you do now?, choose Water thoroughly now, Not yet: check again this evening, or Investigate before watering, then press Make the call; choosing an option only selects it. After Investigate before watering, What first? asks for the first action: Do not water: reduce sun and wind exposure, Empty the standing water and let it drain, Pause feeding and rinse the salts out with plenty of plain water, or Get help with a diagnosis (Course 07). Choose one and press Choose this first step. The feedback names the evidence that settles each case, and the Detective's notebook writes up each case you close. Try these:

1. Start with Trident, 6 pm, the balcony trident from the prediction above. Open Chopstick in the core and Look again at dusk before you make the call.
2. In each of the other cases, open at least one card that reads below the surface and one about what changed, such as Recent history, before you make the call.
3. When a case closes, read The evidence that settles it, and open any card it marks as not opened. Then press the button that names the next case, and keep going until all six are closed and the 6 cases closed card appears.

Two of the six cases did not call for Investigate before watering. The balcony trident checked moist, nothing else was wrong, and it firmed by dusk: Not yet: check again this evening. The nursery juniper was light, with the chopstick dry along its whole length: Water thoroughly now. It was the one tree that did not droop; its dull, grayish color was the warning.

The other four drooped, yellowed, or browned over a moist or wet root zone, and each needed a different first step. The trident trimmed two weeks earlier needed less sun and wind, not water. The ficus, with water standing in its outer pot and a faint sour smell, needed that water emptied, and then watering at the sink only when a check said the root zone was drying. The Japanese larch (Larix kaempferi), fed at full strength every week, needed feeding paused and the salts rinsed out; its crust alone could have been hard water. The Chinese elm, still drooping at dawn and smelling sour two days after its cover pot was emptied, needed help with a diagnosis. In all four, the chopstick and the lift ruled out water, and the first step came from what had changed or what stood under the pot: Recent history for the repotted trident, Look under the pot and Smell the mix for the ficus, Recent history and the crust at the Drainage holes for the larch, and Smell the mix and Recent history for the elm.

Between too dry and too wet

Together, the two halves of this lesson describe a cycle. After a thorough watering, a pot that drains is Wet for a short time, then settles at Moist as its large pores drain and fill with air. Over hours or days the tree draws it down to Drying, the moment to water thoroughly again.

Roots work best at Moist and keep working as it dries toward Drying, with water films to draw on and air in the pores; Course 01's slider shows it particle by particle (Course 01, Lesson 2). The cycle overshoots into Dry when checks are too far apart for the weather, and stalls at Wet or Saturated when water cannot leave or keeps arriving too soon.

What to do when a tree droops