Rain, water quality, and unusual conditions

Mid-June on the west balcony, the instructional example this course returns to. At four in the afternoon a downpour arrives with almost no wind, and for most of an hour the rain falls straight down on the open front half of the balcony. The concrete there darkens and puddles, water streams off the slatted shelf just inside the railing, and the shallow tray under the shelf fills to the brim. Under the balcony above, the floor along the back wall stays dry, and so does the trident maple on its bench against the wall.

On the shelf, the two junipers glisten. But each spreading canopy, about 30 cm (12 in) across, covers most of its pot, and much of the rain that lands on it drips off the outer needles, past the rim, into the tray. This morning, a day after their last thorough watering, all three pots checked Moist. Tomorrow is forecast hot and sunny, and with the whole balcony dripping, skipping tomorrow's check looks reasonable.

This lesson takes the three things that most often tempt an owner to change the plan without checking: rain, the water itself, and unusual weather such as heat waves and strong wind. Every case is set in the course's example city, a temperate Northern Hemisphere city at about 40° N (USDA zone 7a); in the Southern Hemisphere, swap north and south, and shift the months by half a year.

Did the rain reach the roots?

A drenched balcony is not a watered pot. What counts is how much water reached each root zone, and only a check below the surface shows that (Lesson 2).

A pot catches only the rain that falls on it

A pot is a small target. It catches only the rain that falls on its own surface, usually a fraction of a thorough watering. One centimeter (0.4 in) of rain puts about 0.3 L (10 fl oz) into a 20 cm (8 in) pot, less than a sixth of the west balcony's 2 L (about half a gallon) can, and less still under foliage. Rain falls in every season in the example climate, and it rarely waters a pot reliably (Course 02, Lesson 1).

How the rain falls matters as much as how much. A light shower wets the surface and the top layer and stops there, so the surface looks dark and a fingertip finds damp mix while the root zone below stays as dry as before. A downpour delivers more, but fast: on a dried-out root ball, above all a peat-rich one with water repellency, much of it runs down the easy paths and out of the holes, the same channeling a fast pour from a can causes (Lesson 3). The rain that does most good is long and steady, on a pot whose surface is open to the sky. A steady day of it can wet such a pot right through, and rinse some dissolved salts out on the way.

Overhangs, foliage, and wind

Some pots get almost none. A rain shadow is an area that rain does not reach because something blocks it (Course 02, Lesson 5). On the west balcony, still rain never reaches the back wall under the balcony above, where the trident stands, and the corner by the air-conditioner condenser gets almost none in any weather. Summer storms in the example city often come on a west wind, though, which slants rain in under the overhang. It usually wets only the top layer of the trident's pot, but it can fill the tray under its grid. Empty that tray too, even though the grid keeps the pot above the water.

A tree's own foliage can shelter its pot the same way. Interception is rain caught on foliage before it reaches the pot: some evaporates from the leaves, and a canopy wider than its pot drips much of the rest off its edges, past the rim. Both junipers are low and spreading, about 30 cm (12 in) across over pots of 20 and 22 cm (8 and 9 in), so their canopies cover most of the surface. Watering them means lifting the foliage pad aside, and rain cannot do that.

Trays and cover pots fill too

Rain fills a tray far more reliably than a pot, because the tray catches rain over its whole area along with the drips the foliage sheds, so a full tray under the shelf says nothing about the root zones above it. Empty it after rain, as after every watering (Course 02, Lesson 7). A tray or cover pot that a pot stands in matters more: rain fills it just as a watering does, and the bottom of the root ball sits in that water until someone tips it out (Course 02, Lesson 7). The worked example in Lesson 4, the small backyard's Chinese elm forgotten in a cover pot through a week of rain, shows what follows.

After rain, check as if it had not rained

After any rain, check each pot below the surface at the next routine check, as if it had not rained: the lift test, then the chopstick, partway between trunk and rim (Course 01, Lesson 2). Start with the pots most likely to have been missed, in rain shadows and under dense foliage, and keep doing it in winter.

The check can also find that the rain did the job. After a steady day of still rain, a pot open to the sky, under a light canopy that leaves most of its surface showing, can be heavy the next morning, with the chopstick damp and cool on the lower half. Then the call is Not yet: check again tomorrow morning, even if the pot was due for water before the rain.

Rain on the balcony, pot by pot

Did the rain reach the roots? shows the west balcony in side section, with its three pots in their summer spots: the nursery juniper and the gift juniper on the open shelf, and the trident maple on its bench against the back wall, under the overhang. Under Rain, choose Light shower, Steady day of still rain, Storm on a west wind, or A downpour after a hot, dry week. Each rain opens with a line on when the pots were last watered. While it falls you see only what the owner would see from the door: the angle of the rain, the overhang, the junipers' canopies, and the tray. Then:

1. Press Start the rain and watch where it falls. Back, Pause, and Next hold a moment for a closer look.
2. When the rain is over, answer Which pot would you check first?, or press Skip.
3. Press Check below the surface to open all three pots, and read what the rain did in each one and what the chopstick shows. On a phone, pick a pot with Pot.
4. For each pot, choose Water thoroughly now or Not yet: check again later, and read why. If the tray filled, press Empty the tray.
5. Press the button for the next rain, such as Next rain: Steady day of still rain, and repeat until all four rains are checked.

From the door, the balcony looked wet after every rain, and the tray under the shelf caught water every time, filling to the brim in all but the light shower. Below the surface, the rains differed. The Light shower barely touched the nursery juniper, dampened only the top of the gift juniper's coarse ring, and never reached the trident. The Steady day of still rain lasted longest, yet it never reached the trident at the back wall either, so after a whole day of rain its pot still needed water, and in the junipers' pots only the check could say how far down it went. The Storm on a west wind reached the trident but wetted only the top layer of its pot. A downpour after a hot, dry week ran down the gift juniper's coarse ring and out of the holes, past its dried core, the channeling of Lesson 3: the tray filled, and the core stayed Dry. In every rain, the junipers' canopies shed much of the water past their rims. Rain counts only as far as a check below the surface finds it, and the check decided every call: a pot it found Moist could wait, and a pot it found Drying or Dry needed a thorough watering now, however wet the balcony looked.

What is in the water you pour

Most tap water suits most trees. City water is treated to be safe to drink, and the trees in this course's cases grow well on it. It is disinfected with chlorine or with chloramine, a longer-lasting compound of chlorine and ammonia, and at tap levels neither is generally a problem for established trees. Three other things can change a decision: how hard and alkaline the water is, whether a softener has treated it, and how much dissolved salt builds up in the pot, from the water or from feeding.

Hard water and its white crust

The example city's tap water is moderately hard. Water hardness is the amount of dissolved calcium and magnesium in water. When the water is heated, they come out as limescale in kettles, and when it evaporates, they stay behind as a white crust. Over months, unglazed terracotta shows the crust most, because water seeps through its walls and dries on the outside. By midsummer, the balcony trident's pot, watered from the kitchen tap every time, has a faint white bloom along its rim and upper wall.

The crust itself does no harm, and on a tree growing well it mostly shows minerals building up, so keep watching (Course 01, Lesson 10). Fertilizer leaves a similar crust of soluble salts, and the tree's history is the most practical way to tell them apart: how hard the water is, and how much the tree has been fed. A crust that comes with browning leaf edges, or with wilting in a moist pot, after regular or heavy feeding points to salts building up in the root zone, which calls for leaching (below) and a look at the feeding (Course 05, Mineral Nutrition and Fertilizing). The balcony trident shows neither sign: it has had moderately hard water at every watering and is growing well, so its crust is one to watch, not treat.

Alkalinity and acid-loving trees

Limescale is mostly calcium carbonate, so water that leaves it is also carrying dissolved carbonates and bicarbonates, which neutralize acid. That is alkalinity. It is not the same as pH, which measures how acidic or alkaline the water is; alkalinity measures how much acid it can neutralize. Each watering adds a little, and over many waterings it slowly raises the substrate's pH (Course 01, Lesson 10). Hard water is usually high in alkalinity, which is why limescale in the kettle is a fair first sign.

For the junipers, the trident maple, and the ficus, the drift is usually slow enough to watch rather than treat. It matters most for acid-loving trees, such as azaleas, and for small pots watered often. On moderately hard water, an azalea's newest leaves may begin to pale between green veins late in a season or the next year, sooner in a small pot watered daily, and later or not at all when plenty of rainwater is mixed in. That pattern, chlorosis between the veins of the newest leaves, is a clue to iron locked away by a high pH, not a diagnosis. Mixing in rainwater is a safe first step, but find out the substrate's pH before acidifying, feeding, or changing the mix (Course 04, Soil, Substrates, and the Root Environment).

Softened water

The small backyard's house has a water softener on its indoor supply, and the outdoor hose tap bypasses it. A softener swaps the calcium and magnesium for sodium (or, in some units, potassium). Softened water leaves no limescale, but sodium builds up in a pot as the water evaporates, where it can harm roots and brown leaf edges, so softened water is not used for routine watering. Softening does not remove the alkalinity either, so it does nothing for an acid-loving tree on hard water. The answer is water the softener does not treat: the household's trees outdoors get the hose, and its ficus, indoors from about early October to late May, gets water carried in from the hose tap or the rain barrel, never from the softened kitchen tap. In midwinter, if the hose tap is shut off against frost and the barrel drained, fill jugs at the hose tap before the freeze and keep them indoors, or have an indoor tap fitted that bypasses the softener. Water carried in from outside should reach room temperature before it goes on the ficus.

Rainwater and reverse-osmosis water

Rainwater is soft, low in dissolved salts, and slightly acidic, so it leaves no crust and adds no alkalinity. The small backyard keeps it in a covered barrel fed by the garage downspout: a screen at the inlet keeps out leaves and mosquitoes, and the lid keeps out the light algae need. Roof water carries some dust and debris, so it is for the trees, not for drinking. Lesson 6 returns to the barrel as a store for dry spells.

Reverse-osmosis water, from a filter that forces water through a very fine membrane, is much like rain: very low in salts and alkalinity, and carrying almost none of the calcium and magnesium that tap water supplies in small amounts. A tree watered only with rainwater or reverse-osmosis water depends on feeding for them (Course 05). Neither is needed on ordinary city water. They earn their place where the tap is softened, and for an acid-loving tree on hard water, where mixing in rainwater slows the drift in pH.

When a water test is worth it

You rarely need to test city water yourself, and most of the answers are cheap:

• The water utility's annual report, usually online, gives the city water's pH and dissolved salts, and often its hardness, alkalinity, and sodium. For most owners on city water, with trees growing well, it is enough.
• Test strips, sold for aquariums and pools, read hardness, alkalinity, and pH. A hardness strip also shows which taps a softener treats, because softened water reads close to zero. For sodium, use the report for unsoftened city water, or a lab.
• A conductivity meter gives dissolved salts as electrical conductivity (EC): the higher the reading, the more salt. A report may give EC or total dissolved solids, which rise and fall together. A probe moisture meter is not one, because it responds to salts and water at once (Lesson 2).

Testing earns its cost when the answer would change what you do. That means an acid-loving tree on hard water, a house with a softener (to find which taps it treats), and a crust with browning leaf edges on a tree that has hardly been fed, where the salts may be coming from the water itself. Above all it means a private well, about which nothing is known until a lab tests it.

Leaching salts out of a pot

When salts are suspected, rinse them out with plenty of plain water (Course 01, Lesson 10). That is leaching, sometimes called flushing, which has nothing to do with a growth flush (Lesson 1). Water thoroughly until water runs from the holes, wait a few minutes, then water again, so that plenty runs through the root zone and carries dissolved salts out. Use water that has not been softened: rainwater is ideal, and ordinary tap water is fine. On the west balcony, leach at the sink, or with the pot on a grid in a basin, so that none of the water reaches the balcony below.

Leaching is a response, not a routine: it rinses out nutrients too, and every thorough watering already carries some salts out. Pause feeding while you find out why the salts built up (Course 05), and if the browning, or wilting in a moist pot, continues afterward, look beyond the water (Course 07, Tree Health and Diagnosis).

Proportionate answers to water problems

What is in your water? begins as a sandbox and ends with five cases to judge. Choose a Source and a Tree, and under Pot, a Training pot or a Small pot; for tap and softened water you can also switch on Mix in rainwater. Before a run, you can answer After a season of this water, what builds up in the pot?, or press Skip. Then press Run a season to watch months of watering pass in seconds, from early spring to the next spring: what builds up on the pot and in the root zone, and how the tree responds. Back, Next, and Replay return to any stop. What a test would show gives the readings a test would find for that source, and where each would come from. Then:

1. Run City tap, moderately hard with the Trident maple, then with the Azalea, acid-loving. Run the azalea again in the Small pot, and once more with Mix in rainwater on. Then switch Source to Water from a softener, Rain barrel, Reverse-osmosis filter, and Private well, untested, and compare What a test would show.
2. Work through the five cases. For each, answer What do you do about the water? with the response that fits the evidence, and read why; a wrong choice can be changed. Solving all five finishes the exercise.

On city tap water, a white crust built up on the pot and the substrate's pH drifted slowly toward alkaline, but only the acid-loving azalea showed it in its leaves. Its newest leaves paled between the veins the next spring in the Training pot, sooner, in late summer, in the Small pot watered every day, and not at all with Mix in rainwater on. Softened water left almost no crust, but sodium built up in the mix, and without rainwater mixed in, the leaf edges, or the juniper's needle tips, browned. Its pH still drifted, because a softener leaves the alkalinity in place. Rainwater and reverse-osmosis water left almost nothing behind, and the well stayed unknown until tested.

Each case called for the smallest change its evidence supported:

• Use a tap that bypasses the softener for the household's ficus, whose first waterings indoors came from the softened kitchen tap.
• Keep using it, the city tap, for the balcony trident growing well under its white crust, and keep watching the leaves.
• Leach now and then for a neighbor's boxwood, fed at full strength every week, with a crust and browning leaf edges, and pause feeding (Course 05).
• Get a water report or test it first for a friend's private well that nobody has tested, before relying on it.
• Mix with rainwater for the azalea whose newest leaves yellowed between green veins on hard tap water, and find out the substrate's pH (Course 04).

Heat waves: check morning and afternoon

A heat wave changes the pace, not the decision. In the example city, July afternoons are often 30 to 33 °C (86 to 91 °F), and in a heat wave they stay near 36 °C (97 °F) for days. The west balcony's sun arrives with the hottest, driest air of the day, so evaporative demand peaks while the pots are in the sun (Course 02, Lesson 5), and a pot that checked Moist in the morning can be dry by late afternoon. While the heat lasts, the summer plan from Course 02, Lesson 5 holds, and the checks double:

• Check every pot morning and afternoon, with the lift and the chopstick, and water thoroughly whenever a check says so. When the morning check says water, water then, so the tree starts the hottest hours with a full root zone. The afternoon check often finds a pot that needs water again, and watering in sunshine rarely harms the leaves (Lesson 3).
• Keep the pot walls shaded, and keep the two junipers side by side on the shelf, so their pots shade each other's walls, because a sunlit pot wall can heat the roots far above the air (Course 02, Lesson 5). The trident stays in its spot at the back wall; moving all three there is for time away, when no one can check every day (Lesson 6).
• Put shade cloth over the trident on the hottest afternoons to protect its leaves from leaf scorch. The sun-loving junipers stay in the sun.

Judge a drooping trident by its root zone: limp at 6 pm over a moist root zone and firm by dusk is temporary wilting, not a call for water (Lesson 4). A juniper barely droops, so judge it by its pot.

Very small pots need more than extra checks. A cutting in a 9 cm (3.5 in) pot, or a tree in a bonsai pot 5 cm (2 in) deep, can dry out in hours, between a morning check and an afternoon one. For the days of a heat wave, move such a pot into bright shade, out of the wind, and check it more often. This is not the myth in Course 02, Lesson 5 coming back: the myth moves a tree into shade instead of fixing what is overheating, while here the trouble is a reserve too small to last between checks.

Three days of heat on the balcony

Three days of heat runs the west balcony through a heat wave near 36 °C (97 °F), windy on the second day, with the pot walls shaded and a thorough watering whenever a check finds a root zone Drying or drier. For each day, set Checks to Morning only, Morning and afternoon, or Morning, afternoon, and evening. Two switches hold for the whole wave: Group the pots, which brings the two junipers close together on the shelf and leaves the trident at the back wall, and Shade cloth over the trident on the hottest afternoons. A third, Move the junipers under the overhang, is there to test. Before each run, you can answer Which pot runs out first on day 2?, or press Skip. Then press Run the heat wave, and use Pause, Back, Next, Replay, or Skip to the end to look closely at any moment. Find both challenges to finish. Try these:

1. Run the wave with Checks set to Morning only every day, and watch each pot's moisture line fall toward Dry. Note where a line crossed into Dry between checks.
2. Switch days to Morning and afternoon until all three pots come through, for the challenge Bring all three through.
3. For the challenge Find the day the afternoon check mattered most, answer Which day did the afternoon check matter most? after a run. To test a day first, press Day 1, Day 2, or Day 3 at the bottom of the challenge, which loads a plan with afternoon checks on every day but that one, and run it.
4. Then try Move the junipers under the overhang, and see how it is graded.

The moisture lines fell fastest in the afternoons, while the sun was on the balcony, and fastest of all on the windy second day. With Morning only, a pot could check Moist in the morning and cross into Dry by mid-afternoon, hours before anyone looked again; an afternoon check caught it while it was still Drying, in time for a thorough watering, and each check watered only the pots that needed it. With afternoon checks on the other two days, skipping the afternoon check cost the most on day 2: with no switches on, all three pots went dry, against two on day 1 and one on day 3. Group the pots and Shade cloth over the trident on the hottest afternoons slowed the drying, but with the junipers in the sun, no plan brought all three through without an afternoon check every day, so they are additions to that check, not substitutes for it. Move the junipers under the overhang slowed their drying too, but it was graded Not needed: it keeps the sun-loving junipers out of the sun they need, and shading the pot walls already protects the roots.

The leaves were no guide to the root zones. The junipers never drooped; they turned dull and grayish only after their pots had gone dry. Without shade cloth, the trident drooped in the late sun even over a moist root zone, and recovered by evening.

Wind, fast drying, and frozen pots

Strong wind

Strong wind dries foliage and pots fast. It thins the boundary layer, the still, damp air around each leaf, so water loss climbs, most sharply in sun and heat, and it dries the pot too, above all unglazed terracotta (Course 02, Lesson 5). Add an afternoon check on any windy day, in spring and autumn as well as summer, starting with the pots on the open shelf, and make sure every pot is still tied on: four floors up, a falling pot endangers people below.

A pot that dries faster than expected

Sometimes a pot dries far faster than the weather explains: light by noon on an ordinary day, or dry at the morning check after a mild night. Confirm it with the lift and the chopstick, not the surface. In the gift juniper's two-zone root ball, the coarse ring dries ahead of the old core in summer, so a lighter pot or a dry ring is expected; the chopstick in the core decides (Lesson 2). When the checks confirm it, water it thoroughly now, in slow passes, or by immersion if the root ball has shrunk from the wall or water runs straight through (Lesson 3). Only then look for the cause, before you change the routine:

• Channeling. Water out of the holes within seconds, and a pot barely heavier afterward, mean the water ran past the root ball. Watering more often in the same way will not fix it.
• Roots filling the pot. A pot full of roots holds less water and dries faster. Roots matting at the holes and circling the wall are the sign that a tree may be root-bound, a question for the next suitable time to repot (Course 11, Repotting and Root Development).
• A hotter or windier spot. A move, a shade board that has fallen, or the condenser running beside a pot all raise the pace.
• A growth flush. A burst of new leaves adds leaf area week by week, and the pot keeps drying faster from then on (Lesson 1).

Note what you found, with the date and the weather, in a watering log: one odd day is not a pattern, and the log shows which it was (Lesson 7).

Frozen pots in winter

Winter brings the opposite problem. A frozen root ball cannot take up water, and water poured onto it cannot soak in. Check pots on milder days, once they have thawed, and water a dry one then, so that it goes into the next freeze moist rather than dry (Course 06, Seasonal Care and Timing). Evergreens also lose water to winter sun and wind while their roots are frozen, which is winter desiccation (Course 02, Lesson 5). The rain shadow lasts all winter too, so pots under an overhang still need checking after winter rain.

Checks for rain, heat, and water