How to water effectively

April on the west balcony, the main instructional example in this course. Like every case here, it is set in one temperate Northern Hemisphere city at about 40° N (USDA zone 7a). The gift juniper has lived on the balcony's secured slatted shelf since January, in the plastic training pot it was slip-potted into in December: an old, peat-rich root ball inside a ring of coarse fresh mix (Course 02, Lesson 7).

On a mild, breezy morning, its owner lifts the foliage aside and pushes a chopstick into the old core, partway between trunk and rim. After a few minutes it comes out dry and pale along its whole length, and the pot feels lighter than after watering (Lesson 2). The core needs water now. The owner waters through the can's fine rose, and within seconds water is running from the holes into the tray under the shelf. They have always stopped once water runs from the holes, so they stop.

The next morning, the chopstick in the core comes out dry and pale again. The coarse ring had carried the water straight down to the holes, and the dried peat had shed what fell on it. The tray said the pot had been watered; the roots in the core got almost none of it. This lesson is about telling those two apart: what a thorough watering does, how to apply one with a can, a hose, or a basin, how water slips past a root ball, and the checks that show it wetted through.

What a thorough watering does

A thorough watering is water applied evenly across the whole surface, slowly enough to soak in, until it runs from the drainage holes, and then confirmed by a check that the root ball wetted through. It usually takes two or three passes, a minute or two apart. Course 02 set the aim: every thorough watering should wet the whole root zone and end with water running from the holes (Course 02, Lesson 1). The gift juniper's pot showed that the second half can happen without the first, so this lesson adds the check.

A thorough watering does three things for the roots:

• It wets the whole root ball. Fine roots grow all through it, and each takes up water only from the mix around it. A root in a dry pocket gets little from wet mix a few centimeters away, because water moves sideways only slowly through drained or dry mix, and fine roots and root hairs that stay dry die and do not revive (Course 01, Lesson 2).
• It drains, and draws air in behind it. For a short time afterward the substrate is saturated, every pore full of water. In a pot that drains, that is normal: as the extra water runs out, it pulls fresh air into the larger pore spaces behind it (Course 01, Lesson 7).
• It carries some dissolved salts out. Water that runs through takes some of the soluble salts left by tap water and fertilizer with it, while small sips that never run through leave them to build up (Course 01, Lesson 10). Watering heavily on purpose to wash salts out, leaching, is part of Lesson 5.

Water that drains out of the holes is the runoff. However much of it there is, a thorough watering is not overwatering. Overwatering is not a large amount poured at once; it is a root zone kept wet so constantly, or draining so poorly, that air never gets back in (Course 01, Lesson 7).

Technique: slow passes over the whole surface

The method is the same with a can, a hose, or at a sink: spread the water over the whole surface, only as fast as it soaks in, in passes. The pot and the place decide the tool: a can with a fine rose for most pots, and wherever the water has to be carried; a hose wand on a gentle shower in a yard where the runoff can soak away; and, for a root ball that is bone dry or has shrunk from the wall, immersion in a basin, described under rewetting below.

A fine rose, held low

A fine watering rose breaks the flow into a gentle shower of small drops. Hold it low, a few centimeters above the mix, so the drops land softly instead of splashing grit out or digging into the surface, and keep it moving. Cover the whole surface on every pass: the rim all the way round, the middle, and the base of the trunk, which is easy to miss because the trunk and lowest branches are in the way.

Both junipers on the west balcony spread about 30 cm (12 in) across, wider than their pots of 20 and 22 cm (8 and 9 in), so their foliage hides most of the surface. Water poured onto it runs to the edges and drips off past the rim, as rain does (interception, Lesson 5), so lift the foliage pad aside with one hand and water under it with the other.

Slow passes, a minute or two apart

Pour only as fast as the surface takes water in; if water starts to pool, pause until it has soaked away. Go over the whole surface, wait a minute or two, and go over it again: a damp surface takes water in better than a dry one, so the later passes soak in instead of running off. Keep going until water runs from the holes, usually on the second or third pass, then stop pouring and check the watering, as described later in this lesson. Drips during the first pass are a reason to check sooner, not proof of a good watering: if the pot already feels heavy, as if just watered, it is done; if it is barely heavier, the water has found an easy path past the roots. If water pools at every watering, however slowly you pour, the mix itself may be the problem (Course 04, Soil, Substrates, and the Root Environment).

Water the whole surface puts that habit into practice on the gift juniper in late May, when new roots have grown out into the coarse ring. Its chopstick came out dry, with only the tip faintly damp, and the pot is light. You look down on the pot, mostly hidden under the canopy, and choose where the rose goes on each pass with seven numbered zone buttons: the four quarters of the rim, two inner zones between the trunk and the rim, and Around the trunk. Before the first soak, an optional quick call asks how far one slow pass will soak into the old core: answer it, or press Skip. Zones under the foliage unlock when you press Lift the foliage pad, and End the pass cuts through the pot to show where the water went. You have up to three passes, with a pause of a minute or two before each new one. The exercise ends when every zone is wet to the bottom, or after the third pass.

1. On the first pass, try Pour over the foliage once, then water only what you can reach without lifting the foliage, Rim, front and Rim, right, and press End the pass. Watch the drainage holes.
2. Press Pause, then pass 2, then Lift the foliage pad, and wet every zone by the end of the third pass.
3. For the optional challenge, press Try again and do it in two passes.

On the first pass, the water on the rim ran straight down the coarse ring and along the layer under the core to the holes, so the holes dripped while the old core beside the ring was wet only partway: water at the holes showed only that water reached the bottom. The water poured over the foliage barely wet the mix, because most of it ran off the needles and dripped past the rim. One slow pass soaked only partway into the peat. After the pause, the damp surface took the next pass in faster and carried the water to the bottom, so every zone needed water on two passes to wet through, which is why a thorough watering is usually two or three passes.

Zones under the canopy stayed dry until the pad was lifted aside, and each zone the rose missed showed in the cutaway as a dry wedge reaching down from the surface, while a zone watered on one pass only was wet near the top and dry below: water soaks down more readily than it spreads sideways, so a patch missed on top stays drier below. In late May that matters most at the rim, where new roots have grown into the ring. When wedges were left, the lift and a chopstick in one spot could disagree, and then the rule is to add a check before you decide.

The same method on the balcony, at the sink, and with a hose

Each place changes where the water comes from and where the runoff goes; the passes stay the same.

On the balcony: carry the water, catch the runoff

The west balcony has no outdoor tap, so every watering is carried from the kitchen in a can of about 2 L (about half a gallon), and drips land on the neighbors' balcony below (Course 02, Lesson 7). Slow passes suit both limits: more of each canful stays in the root ball, and the runoff drips into the tray under the shelf, or the trident maple's tray under its grid, without overflowing. Immersion and leaching, which release far more water than a tray can hold, happen in a basin or at the sink.

Indoors: water at the sink

At the bright window, runoff on the sill would reach the radiator, and the ficus's plastic growing pot sits inside a glazed outer pot with no hole, a cachepot. So its owner carries the growing pot to the kitchen sink, waters it there in slow passes until water runs from the holes, and lets it drain until the drips stop before it goes back. An hour later, they tip the outer pot and pour away any water that has collected (Course 02, Lesson 7).

In the backyard: a gentle wand on low flow

The small backyard has a hose with a shower-head wand, and its runoff soaks into the lawn. Set the wand to a gentle shower on low flow and use it like a rose: low, moving, in passes. Never use a jet: a hard stream digs out the mix, bares roots, and cuts a path that later waterings follow straight down. Hose water left lying in the sun can be hot enough to harm roots, so run it onto the lawn until it comes out cool on the back of your hand. For the smallest pots, such as the Chinese elm's 15 cm (6 in) nursery pot, the can is easier to control.

Infiltration, wetting fronts, and channeling

When water soaks into the surface and moves down into the root zone, rather than pooling, running off, or slipping past down the sides, it is called infiltration. As it soaks down, the boundary between mix that water has reached and mix still dry moves down with it: the wetting front. In a mix that takes water well, the front moves down fairly evenly, spreading only a little sideways, and when it reaches the bottom, water starts to drip from the holes. That is the left-hand pot at the top of this lesson.

Water also takes the easiest path. When a few paths through a pot are much easier than the rest, most of the water runs down them and out of the holes without wetting the root ball. This is channeling, and the easy paths are familiar:

• A gap at the wall. A peat-rich or bark-based root ball that dries out shrinks from the pot wall (a coarse mineral mix does not), and water poured on top runs down the gap (Course 02, Lesson 7).
• A crack or a dug-out hole. A dried root ball can crack, and a jet digs a hole that later waterings follow straight down.
• A dry, water-repellent core. Peat-rich mixes shed water after they dry out, until they slowly rewet. That is water repellency: water beads on the dry peat and runs off to wherever it can soak in.
• A coarse ring around a fine core. In the gift juniper's pot, water on the ring runs straight through, and water shed by the dry core joins it.

Signs that water ran past the roots

Channeling happens out of sight, but it leaves three signs you can read from outside:

• First drips within seconds from a pot that was light.
• A pot barely heavier after the watering, when it should feel heavy, as if just watered.
• A dry core: a chopstick pushed into the core and left a few minutes still comes out dry.

Either of the first two signs alone is a reason to check further, not proof: quick drips can come from a coarse, fast-draining mix that is soaking water in, and a small, shallow pot gains little weight even when it is watered well. A dry core, or quick drips together with a pot barely heavier, shows that the water ran past, and then the root ball needs more water, given slowly or by immersion, as the next section describes.

Check your prediction in Watch a watering soak in. Choose a Root ball and a way to water under How. Before the first pass of each setup, predict whether it will wet the root ball through (Yes, No, or Not on the first pass), or press Skip. Then press Water and watch the wetting front, the drips into the tray, and the First drips marker. After each pass, press Check it to lift the pot and read a chopstick in the core, then Another pass if the run allows one. For immersion, press Lower into the basin, press Hold it down if the root ball floats, use Check it to read the core as the bubbles rise, slow, and stop, and press Lift out and drain when you choose. Back, Next, and the row of moments under them step through a run, and Start this setup over runs it again. Each new outcome is added under Outcomes found: find all seven to finish, or five once you have made three right calls with the X-ray view off. Try these:

1. With The gift juniper's two zones and Fine rose, slow passes, water three times across the whole surface, pressing Check it after each pass. Then press Start this setup over and, on the second and third passes, press Aim the next pass at the core. Last, try Immersion in a basin: lift it out once while bubbles still rise, then start the setup over and lift it out only after Check it finds the core wet.
2. With Dry and shrunk from the wall, compare the first slow pass with the third, then try Immersion in a basin.
3. With Moist, open mix, try A quick splash, Spout in one spot, and Hose jet against Fine rose, slow passes.
4. Turn X-ray view off. After each pass, press Check it for the lift, and choose under Your call from the surface, the drips, and the lift before the inside is shown.

With Moist, open mix, which takes water readily, Fine rose, slow passes wet the root ball through in one pass, with the first drips only after a while; of the pours, it was the only one that did. A quick splash wet only the top; Spout in one spot poured more onto one patch than it could take in, so water pooled there and went down one narrow path while the rest stayed dry; and Hose jet washed mix out, bared roots, and cut channels straight to the holes. With Dry and shrunk from the wall, the first slow pass ran down the gap within seconds and left the pot still light; by the second, the wetted surface had started to take water in, and by the third the root ball had swelled back against the wall and wetted through.

With The gift juniper's two zones, passes over the whole surface ran off the dry peat and down the coarse ring every time, with drips within seconds, until Aim the next pass at the core kept the water on the peat: two aimed passes wet the core through by the third. In the basin, the bubbles rose fast, slowed, and stopped, and immersion rewet every root ball left in long enough, the shrunk one once it was held down. But the gift juniper's peat core stopped bubbling before it was wet through, so lifting it out while bubbles still rose, or as soon as they stopped, left part of it dry: the chopstick in the core decided when to lift it out.

With X-ray view off, the surface, the drips, and the lift settled the call for a pot of one even mix: late drips and a pot heavy, as if just watered, meant it was wet through, and quick drips, no drips, a pool, or a pot still lighter than after watering meant it was not. In the gift juniper's two zones, the damp ring could make the pot feel heavy over a dry core, so when the drips and the lift looked good, the honest call was Can't tell yet: check the core.

Rewetting a dry or water-repellent root ball

A peat-rich or bark-based root ball that has dried out, or has a dried core, will not take water the ordinary way, and pouring harder only sends more of it down the easy paths. Both ways to rewet it rely on time: the water has to stay against the dry mix long enough for it to start taking water in.

Slow passes aimed at the dry part

When the dry part is a core in a pot that otherwise takes water, as in the gift juniper's pot in April, aim the water at the core. Lift the foliage aside, hold the rose low over the old root ball, between the trunk and the ring, and pour a little at a time, slowly enough that the water sits on the peat and starts to soak in instead of running off to the ring. Pause a minute or two, and do it again. It can take several passes before the peat begins to take water in; after that it soaks it up readily. Then read a chopstick in the core; if it is still dry after three or four passes, immerse the pot.

The same small, slow passes can rewet a root ball that has shrunk from the wall, if the rose stays over the root ball itself, away from the gap at the rim.

Aiming at the core is a way to rewet it, not this pot's everyday method. Once the core takes water readily again, go back to passes over the whole surface: by late May, with new roots in the ring, the rim needs water as much as the core does. Either way, the core's chopstick decides when to water, as in Lesson 2: a dry core needs water now, even when the damp ring makes the pot feel only a little lighter than after watering.

Immersion in a basin

When the whole root ball is bone dry or has shrunk from the wall, immersion is the surer way, and it is the next step when three or four slow passes have not wetted a dry core. This is immersion watering: standing the pot in a basin of water that comes up to the surface of the mix until the root ball is wet through, then lifting it out to drain fully. It is the soak Course 02 promised for a bone-dry, shrunken root ball (Course 02, Lesson 8).

1. Lower it in gently, until the water reaches the surface of the mix. A dry root ball is light and can float, so hold it down with a hand on the rim of the pot, never on the trunk.
2. Watch the bubbles. As water pushes air out of the dry mix, bubbles rise fast, then slow, then stop.
3. Check the core when they stop, with a chopstick, because a water-repellent core can stop bubbling before it is wet. If it is still dry, leave the pot in longer and check again.
4. Lift it out and let it drain fully, on a grid over the basin or in the sink, before it goes back to its spot.

Immersion usually takes 10 to 30 minutes, and up to an hour for a bone-dry, peat-rich root ball. Never leave a pot in for more than a few hours: immersion rewets a root ball, and roots left standing in water run short of air (Course 02, Lesson 7). A pot too big for any basin gets slow passes instead, and more of them.

Rewetting brings the water back, but not every root. Fine roots that died in the drying do not revive, so a tree that dried right out may grow less for a while (Lesson 4).

Which method for which pot

The root ball you find decides the method, and the method decides which check confirms it:

After watering: confirm, then empty the tray

Three checks that it wetted through

Water at the holes is where checking starts, not where it ends. Three checks confirm a watering:

• The lift. Lift the pot, or tilt one edge of a heavy one. It should feel heavy, as if just watered: the weight you learned for this pot with the lift test (Lesson 2). Light, or only a little heavier than before you watered, means the water probably ran past: read a chopstick in the core.
• The chopstick in the core. When the lift leaves doubt, or the root ball has an old core that dries at its own pace, push a chopstick into the core, partway between trunk and rim, and read it after a few minutes. Dark, cool, and wet along its whole length means the core took water.
• The drips. A root ball that is taking water in soaks up a good deal before much drains, so the first drips come after a while, and after you stop pouring they keep falling for a while and slow to a stop. Drips that came within seconds and stop almost as soon as you stop pouring, from a pot that still feels light, point to a channel. If no water comes out after several passes, tilt the pot and check that the holes are open (Course 02, Lesson 7).

For a pot you know, in a mix that takes water, the lift is usually enough. For the gift juniper, the chopstick in the core decides, because the lift cannot say where in the pot the water went.

Trays and cover pots

Once the dripping stops, empty the tray, the saucer, or the outer pot. A pot left standing in its own runoff has become a pot in a cover pot: the bottom of the root ball sits in water and stays saturated (Course 02, Lesson 7). On the west balcony, that means both trays, after every watering; at the bright window, the outer pot, checked an hour later. Outdoors, a saucer or cover pot fills with rain as well, so check it after rain too.

What time of day to water

The check decides whether to water; the time of day decides how well the water serves the day ahead.

• In summer, check in the morning. A pot watered in the morning goes into the afternoon full, when evaporative demand, the air's pull on the water in the leaves, is highest (Course 01, Lesson 8). On the west balcony the direct sun arrives in early afternoon, so the morning check, and a watering when it calls for one, comes before the hardest hours.
• Water a dry root zone whenever you find it. That includes midday: waiting for a cooler hour leaves the roots dry through the hottest part of the day, and that does real harm.
• Late in the evening, water the root zone, not the foliage. Leaves that stay wet through a warm night are more open to some leaf diseases (Course 07, Tree Health and Diagnosis), and the roots are what need the water.

A watering you can confirm