Reading moisture below the surface

Late March on the west balcony, an instructional example this course returns to: a fourth-floor balcony facing west in a temperate Northern Hemisphere city at about 40° N (USDA zone 7a). The gift juniper's spreading foliage hides most of its pot. In December it was slip-potted, root ball and all, into a training pot, so an old, peat-rich root ball now sits inside a ring of coarse fresh mix (Course 02, Lesson 7). Since January, its owner has judged it by a fingertip and a chopstick near the rim, the easiest spots to reach under the foliage.

This morning, with the foliage lifted aside, the surface is pale and looks dry. A probe moisture meter a friend lent reads “DRY” near the rim, yet a chopstick left a few minutes in the same spot comes out damp and cool on the lower half. The pot feels lighter than after its last watering, but not light. Then the owner tries a new spot, partway between trunk and rim, in the old root ball itself: there the chopstick comes out dry and pale along its whole length.

Five readings from one pot, and they do not agree. Should the owner water now, or wait? This lesson goes below the surface: what each check reads and where it can fool you, how to put checks together, and where to check in a root ball that dries unevenly. If you need to decide about a tree of your own today, read the next section, then the two checks at the end of this lesson, which work for any pot that drains.

What you are checking for

The question is how much water the root zone, the substrate where the roots are, still holds. Course 01, Lesson 2 introduced the basic checks; this lesson takes each one further, including what it cannot tell you.

This course uses six words for how moist a root zone is, from driest to wettest:

• Bone dry: pale and loose, or shrunk away from the pot wall. Fine roots are dying, and a peat-rich mix sheds water.
• Dry: dry all the way down. It is late, and the tree needs water now.
• Drying: dry through most of its depth, with only the bottom still faintly damp, and the pot feels light. For most trees in active growth, this is the usual moment a check says water.
• Moist: damp from partway down to the bottom, with water in the small pores and air in the large ones. This is the healthy middle.
• Wet: just watered and draining, which is normal for a short time.
• Saturated: every pore full of water, with no air. It is normal only briefly, right after watering in a pot that drains, and harmful when it lasts.

Every check ends in one of three decisions:

• Water thoroughly now. Wet the whole root ball, slowly, until water runs from the drainage holes, then confirm that it soaked in. That is a thorough watering, and Lesson 3 shows how.
• Not yet: check again later. Do not water now; look again this evening or tomorrow morning.
• Investigate before watering. This is for three situations: the tree and the root zone disagree (a tree that droops, dulls, or yellows in a pot that checks Moist or wetter), the pot has been standing in water, or it stays Wet far longer than the weather explains. It means looking closer, at the middle of the root ball, the holes, underneath the pot, the smell, and what changed recently, before you decide. It never means watering by default (Lesson 4).

For an established tree in active growth, the routine is a morning check, not a morning watering: water only when the check says the root zone is drying, wet it thoroughly, and confirm that it soaked through and drained (Lesson 1).

Why the surface misleads

The surface is the first part of a pot to lose water. Sun, warm air, and wind dry the top layer, so it can look dry for a day or more over a root zone that is still moist, while the wettest layer is usually at the bottom (Course 04, Soil, Substrates, and the Root Environment, explains why). It can mislead the other way too: a light shower, or mist drifting down from the leaves, can darken the top layer over a root zone that is drying. A glance reads only the top few millimeters.

Often you cannot see the mix at all. Moss can stay green over a root zone that has dried below it (Course 01, Lesson 2). A top dressing is a thin layer of grit, fine gravel, or moss spread over the mix to keep the surface from washing out. It hides the mix too, and so does loose decorative gravel: brush a little aside, or check beside it. A spreading canopy like the gift juniper's hides the surface as well, so lift the foliage aside before you look, touch, or push in a chopstick.

Two setups hide the root zone from most checks, and both are common on trees sold as gifts. A crust of pebbles glued over the surface sheds water and hides the mix, so there is nothing to see or touch, and a chopstick can go in only through the gap at the rim (Course 02, Lesson 7). A pot with no drainage hole keeps water standing in its bottom, out of sight, so the surface can look thirsty, and a chopstick that stops short of the bottom can come out nearly dry, while the roots below sit in water (Course 02, Lesson 7). Fix the pot first: if it has no hole, slip the root ball into a pot that drains, then soak the crust off. Until then, the lift and the tilt still help: a heavy pot, or water that trickles out when you tilt it over a sink, means add no more.

The checks, one by one

Seven checks are worth knowing: the surface, a fingertip, a chopstick, the lift, the drainage holes, a probe meter, and the leaves. Each reads something real, and each has a blind spot.

Look at the surface

Lift any foliage aside and look at the color and texture: damp mix is darker, dry mix paler and looser. A surface still dark the morning after a watering usually goes with a moist root zone, and a pale surface shrinking away from the pot wall warns that the whole root ball may be dry. Neither settles it, so let the surface start a check, never end one.

Feel with a fingertip

Push a fingertip into the mix near the rim, down to the first knuckle, and feel for coolness and damp. Near the rim you touch only mix, away from surface roots and the trunk, so the check does no harm; do not dig. A fingertip reads only the top few centimeters at one spot, so where the edge of a pot dries at a different rate from the middle, it reads only the edge.

Read a chopstick

The chopstick is the check that reaches the roots. Use a plain, unvarnished wooden chopstick or skewer, because varnish stops the wood from taking up water. Push it gently and deep into the root zone, partway between trunk and rim, and if it meets a root, move it a little rather than force it. Leave it a few minutes so the wood can draw in moisture, then pull it out and read it along its whole length, from the tip up to where it left the surface:

• Dry and pale along its whole length: the root zone is dry, or bone dry.
• Dry, with only the tip faintly damp: drying. Most of the depth has dried.
• Damp and cool on the lower half: moist.
• Dark, cool, and wet along its whole length: wet, as after a watering, or saturated if it stays that way.

Check a surprising reading in a second spot, on the other side of the trunk, and let a damp chopstick dry before you use it again, so yesterday's water does not color today's reading. A chopstick reads one spot, so in a root ball that dries unevenly, where you push it decides what it tells you.

Lift the pot

Water is heavy, and little else in a pot changes from day to day, so weight shows how much water the whole pot holds. Judging moisture this way is the lift test, and it has to be learned for each pot. Lift the pot right after a thorough watering, once the drips stop: that is your heavy reference. Lift it again on a morning when the chopstick comes out dry with only the tip faintly damp: that is your light reference, how the pot feels when it is dry enough to need water. If you did the activity at the end of Lesson 1, “Lift one pot morning and evening,” its first lift is already your heavy reference. Use the same grip each time, so you compare the pot and not your arms, and tilt one edge of a pot too heavy to lift. Against those two references, a lift reads one of four ways, from lightest to heaviest:

• Light: at or below your light reference. Drying or drier when the chopstick agrees, and time to water.
• Lighter than after watering: between the two references. Moist or drying: the lift cannot settle it, so let the chopstick decide.
• Heavy: a little below your heavy reference. Moist.
• Heavy, as if just watered: at your heavy reference. Wet.

The lift cannot say where the water is: it weighs the whole pot, so it cannot tell an evenly moist pot from one with a damp part and a dry part. It also tells you little where water is a small share of the weight, as in a small, shallow bonsai pot. Its strength is speed: after a move, the pot's weight is the quickest way to learn how fast it now dries (Course 02, Lesson 3).

Tilt to see the drainage holes

Tilt the pot and look at the mix in the drainage holes, then touch it. This is the bottom of the pot, usually the wettest layer, so in a pot of one even mix it answers one question well: if even the mix at the holes is dry, the rest of the root zone is drier still, and it needs water now. A damp bottom says little about the middle and the top. And where a root ball sits on a layer of different mix, as after slip-potting, the holes read that layer, not the root ball above it. While you are there, look for white root tips, a sign of growing roots (Course 01, Lesson 2), and for water standing in a tray or cover pot, which calls for investigating before the pot gets any more water (Course 02, Lesson 7).

Why a probe meter misleads

A probe moisture meter looks like the answer, but it does not measure water directly. It reads an electrical property of the mix around its tip, which responds to soluble salts in the mix and to how well the probe touches the particles, as well as to water (Course 02, Lesson 7). In a coarse bonsai mix, with large air-filled pores between the grains, the probe touches little, so damp mix can read dry; in a mix that has been fed, salts can make a drying mix read moist. Near the gift juniper's rim, the meter read “DRY” where a chopstick came out damp.

Look at the leaves

Leaves are worth a look at every check, but they are the slowest signal. A broadleaf tree such as a trident maple droops when water runs short, but only after the root zone has been short for a while. Conifers such as junipers barely droop: a thirsty juniper may show only dull, grayish foliage (Course 01, Lesson 8), and it can stay green for weeks after its root ball has dried and its fine roots have died (Course 01, Lesson 14). Waiting for the leaves means waiting too long. And a droop does not always mean dry: wilting in a pot that checks Moist or wetter calls for investigating, not water (Lesson 4).

Putting checks together

No single check is enough. Two checks that read different things, such as the lift, which weighs the whole pot, and the chopstick, which reaches the root zone at one spot, cover most of each other's blind spots. When they agree, act. When they disagree, add a check, such as a second chopstick spot or the drainage holes, rather than pick the answer you would prefer. Read together, they point to a moisture level and a decision:

• The chopstick comes out dark, cool, and wet along its whole length, or the pot feels heavy, as if just watered: wet. Not yet: check again later. If it stays that way for days in warm weather, or the tree droops, the decision becomes Investigate before watering.
• The chopstick is damp and cool on the lower half, or the pot feels heavy: moist. Not yet: check again tomorrow morning, or this afternoon on a hot, windy day.
• The pot feels lighter than after watering: moist or drying. The lift cannot settle it, and the chopstick decides.
• The chopstick is dry, with only the tip faintly damp, and the pot feels light: drying. Water thoroughly now.
• The chopstick is dry and pale along its whole length, and the pot feels light: dry, or bone dry. Water thoroughly now: you are late. If the root ball has shrunk from the pot wall, water runs straight down the gap: rewet it slowly in several passes, or stand the pot in a basin of water up to the surface of the mix until the root ball is wet through, usually 10 to 30 minutes, up to an hour for a bone-dry, peat-rich root ball, then let it drain (immersion, Lesson 3).

The surface, the fingertip, and the leaves add context, and a probe meter is the last reading to trust. Sometimes the disagreement is the finding: when a chopstick says moist in one spot and dry in another, no check is broken. The root ball is drying unevenly, and the question becomes which part most of the roots draw on.

The lift takes practice. In Learn a pot's weight, a gauge stands in for your hands. Start with the Nursery pot, then press Lift just after watering and Lift when dry: two pencil marks appear on the gauge, labeled “Just watered” and “Dry.” They stand for your heavy and light references; on your own pot, take the light one on the morning the chopstick says water, before the root zone is truly Dry. The Large training box is too heavy to lift, so there the buttons read Tilt just after watering and Tilt when dry. Then judge three mornings for each pot:

1. Read when the pot was last watered and the weather since, then press Lift (Tilt one edge on the box) and see where the needle settles against your marks.
2. Make your call from the lift alone, while the pot is still closed: choose Water thoroughly now, Not yet: check again later, or Can't tell from the lift: use the chopstick, then press Make the call. Only then does the lift appear in words, with the pot opened and a chopstick to confirm it.
3. Press Next morning, and after a pot's third morning, Next pot: Terracotta training pot, and so on through the Shallow bonsai pot and the Large training box. The exercise finishes when all four pots are judged.

The lift was clearest where water made up a large share of the weight: in the Nursery pot, the nursery juniper's light plastic pot, the marks sat far apart, and the trident's heavier Terracotta training pot, its marks a fair way apart, still read clearly on every morning. In the Shallow bonsai pot the marks sat close together, so two of its three mornings landed between them, where the lift says only Lighter than after watering. That can mean Moist or Drying, and there the chopstick decided, once to wait and once to water. The Large training box could only be tilted, which feels only part of the weight, so it left most of the work to the chopstick and the drainage holes.

When the lift said light, a chopstick still confirmed it before you poured; when it said heavy, you could wait without one. In every pot the lift showed how much water it held, never where, and the marks held only for the pot they were learned on.

Uneven root balls

Most pots of one even mix dry from the top down and fairly evenly from side to side, so one chopstick spot partway between trunk and rim is a fair guide to the root zone, with a second spot to confirm a surprising reading. Some pots do not dry that way, and the gift juniper's is one.

The gift juniper's two zones

The old root ball keeps the shape of the shallow oval pot it came from, about a quarter as deep as it is long, so its dark, fine, peat-rich mix fills most of the round training pot, about 22 cm (9 in) across and noticeably wider than deep. A thin layer of coarse fresh mix lies under it, and a ring of the same mix, a finger's width or two wide, runs around it, wider at the sides.

Right after a thorough watering, the peat-rich core holds far more water than the coarse ring, which drains within minutes to moist. Then the two zones lose water by different routes. Until about April, nearly all the roots are in the core, and they draw it down. The ring, with few roots yet, loses water only from its narrow strip of surface: the plastic wall lets none through, and once drained, the ring passes little water sideways into the core. So in late winter and spring, on sunny, breezy days, the core can dry while the lower ring stays damp. A peat-rich core that dries past a point also sheds water, a behavior called water repellency, and the next watering can then run down the ring and out of the holes without wetting the core: a kind of channeling that Lesson 3 shows how to spot and fix.

Through spring and early summer, new roots grow out into the ring, and the pattern flips: they spend the ring's small store first, so the ring dries ahead of the core, while the core's larger store keeps it moist longer, most of all in cool weather. The drainage holes, under the coarse bottom layer, read that layer rather than the core: in spring they can be damp under a dry core, and in summer dry under a moist one.

The rule does not flip. The core still holds most of the roots and most of the water, so in both seasons the chopstick goes into the old core, partway between trunk and rim, and its reading decides, as Course 02, Lesson 7 advised. A dry core needs water now, poured slowly and aimed at the core, even when the pot feels only a little lighter than after watering. A ring that dries ahead of a still-moist core is no reason to water: watering by the ring would keep the core wet and short of air (Lesson 4). One even mix throughout is the lasting fix, at a later repotting (Course 11, Repotting and Root Development).

Root-filled pots and fresh mix

A pot full of roots, circling the wall and matting at the bottom, is called root-bound. It dries fast, because roots draw on every part of it and take up room that would otherwise hold water. A dense mass of roots can be hard to push a chopstick into, and water may run down its sides rather than soak in (Lesson 3), so lean on the lift and check it more often, in summer heat sometimes twice a day.

The opposite is fresh mix that no roots have reached yet, after slip-potting or in a pot much bigger than its roots. Nothing draws on it from inside, so it dries slowly, mostly from its surface (and, in an unglazed pot, through the wall), and a check made in it can say wait while the roots beside it run dry. That is one reason a much bigger pot is not kinder to a tree (Course 02, Lesson 7). Wherever a root ball dries unevenly, check where most of the roots are.

Six pots, checked below the surface

In What is under the surface?, each pot's cutaway is hidden under a cover, so you find out what is inside the way you would at home: with checks. Start with the New juniper, then use any checks, in any order: Surface, Fingertip, Chopstick, Lift, Drainage holes, Probe meter, and Leaves. Each one adds a card to the Evidence tray.

1. Use at least two checks that read different things before you decide. Chopstick asks where to push it in: Near the rim, Partway between trunk and rim, or Close to the trunk.
2. Make your call while the cover is still on: under Decide, choose Water thoroughly now, Not yet: check again later, or Investigate before watering, then press Reveal to peel off the cover and see the cutaway and how your call compares. A right call that rested on one check, or on checks that pointed elsewhere, is marked a lucky call.
3. Answer Which check would have misled you here?, trying again until you find one, then press Next case: Backyard trident, and so on through the six. The exercise finishes when all six cases are solved and a blind spot is found in each, and a case file sums up each check's blind spot.

In four of the six pots, the surface pointed the wrong way: pale over a moist root zone, green moss over a dry one, damp after a light shower over one that was drying, and pale, dry-looking mix in the trident's terracotta pot the day after a thorough watering. The probe meter misled three times: salts in the mix made it read “MOIST” in the mossy, dry pot, and coarse mix made it read “DRY” in damp or moist mix. Each case was settled by two checks that read different things and agreed, or by a third when two disagreed; a right call made on one check, or on checks that pointed elsewhere, was only a lucky call.

The pale crust over a moist root zone and the trident watered the day before both called for Not yet: check again later. In the gift juniper's pot the spot mattered: the fingertip, the chopstick near the rim, and the drainage holes all read the damp ring, the lift could not tell ring from core, and only a chopstick in the old core found the dry root ball, so the call was Water thoroughly now, slowly and aimed at the core. The juniper left in a full tray was the one case for Investigate before watering: a pot as heavy as if just watered, water running from the holes into the full tray, and dulling needles. Every check, even the chopstick in the wrong spot, misled in at least one pot, which is why no single check is enough.

A one-minute morning round

A check only works if it happens, so make it short enough to do every morning. For each pot, it takes about a minute of hands-on time:

1. Look at the leaves and the surface, lifting any foliage aside, and glance at the tray or cover pot for standing water.
2. Lift the pot, or tilt one edge, and compare it with your two references.
3. Push in the chopstick at the pot's checking spot, partway between trunk and rim, or into the old core in a two-zone pot, and leave it a few minutes.
4. Read the chopstick along its length, and decide: water thoroughly now, not yet, or investigate before watering.
5. Write one line: the date, the weather, the readings in words, and the decision. A few weeks of these lines make a watering log that shows how fast each pot dries in each kind of weather (Lesson 7).

With several pots, do the first three steps for each pot in turn, then read the chopsticks in the same order, so each has had its few minutes. Mark each pot's checking spot, for example with a dot of paint on the rim, so every reading comes from the same place. This is the arrival-day check (Course 02, Lesson 8) made into a habit. On hot, windy summer days, add an afternoon round, because a small pot can dry out in a single day (Course 02, Lesson 1).

Two checks, then a decision