Drought and an established tree: where to water, how much, leaf scorch, and the year after

Water an established tree in a drought over the whole ground under its crown and a few feet past the branch tips, never at the trunk. Soak it slowly until the soil is moist 6 to 12 inches down. Soak it again once the top 6 inches have dried, which in a drought comes every one to two weeks. A tree that has stood for years needs this only in a long dry spell. Its thirst shows as wilted afternoons, scorched leaf edges, early leaf fall and, the next spring, a thin crown.

The signs, from wilting to a thin crown

As the soil dries, the signs come in this order:

1. Wilting that recovers overnight. Leaves droop in the afternoon and look normal by morning. On a tall tree it starts at the top center of the crown, where it is hard to spot, so check the soil instead.
2. Scorch along the edges. In July and August the tips and edges of the leaves, last in line for water, yellow and then turn dry and brown. In hard heat the tissue between the veins dies too. It is worst on the side facing wind, strong sun or heat thrown back by a wall or pavement.
3. Early color and early leaf fall. The leaves yellow or turn their fall color early, and many drop.
4. A thin crown the next spring. A tree forms next year’s buds in summer, and a dry summer leaves fewer and smaller ones. The crown comes out sparse, its leaves undersized.

Some trees show drought sooner. American beech suffers in drought, river birch drops its leaves, and Ohio buckeye scorches. Even so, a big, healthy tree needs no water in the first dry week:

“Large, mature shade trees and shrubs can be left alone unless the drought is severe or the trees begin to wilt or the root systems have been recently disturbed.” (University of Maryland Extension, Watering Trees and Shrubs)

Where the roots are, and where the water goes

Tree roots run shallow and wide. Most of the roots that take up water lie in the top 18 inches of soil, and they reach from near the trunk to well past the drip line. Water that whole area, under the crown and a few feet beyond, and give the most around the drip line, where the roots best able to take up water lie. Water at the base of the trunk does little good and can lead to decay.

How much water, and how often

Wetting the soil 6 inches deep takes 1 to 2 inches of water, about 65 to 130 gallons per 100 square feet. Run a sprinkler set low under the crown until an inch stands in a tin can, then move it. Or lay a hose at the drip line, its stream as thick as a little finger. Leave it 20 to 30 minutes in each of three or four places around a medium or large tree. A soaker hose in a spiral under the crown is slower and often more efficient.

In gallons, Colorado State Extension gives an established tree about 10 gallons per inch of trunk diameter every two weeks, the figure our watering calculator uses. Set to a tree planted five years ago with a 1-inch trunk, the calculator counts it established and gives 10 gallons. Measure the trunk again, since it has thickened: at 6 inches across, give about 60 gallons.

At least four hours later, push in a long screwdriver. It slides through moist soil and stops at dry, and it should go 6 inches down. Soak again when the top 6 inches have dried, usually every one to two weeks in a drought. A light daily sprinkle does harm, keeping a shallow layer of soil saturated until the roots in it decline for want of air.

Mulch, and what to leave undone

Mulch holds water in. Spread 2 to 3 inches of wood chips or leaves toward the drip line, 8 to 12 inches off an old trunk, and water through it into the soil. Let the lawn go brown: it greens again in fall, and the tree comes first.

Leave three jobs until the rain returns. Fertilizer pushes new growth that needs water the tree lacks, and its salts can harm dry roots. Prune only broken, storm-damaged or diseased branches, cut at the branch collar. Digging, trenching or heavy traffic under the crown cuts or packs roots, and a tree short of roots shows drought even in moist soil.

The year after a drought

Drought does much of its harm after the rain returns:

“In many cases, drought kills the fine feeder roots that trees use to take up water and nutrients. Until these feeder roots can be fully replaced, the tree remains under stress and cannot support a full canopy of leaves.” (Texas A&M Forest Service, Effects of Drought on Trees)

A tree in that state is open to borers, root rots and canker fungi, which can kill it after the drought has ended. Scattered trees die for several years afterward. Among the trees we grow, a drought leaves these open to a pest:

Watch the tree through the next summer for twigs and limbs dying back, bark sloughing off, and sawdust at the base or pitch on the bark. Water it again in any dry spell that summer.

A new tree, and evergreens before winter

A tree planted in the last year or two is different. Its roots are still close to the planting hole, so water the root ball and a few feet past it, far more often, as the care calendar for a new tree sets out. Its wilting and scorch are often transplant shock, its roots still too few for its crown.

Evergreens lose water all winter, most on sunny, windy days above 40 degrees. After a dry fall, soak them deeply before the ground freezes. In a dry winter, water them once or twice a month on a day above 40 degrees when the ground is not frozen. Otherwise the foliage can brown by spring, the winter burn of late winter.

What to do next

• This week: probe the soil 6 inches down under the drip line, and soak the tree if it is dry there.
• The gallons: the watering calculator tells whether a tree you planted is established yet and gives its dry-spell soak.
• Next summer: watch for dieback, borers and canker, and water in any dry spell.
• Still declining: Blue Whale Trees answers tree questions through its support page. Say when the trouble began, and photos of the whole tree and a close-up can follow by email once we reply.

Common questions

Will a tree recover from leaf scorch?

Usually: scorch alone does not kill an otherwise healthy tree, though the browned tissue is dead. Judge the tree by its leaves next spring.

Why is my tree dropping leaves in August?

In a dry summer it is most likely drought. If the soil 6 inches down is moist, look for roots cut by digging or packed by traffic, which bring the same signs.

Can you overwater an established tree?

Yes: soil kept wet, often by a lawn sprinkler running every day or two, loses its air, and the roots in it decline. Soak deeply, then wait until the top 6 inches are dry.

Should I water trees in fall after a drought?

Yes, as long as the ground is not frozen, and evergreens most of all, since they lose water all winter. Weather and the soil decide when to stop, not a date.