Drought stress

The test that comes first

A wilting plant may be short of water or it may be drowning, and the two look identical from above. Roots killed by waterlogging cannot take water up, so the leaves go slack while the compost is sodden.

Put a finger into the compost to the second knuckle before you do anything else. Dry, light, shrunk away from the side of the pot is this page. Wet, heavy and sour-smelling is root rot, and watering it again will finish the plant.

That one check takes three seconds and it decides which of two opposite treatments you apply.

What wilting is

Wilting is a control mechanism rather than damage. A plant holds its shape by water pressure inside its cells, and when the roots cannot supply enough, the cells lose pressure and the tissue goes limp. At the same time the pores on the leaf surface close to stop losing more.

A plant that has wilted mildly and recovers by morning has lost a day’s growth and nothing else. Photosynthesis stops while the pores are shut, so a plant that wilts every hot afternoon grows slowly all summer without ever looking obviously ill.

Severe or repeated wilting is a different matter. Air bubbles form in the water columns running up the stem and break them, so those pathways stop working permanently. Fine roots die back. That is why a plant allowed to dry to collapse three or four times stops bouncing back the way it did the first time.

Why a watered pot can still be dry

Compost that has dried hard becomes water-repellent, and it shrinks away from the sides of the pot as it goes. Pour a can over the top and the water finds the gap, runs straight down the inside of the pot and out of the drainage holes.

The pot then feels heavier, water has clearly come through, and the root ball in the middle is still bone dry. People kill plants this way while watering them diligently.

The fix is immersion. Stand the pot in a bucket of water up to the rim and wait until the bubbles stop, which usually takes twenty to thirty minutes. Then let it drain, loosen the compost surface, and press it back against the sides of the pot so the channel closes.

Where the damage shows

Leaf edges and tips go brown and crisp first, because they are furthest from the water supply, and the boundary between brown and green is sharp. Older leaves yellow and drop as the plant sacrifices them to keep the growing tip supplied. Flowers and small fruit are shed for the same reason.

Outdoors, watch for premature autumn colour. A birch, a maple or a magnolia turning in early August has been short of water since June, and a shrub that drops its leaves early is telling you about a drought that already happened.

Some effects arrive a year late. Camellias and rhododendrons form next year’s flower buds in mid to late summer, so a dry August produces a poor spring, and the owner looks for a cause in March that occurred seven months earlier.

What to do about it

Water fewer times and much more thoroughly. A shrub wants ten litres delivered slowly to the root zone rather than a can waved over a border. That draws the roots down to where the moisture lasts, which is what carries the plant through the next dry spell without you.

Water the soil rather than the leaves, and put a mulch on afterwards while the ground is wet.

For containers, learn the weight. Lift the pot when it has just drained and again when it is dry, and after a week you will know what your plants need better than any moisture meter or schedule can tell you.

Keep watering anything planted in the last two years. A newly planted tree has a root ball no bigger than the pot it came in and a canopy that transpires like a tree, and the two-year rule is what separates the trees that establish from the ones that die in their second July.

Two things worth ignoring

Watering in sunshine does not scorch leaves. The idea that droplets act as lenses and burn the foliage has been tested and does not hold on flat leaves. It does waste water to evaporation, which is the real reason to water early or late.

Little and often is the wrong instruction outdoors. It produces a shallow root system living in the top few centimetres of soil, which is the part that dries first, so the plant becomes more dependent on you each season rather than less.

What it gets mistaken for

Root rot causes the same wilt with wet compost, blackened soft roots and a sour smell. Test the compost before deciding.

Nutrient shortage yellows the older leaves while the plant stays turgid and the compost is moist. Feeding a plant that is short of water makes things worse, since fertiliser salts draw water back out of the roots.

Leaf scorch from sun or wind marks the exposed side of a plant and the leaves facing the light, in patches rather than uniformly at the leaf edges.

Vine weevil grubs and root rot both remove a container plant’s ability to take up water, and both look like drought. Tip the pot out and look before you conclude anything.

Hydrangeas and courgettes wilt every hot afternoon regardless of how well watered they are, because their leaves lose water faster than any root system can supply it at two o’clock. Judge those two in the early morning, and if the plant is upright at seven it does not need more water.