Temperature, air movement, and humidity

Instructional example: the west balcony from Lesson 1, on a July afternoon. Its owner has stood the nursery juniper and a trident maple on the concrete floor in the middle, where the sun is best. The weather app says 32 °C (90 °F).

On the balcony, that number is true almost nowhere. The sunny side of the juniper's dark pot is too hot to keep a hand on. Beside the air-conditioner condenser, the fan blows air hotter than the forecast. Under the balcony above, the floor along the back wall is still dry after last night's storm. By the end of the month, the trident maple's sunny-side leaves have brown, crisp edges.

The forecast is not wrong: it describes air measured in the shade, about 1.25 to 2 m (4 to 6½ ft) above open grass, so that places can be compared. A tree lives somewhere else: at pot height, beside a wall, over concrete, in the sun or out of it.

Every spot has its own climate

A microclimate is the climate of a small area, such as one corner of a balcony or the strip of ground along a wall, where it differs from the weather around it. A few features cause most of the differences:

• Walls and paving soak up sun and pass the heat to whatever is near them, into the night. A heated building's wall also leaks warmth on cold nights.
• Overhangs and roofs cut sun and rain, and on clear nights they hide what is under them from the open sky, which draws heat away.
• Corners and screens funnel or block the wind. An open corner can be the breeziest spot; a closed one can trap still, hot air.
• Height adds wind, on a fourth-floor balcony or a shelf at railing height. On clear, still nights over open ground, though, the coldest air lies right at the ground.
• Low ground collects cold air, which drains downhill.

The cases in this course are in the Northern Hemisphere, where south-facing walls and slopes are the sunny side; in the Southern Hemisphere, swap north and south. A west-facing spot gets the afternoon sun in either hemisphere.

Lesson 4 mapped light. Heat, cold, wind, and moisture matter too, because each changes how fast a tree loses water and how well its roots keep up (Course 01, Lesson 8).

Heat builds through the afternoon

The west balcony gets about six hours of direct summer sun, from early afternoon to sunset. An east-facing doorstep might get almost as many, from sunrise to late morning. For light, the two are similar. For heat, they are not.

At the same height in the sky, the sun delivers about the same energy morning or afternoon; what differs is everything around it. Morning sun meets air and surfaces that cooled overnight, and leaves that refilled with water. Afternoon sun shines while the air is at or near its daily peak, which usually comes in mid to late afternoon, after walls and paving have heated for hours, and while the air is driest. So west-facing spots run hottest, and their sun lands when evaporative demand, the air's pull on the water in leaves, is highest.

Hard surfaces add reflected heat: the extra heat a tree gets from sunlit walls, glass, and paving nearby, as bounced sunlight and as heat those surfaces give off once warm, even after sunset. The small backyard's patio, paved and along the house's sunny south wall, gets plenty of both.

The leaves pay in water. Heat and dry air speed transpiration, and when loss outruns what the roots supply for long enough, the leaf tissue farthest from the water supply dies first. Leaf scorch is the result: leaf edges and tips turn brown and crisp, worst on the sunny or windward side. The trident maple is prone to it in strong afternoon sun and wind. Scorch is a symptom, not a diagnosis: a dried-out root ball, damaged roots, or salt buildup can brown leaf edges too (Course 07, Tree Health and Diagnosis).

Hot roots in a small pot

In the ground, roots sit in a large mass of soil that warms and cools slowly, so at root depth it rarely gets as hot as the air. Roots are adapted to that steadiness, and fine roots are generally more easily injured by heat than leaves.

A pot in the sun has no such buffer. Its sunny wall heats first, and a dark surface, a small pot, and hot paving underneath all make it worse. Nursery studies have measured more than 45 °C (113 °F) just inside the sunny side of black containers on summer afternoons, with the air in the low 30s °C (around 90 °F). Fine roots of many temperate trees are injured when the root zone stays above roughly 40 °C (104 °F) for hours, though the limit varies with the tree and how long the heat lasts, and a dry root zone heats faster than a moist one. Injured roots take up less water, so a tree can then wilt or scorch in a moist pot, as a freshly repotted tree can (Course 01, Lesson 8).

For a sun-loving tree, the fix is targeted: the juniper's foliage needs the sun, and its pot wall does not. Shading the pot, not the tree, protects the roots without taking light from the leaves: a board or other pots on the sunny side, or a pale sleeve or wooden box around the pot. Lifting the pot off hot paving onto a slatted shelf helps as well, though it does not keep the sun off the pot wall.

Check your prediction in A hot afternoon, from the roots' point of view. It starts with a Dark glazed pot on the Concrete floor and no shading. Press Play or step with Next, and compare Root zone, sunny side with Air in the shade until How this setup did appears at sunset. Then change one setting at a time and run the day again (the dashed line keeps your previous setup), and try the three challenges under Try these.

With nothing done, the leaves had full sun and looked fine, but the root zone on the sunny side reached Too hot for roots in late afternoon and stayed there through sunset, while the air never went above Hot. Sun on the pot wall was the main source: the Slatted shelf alone still left a dark pot too hot for most of the afternoon. Pot shaded kept the roots safe with either pot on either stand, and the leaves kept every hour of sun. Shade cloth cut water loss but also about half the light, and over a dark pot on concrete the roots still overheated. With no shading at all, only the Pale unglazed pot on the shelf stayed out of danger, and it dries out faster.

Cold: pots, clear nights, and low ground

In winter the comparison reverses. As Lesson 1 showed, soil in the ground buffers the roots, but a pot above ground is exposed on every side and, over a cold spell, can freeze through, starting at the wall, where many fine roots grow. Roots are also generally less cold hardy than shoots, because they build much less cold acclimation than branches and buds (Course 01, Lesson 13), and the hardiness zones on plant labels describe trees planted in the ground, so a tree rated for much colder winters than yours can still lose its roots in a pot. Winter protection belongs to Course 06, Seasonal Care and Timing. What this lesson adds is where on a site the cold is worst.

The coldest nights are clear and still. Ground, pots, and leaves radiate heat to the open sky, and with no wind to mix the air, the air just above the ground can end up several degrees colder than at head height. That is why frost can whiten a lawn while the official temperature stays above freezing. An overhang or a canopy that hides part of the sky slows the loss.

Cold air is also denser than warm air, so on still nights it drains downhill and pools in low ground. A frost pocket is a low spot where this happens: frost comes earlier in autumn, later in spring, and harder there than a few steps upslope. The small backyard's southeast corner, its lowest point, is one. Its patio, against the house's south wall, is the opposite: the wall stores the day's sun and leaks a little warmth from the house. A warm wall has a catch, though: in late winter it can wake buds early, putting tender leaves out in time for the last spring frosts, usually in late April in the example climate.

Wind: useful air and drying gusts

A gentle breeze helps. It thins the humid boundary layer around each leaf (Course 01, Lesson 8), carrying away heat and bringing in carbon dioxide, and it dries leaves after rain. Without it, heat and humidity build up, as in the still corner by the west balcony's privacy screen.

Strong or drying wind raises water loss, sharply so with sun and heat, tears soft leaves, dries pots, and blows them over. Four floors up, a falling pot endangers people below: fasten any pot that stands high, and never stand pots on the railing (Lesson 7 covers stable setups).

A windbreak is anything that slows the wind behind it: a fence, a screen, a hedge, or netting. The best ones let some air through. A solid barrier sends the wind tumbling over its top in gusts and eddies, with still air close behind it; a screen with roughly half its area open slows the wind over a longer stretch and keeps air moving. Netting across part of the balcony's open south end, if the building allows it, would calm that end without making it airless.

Winter wind and evergreen foliage

In winter, evergreen foliage keeps losing water through its stomata and waxy cuticle on sunny or windy days, and if the root ball is frozen, nothing replaces it. The result is winter desiccation, or winter burn: foliage on the sunny or windward side dries and browns, often noticed only in spring, when it stays brown while the rest of the tree greens. A juniper's even, purplish bronze in cold weather is different, a normal winter color that fades in spring (Course 01, Lesson 13). A potted juniper in the balcony's windy south corner is a classic case for winter burn; shelter from winter wind, and a root ball that goes into freezes moist rather than dry, are the main defenses (Course 06).

Humidity: what it changes, and what misting cannot

The drier the air, the faster leaves and pots lose water. Relative humidity measures that dryness: the amount of water vapor in the air, as a percentage of the most the air could hold at its temperature. Warm air can hold far more vapor, so relative humidity falls as air warms, outdoors through each afternoon and indoors whenever cold air is heated. Air at 0 °C (32 °F) and 80 percent relative humidity, heated to 21 °C (70 °F), ends up near 20 percent, so the bright window's room is very dry in winter, and the air rising off its radiator is hotter and drier still.

Misting, a humidity tray (a shallow tray of water and gravel under the pot), and grouping plants all raise humidity, but only a little and only near the plant, because room air mixes away quickly. None of them replaces what matters more: an indoor tree is usually short of light long before it is short of humidity (Lesson 4), and its water comes in through the roots. For the ficus on the bright window, the order is strong light, watering based on checks below the surface, and a spot out of the radiator's rising air. If you use a tray, keep the pot on the gravel above the water, so its drainage holes never sit in it (Course 01, Lesson 7).

Where the rain does not reach

A rain shadow is an area that rain does not reach because something blocks it: the ground under an overhang or eave, beside a wall on the side away from wind-driven rain, or under a dense canopy. On the west balcony, pots against the back wall get little rain, and the condenser corner, with the screen beside it, almost none. After a day of rain, check those pots below the surface (Course 01, Lesson 2) rather than assume the rain watered them, in winter too.

Reading a site spot by spot

One balcony, six climates puts the west balcony's spots side by side. Under Weather, start with Summer afternoon and compare the spots, using the numbered markers or the Spot buttons (1 Back wall to 6 By the screen). Then switch to Rainy day and Winter night. Finally, build Your summer plan: pick a spot and press Juniper here, Trident here, or Avoid this spot.

Spots two steps apart differed. The Railing shelf (4), a slatted shelf at railing height in the open middle, set just inside the railing, had as much sun as the Middle floor (3) or more, but its Heat read High instead of Very high, because the pot sat off the hot concrete. The low sun still heats the pot wall up there, as the afternoon animation showed, so with the shelf secured, the pot tied on, and its sunny side shaded, the shelf is the juniper's best summer spot. The Back wall (1) is shaded through the early afternoon and sheltered from the breeze, which made it the trident maple's best option, though on the Rainy day little rain reached it: a rain shadow. The condenser corner (5) was shaded and still the spot to avoid, with hot, dry air blowing across it on hot afternoons.

The Winter night is clear and still, the kind that brings the hardest frosts, so Wind read Still everywhere and the order changed: the back wall and the condenser corner, under the balcony above, were the least cold, and the shelf and the south corner, open to the sky, the coldest. The screen, which shelters its spot from wind on other days, did nothing to stop the frost. The best summer spot is not automatically the best winter spot.

Watch your own spots

A min/max thermometer records the highest and lowest temperatures since you last reset it. Hang it at pot height and out of direct sun (in sun it reads its own heating), read and reset it at the same time each day, and move it between two or three spots over a week or two. In summer, a probe thermometer pushed into the substrate on a pot's sunny side late in the afternoon shows how hot the roots get. After hot, windy, frosty, or rainy days, note which pots were lightest by evening, any brown leaf edges, where frost lingered, and which pots stayed dry after rain. Those notes become part of your site profile (Lesson 1).

Adjustments and what they cannot replace

A useful adjustment targets the actual problem without taking away something the tree needs.

Placing trees for a hot, windy summer