Why water demand changes

The west balcony, an instructional example that runs through this course, holds three trees this summer. The nursery juniper and the gift juniper stand side by side on the slatted shelf in the open middle, in the afternoon sun and wind. The trident maple stands in its unglazed terracotta pot on a low bench against the back wall, under the balcony above. Like every case in this course, the balcony is in one temperate Northern Hemisphere city, at about 40° N in USDA zone 7a (Course 02, Lesson 1).

On a Monday morning in late June, after a cool, rainy weekend, the morning check finds that none of the three pots needs water. The junipers were watered thoroughly on Friday, and in the cool, damp air their pots still feel heavy. Their dense foliage sheds much of any rain past the rims, so it is the lift, not the weekend's rain, that says they can wait (Lesson 5). The trident, in the back wall's rain shadow, was watered by hand on Saturday and has barely dried since.

By Thursday it is hot, sunny, and windy, about 31 °C (88 °F) in the afternoon. Late that afternoon the nursery juniper's pot is light, and so is the trident's: a tree in full leaf draws water fast, and terracotta loses water through its walls. Both are watered thoroughly. In the gift juniper's pot, a chopstick near the rim comes out dry, with only the tip faintly damp, but one pushed into the old core comes out damp and cool on the lower half. Friday is just as hot, and by evening the nursery juniper's pot is light again.

The gift juniper's old tag said “Water twice a week.” Kept on Mondays and Thursdays, that schedule would have watered the gift juniper's pot on Monday, still heavy, and again on Thursday, with its core still moist, while the nursery juniper, light again on Friday evening, would have waited until Monday. In a cool, gray week in late November, with the trident bare, the same three pots go most of the week on one thorough watering. This lesson explains where a pot's water goes, what sets the pace at which it goes, and why a routine of checks keeps up with that pace when a calendar cannot.

Where the water goes

Water poured into a pot that drains leaves it in three ways. The extra runs out of the drainage holes within minutes, and as it drains, fresh air is drawn into the pore spaces behind it (Course 01, Lesson 7). For a tree in leaf, most of the water the pot holds on to is drawn up through the roots and lost from the leaves as vapor, by transpiration: of all the water a tree absorbs, often well over nine-tenths goes this way (Course 01, Lesson 8). The rest evaporates straight from the surface of the mix and, in an unglazed pot such as the trident's terracotta, through the pot wall as well.

After the runoff stops, the leaves, the surface, and the wall spend what the pot held on to. So how soon a pot needs water again depends on two things: the store, how much water the pot held, and the pull, how hard that water is drawn out. Together they set the pace at which the pot dries: a big store under a gentle pull can last a week, and a small store under a strong pull can be gone in a day.

Six words for the root zone

As the store runs down, the root zone passes through stages. This course names them with six words, from driest to wettest, and uses them the same way in every lesson and interactive, under the label Root-zone moisture:

A pot that drains is Saturated only briefly after a thorough watering, Wet for a short while, and Moist for most of the time between waterings. Drying is the stage to catch. Dry is late, and Bone dry costs fine roots and root hairs, which do not revive (Course 01, Lesson 2). Telling the stages apart from outside the pot is the work of Lesson 2.

What sets the pace

Six things set the pace at which a pot dries. Some change the pull, how hard water is drawn out; some change the store, how much the pot holds; a few change both:

• The tree: how much leaf it has and how fast it is growing (pull).
• The weather: sun, heat, dry air, and wind (pull).
• The spot: how much of that weather reaches the tree (pull).
• The pot: its size and shape (store), and its wall and color (pull).
• The substrate: how much water it holds once drained (store).
• How full of roots the pot is: roots take up room (store) and draw the water out (pull).

Comparing two pots

What makes a pot dry faster? runs the same comparison with the small backyard's trident maple in two pots side by side on the bench in the western third, with morning sun and afternoon shade from the neighbor's maple. Both trees are established and in full summer leaf, and both pots were watered thoroughly at dawn; neither is watered again during the run. Pot A stays as it is, in a medium nursery pot; Pot B starts in a Small, shallow bonsai pot.

1. Choose a kind of day under The day.
2. Make the one-tap call, Which pot will say water first? (A, B, or About the same), or press Skip.
3. Press Start the day. Watch each pot's Root-zone moisture until its flag, Check says water, goes up, and watch for Dried out. Pause, Back, Next, and Replay step through the stops, and Skip to the end jumps to the result. When the run ends, See the result takes you to the result card.
4. Change one of B's settings at a time under Foliage, Pot, Wall, Mix, or Roots (on a phone, pick the setting under Change), then press Start the day again.

Four challenges tick themselves off as you find them, and finding all four finishes the race: make B say water before A by evening (your first run may find this one), make B last longer than A, find a change that matters more on a hot day than a cool one (run the same pot B under Cool and cloudy and then on a hot day), and show why a pot full of roots dries faster.

The small, shallow pot ran short first on every kind of day. It started with a much smaller store, and for the water it held it had more surface open to the air, while the same tree in full leaf pulled on it just as hard. What the day changed was what that cost. On Cool and cloudy, B's check said water on the first afternoon, but a morning check would still have caught it before it dried out, and A's check did not say water until day 4. On the mild day B dried out that afternoon, and on the hot days before midday, long before the next morning's check.

Every setting moved B the way the subsections below explain. Unglazed terracotta, Coarse bonsai mix, and Pot full of roots brought B's check forward; Bare branches, Light foliage, a Large, deep nursery pot, and Few roots in the mix yet pushed it back, sometimes past the whole week. A pot full of roots dried faster because the roots took up room that mix and its water would have filled, and more of them drew on what was left.

A change cost far more on a hot day than on a cool one. Counted in hours, the gap between the pots was shorter on the hot day, but it fell between two morning checks, so B dried out before the next morning's check. The factors multiply rather than add: B in the small, shallow pot with Unglazed terracotta, Coarse bonsai mix, and Pot full of roots said water within a few hours of dawn on Hot, sunny, and windy, while B in a Large, deep nursery pot did not say water within the week on Cool and cloudy. And the pots dried almost entirely in daylight, barely at all overnight.

The tree

A tree's water use rises and falls with its leaves. A tree in full summer leaf has the most stomata open to the air and draws hardest: a trident maple in full leaf can draw its pot down within a day of summer heat. During a spring flush, when many shoots extend and unfold new leaves together (Course 01, Lesson 12), the pull grows week by week. A deciduous tree that has dropped its leaves draws very little, and an evergreen such as a juniper keeps losing some water all year, slowly in cool weather.

The roots matter too. Water enters mostly through young fine roots, so a tree whose roots were recently cut or disturbed draws less, and its pot can stay damp for longer even while its leaves go short, like the repotted maple in Course 01, Lesson 8. Lesson 4 returns to watering such a tree; Course 11, Repotting and Root Development, covers its aftercare.

Trees also show a shortage differently. A trident maple's leaves scorch if its pot dries out, so check it first on a hot day. Junipers tolerate drying better but show stress late and quietly, perhaps only as dull or grayish foliage, so check their root zone rather than wait for the foliage (Course 01, Lesson 8).

The weather and the spot

Sun, heat, dry air, and wind all raise evaporative demand, the strength with which the air draws water vapor from the leaves. They speed evaporation from the pot's surface and wall as well. At the same relative humidity, and other things being equal, air at 30 °C (86 °F) draws water from leaves roughly three times as fast as air at 10 °C (50 °F) (Course 02, Lesson 5), and wind speeds the loss further.

The forecast describes the city; a tree lives in its spot, its microclimate (Course 02, Lesson 5). On the west balcony, the shelf in the open middle takes every hour of afternoon sun and the wind from the open south end, while the back wall is shaded through the early afternoon and sheltered from the breeze, one reason the trident stands there. In the small backyard, the patio along the house's south wall adds reflected heat on summer afternoons, while the western third gets afternoon shade from the neighbor's maple.

Indoors, the year can run the other way round. At the bright window, the heating runs from about October to April and dries the room's air; a board keeps the ficus's pot out of the radiator's hotter, drier rising air. On clear winter days direct sun reaches the sill from late morning to mid-afternoon, and a grow light adds more, while in summer the high sun barely reaches inside. So the ficus can dry sooner in winter than in summer, the opposite of what a calendar suggests.

Every move changes the pace

A move puts a tree in a new microclimate, and its pot starts drying at a new pace (Course 02, Lesson 8). Each hardening-off step into more sun and wind dries the pot faster, so check more often after each step (Course 02, Lesson 3); when the small backyard's ficus moves out for the summer in late May, its pot dries much faster than indoors, so its checks become daily. A move into a cool room does the opposite: in the cool bedroom where the gift juniper spent part of December, its pot dried more slowly, so each check before watering started with the weight of the pot (Course 02, Lesson 3). After a move into more sun or wind, check twice a day for the first few days, until you know the new pace; after a move somewhere cooler or shadier, expect the waterings to come further apart.

The pot

Size and shape set the store. A small pot holds less, and a shallow one, wide for its depth, spreads its water under a large surface open to sun and wind, so either runs short sooner; in summer heat a small pot can dry out in a single day (Course 02, Lesson 1). A small pot is a small reserve that needs closer watching, not less care (Course 01, Lesson 1).

The wall matters too. Plastic and glazed walls let no water through, but unglazed terracotta is porous: water moves into the clay and evaporates from its outer face, so the trident's pot dries faster than a plastic pot of the same size. A dark pot whose wall takes direct sun heats up, which warms the root zone and adds to the drying. Shading each pot's sunny side, as the balcony's owner does, protects the roots and slows the pace (Course 02, Lesson 5).

The substrate

The mix sets how much water the pot holds once it has drained, and how readily it gives it up. Coarse mineral bonsai mixes drain fast and hold less, so they dry sooner; fine, peat-rich mixes hold much more and stay moist much longer, sometimes too long; most nursery mixes, like the nursery juniper's dark, bark-based one, fall between. Why each behaves as it does belongs to Course 04, Soil, Substrates, and the Root Environment. For watering, a pot of coarse mix simply needs checking more often.

How full of roots the pot is

Roots take up room that mix would otherwise fill, and they are what draws the water out. A pot packed with roots holds less and empties faster, and a badly root-bound tree can dry out quickly (Course 01, Lesson 2). Fresh mix that no roots have reached works the other way: nothing draws on it from inside, so it loses water only from its surface and stays damp longer than you expect.

The gift juniper has both in one pot. In December its old root ball was moved whole into a plastic training pot, a correction called slip-potting (Course 02, Lesson 7), so a dark, fine, peat-rich core sits inside a ring of coarse fresh mix a finger's width or two wide. Since spring, new roots have grown out into the ring. They now draw on its small store first, so the ring dries ahead of the core, while the core's much larger store keeps it moist longer, most of all in cool weather. That is what Thursday's check found.

For watering, the chopstick goes into the old core, partway between trunk and rim, and the core's reading decides. A ring that dries ahead of a still-moist core is expected and is no reason to water. Earlier in the year, before roots reached the ring, the pattern ran the other way, which Lesson 2 takes up. One even mix throughout is the lasting fix, at a suitable time (Course 11).

The seasons in brief

Over a year, the same tree's water use follows its leaves and the weather. It climbs quickly in spring, as buds break, leaves unfold in a flush, and the days lengthen and warm. It peaks in summer, in full leaf through long, hot days. It drops away in autumn, as the days shorten and cool and a deciduous tree's leaves color and fall. In winter it is lowest, but not zero: a bare tree still loses a little water through its twigs and buds, the pot's surface and an unglazed wall keep drying, and an evergreen still loses water on sunny, windy winter days (Course 02, Lesson 5). The roots are alive, respiring slowly, and still need moisture and air during dormancy (Course 01, Lesson 13).

Pots dry more slowly in cool weather too, so in November one thorough watering can last the west balcony's pots most of a week, and in midwinter longer. Longer is not never: a root ball left to dry for weeks can kill a resting tree, so checks go on all winter, less often, and a dry pot is watered on a day when its root ball is not frozen (Course 06, Seasonal Care and Timing).

These months and directions 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, and in any climate, go by the stage you can see on the tree rather than by the date (Course 01, Lesson 13).

Checking regularly versus watering automatically

There are two ways to decide when to water. Watering automatically means deciding in advance: every Monday and Thursday, every third day, or every morning, by habit, by a timer, or by a tag. Checking regularly means looking at the root zone on a routine and letting what you find decide. The gift juniper's tag chose the first: “Water twice a week,” a fixed schedule printed for every buyer's home and every week of the year (Course 02, Lesson 2).

Why any fixed schedule fails in both directions

A schedule is set for one pace, and the pace keeps changing. In a cool, rainy spell, a schedule waters pots that are still moist. A pot that drains copes with an extra watering, because the excess drains away and air returns to the large pores. But in cool, wet weather it dries slowly, so a run of extra waterings keeps the lower root zone wet day after day, and the roots there get little air. That is overwatering: not how much is poured at once, but a root zone kept wet so constantly, or draining so poorly, that air never gets back in (Course 01, Lesson 7).

In heat, the same schedule falls behind. The pot dries out between waterings, the stomata narrow and growth slows well before any leaf shows wilting (Course 01, Lesson 4), and a juniper may show nothing until the damage is done.

Moving the schedule does not fix it: more waterings keep up in the heat but make the cool spells worse, and fewer do the reverse. The opening week showed both: two waterings the gift juniper did not need, and a nursery juniper light again on Friday evening with the next scheduled watering not until Monday. Only a look at the pot itself follows the pace. A timer is no exception, because it waters by the clock; it can cover days away only once it has been tested at home, and only while someone checks the pots and adjusts it (Lesson 6).

Twice a week, or when it needs it? follows the nursery juniper on its shelf through an invented two weeks from a Monday in late June, not the week this lesson opened with: a cool, rainy start, mild days and a warm one, three hot, windy days, a thunderstorm, and a cool, cloudy end. The same pot runs in two lanes side by side. Twice a week waters it on Mondays and Thursdays, like the tag; A morning check waters thoroughly only when the root zone is Drying or drier.

1. Make the prediction, Which lane will leave the roots short of water more often? (Twice a week, A morning check, or About the same), or press Skip the prediction. Then press Run it.
2. Watch each morning: the schedule lane reads its calendar, and the check lane reads a chopstick. Compare each lane's tallies as the days go by: Days short of water, Days kept too wet, and Waterings. Pause, Back, and Next let you study a day (on a phone, the day slider too), and Skip to day 14 jumps to the result.
3. Then take on the goal, Find a fixed schedule that matches the morning check. Under Water on these days, tick the weekdays to water on, or tick Every day, and press Run this schedule. Answer What will this schedule get wrong? or press Skip the prediction, then press Run it.
4. Each schedule you run stays pinned under Schedules run so far with its tallies. Three different schedules of your own, other than the tag's Mondays and Thursdays, finish the exercise.

The two lanes had the same pot and the same weather; only the rule changed. Twice a week went wrong in both directions. On day 1, in steady rain, it watered a pot that was still Moist, and with the tree and the air taking up little water, the pot stayed Wet for days, its roots short of air. In the warm and hot days it fell behind: the pot dried out before Monday's watering and again between Monday and Thursday. Each schedule you tried traded days short of water for days kept too wet, or got both, and Every day kept up with the heat only by keeping the pot wet through the cool days at both ends.

No set of weekdays can match the morning check, because the check was never a count of waterings. It watered five times, each on a morning when the root zone was Drying: not at all in the cool, rainy start or the cool end, once before the hot spell, every morning through it, and once after. On the hot days its pot was Drying again by evening, which is why heat calls for an afternoon check as well. Any set of weekdays that covers the hot spell also waters on the same weekdays in the rainy start.

A routine is a schedule of checks

A routine fixes when you look, not when you water. Each morning, lift each pot and compare its weight with how it felt right after a thorough watering and when it was dry: the lift test. Then push an unvarnished wooden chopstick deep into the root zone, partway between trunk and rim (on the junipers, lift the foliage pad aside), leave it a few minutes, and read it along its length (Course 01, Lesson 2). Lesson 2 teaches each check and its blind spots. Each check ends in one of three decisions:

• Water thoroughly now when the root zone is drying: the pot feels light, and the chopstick comes out dry, with only the tip faintly damp. Dry and pale along its whole length means you are late. A thorough watering wets the whole root ball, slowly, until water runs from the holes, and a check afterward confirms it wetted through (Lesson 3).
• Not yet: check again tomorrow morning when the chopstick comes out damp and cool on the lower half, or wetter, and the pot is still heavy. On a hot, windy day, check again that afternoon as well.
• Investigate before watering when the tree and the root zone disagree, such as drooping, dulling, or yellowing in a pot that checks Moist or wetter, when a pot has been standing in water, or when it stays Wet far longer than the weather explains. That means looking closer at the core, the holes, underneath the pot, the smell, and recent changes first, never watering by default (Lesson 4).

When two checks disagree, add one before you decide; in a two-zone root ball, the chopstick in the old core decides. What a bonsai needs every morning in the growing season is a check, and the watering follows only when the check says so: in a hot week perhaps every day, sometimes twice, and in a cool one once or twice.

Start with a morning check