Why is my tree losing leaves in summer, and will they grow back?

Leaves dropping in summer can be a tree’s normal response to drought, or an early sign of something more serious. Here’s how to tell, and what happens next.

Finding a lawn full of leaves in July is unsettling. Sometimes it’s a tree doing exactly what it should to get through a dry spell; sometimes it’s a sign of disease, root trouble, or pests. Here’s how to tell the difference, and what to expect next.

Drought and heat: shedding to save water

Every leaf loses water through its pores, so in a dry summer, some trees drop part of their leaves to cut their water use. Linden, birch, and sycamore are among the trees most prone to it, and as long as the drought doesn’t drag on, health and growth usually aren’t affected (Rutgers Cooperative Extension).

Tulip trees are famous for it: from late June through August, scattered leaves turn bright yellow and fall while the rest of the canopy stays green. It rarely affects the tree’s health, and watering helps (Purdue Landscape Report). River birch does the same, yellowing and dropping its inner leaves in hot, dry weather (Iowa State University Extension). Losing older, inner leaves this way is not a sign of decline; losing leaves from the branch tips is more concerning (University of Tennessee).

On big trees, drought stress often shows first at the top center of the canopy, where it’s hard to see (University of Florida IFAS). Cottonwoods and willows have their own summer habit: they drop whole pencil-sized twigs, more in dry years (South Dakota State University Extension).

Too much water and root problems

Roots need air as well as water. In waterlogged soil, they suffocate, and the tree yellows, wilts, and browns in ways that look a lot like drought. Wet soil also favors root rots (University of Kentucky).

Damage below ground shows up above ground, often slowly. Roots cut or crushed during construction can lead to sparse leaves, scorch, early fall color, and dieback months or years later, often on the side where the roots were damaged (University of Maryland Extension). Newly planted trees commonly wilt, yellow, and drop leaves from transplant shock for two years or more, especially if they were planted too deep or watered too much or too little (Purdue Extension). Roots that circle and squeeze the trunk cause a thin crown, early color and leaf drop, and a slow decline over years (University of Maryland Extension).

Diseases that drop leaves early

After a cool, wet spring, sycamore anthracnose blotches leaves and kills shoots, and sycamores can shed a lot of leaves. They keep pushing out new ones, though, and most have a good canopy again by late June or early July (Iowa State University Extension). Apple scab can strip susceptible crabapples by mid to late summer. It rarely kills an otherwise healthy tree, but repeated severe infections can weaken it (Iowa State University Extension). Marssonina leaf spot drops leaves from poplars, aspens, and cottonwoods in wet weather, starting low in the tree (Purdue Landscape Report), and birch anthracnose and rust do the same on birches (University of Minnesota Extension).

Bacterial leaf scorch, on elms, oaks, sycamores, maples, and others, browns leaf edges in mid to late summer, starting on the lower, inner, older leaves. It comes back every year and can eventually kill a tree (University of Maryland Extension).

Pests

Caterpillars can strip trees bare. In outbreak years, forest tent caterpillar and spongy moth can defoliate whole stands of trees in late spring and early summer (Maine Forest Service; Connecticut Agricultural Experiment Station). Japanese beetles skeletonize lindens and other trees in midsummer, which is usually not fatal to established trees (Iowa State University Extension), and birch leafminer can make a birch look suddenly dried up (University of Minnesota Extension). For a full guide, see what’s damaging these leaves?

Late spring frost

A hard frost after leaf-out can kill tender new leaves overnight. Trees that were healthy beforehand usually start to releaf in about three weeks, so the best advice is to wait (University of Illinois Extension). The second set of leaves can come in slowly and a little smaller, but for trees with good food reserves the damage is largely cosmetic (Purdue University). It isn’t free: research across hundreds of species found late frost depletes trees’ stored reserves and delays the next spring’s leaf-out by about a week on average (USA National Phenology Network). See how weather affects spring buds for more.

Will the leaves grow back?

Often, yes. Healthy, established hardwoods can grow two sets of leaves in one year (Penn State Extension). When frost or caterpillars strip a tree early in the season, backup buds open and a new set of leaves appears, sometimes within weeks (Colorado State University Extension; Maine Forest Service).

Timing matters. Losing leaves early in the season costs the most, because the tree has to spend stored energy on new leaves before they’ve paid for themselves, and that can be serious for small or already stressed trees (Penn State Extension). Losing leaves in late summer matters less, because they were close to dropping anyway (Georgia Forestry Commission). A tree that loses its leaves late usually won’t regrow them until spring, and then leafs out on schedule (Mississippi State University Extension).

The real danger is repetition. Year after year of defoliation, especially combined with drought, drains a tree’s reserves, leading to smaller leaves, dieback, and in some cases death within a couple of growing seasons (Colorado State University Extension; Maine Forest Service).

How to check whether a tree is alive

Scrape a little bark on a twig with your thumbnail. Green underneath is a good sign; brown and dry suggests that part is dead. A year-old twig that snaps cleanly when bent is likely dead (NC State Extension). Warning signs that need a professional look include mushrooms or shelf fungi on the trunk or base, dead limbs, cracks, cavities, and cankers. For trees near homes, driveways, or play areas, have a certified arborist inspect them (UC IPM).

How to help a stressed tree

Water deeply, not often. Most of a tree’s roots are in the top foot or two of soil, so water slowly enough to soak that zone (The Morton Arboretum). During a drought, water slowly and deeply enough to moisten roughly the top 12 inches of soil under the canopy. Every two to four weeks is a useful general guide for mature trees, but adjust for rainfall, soil moisture, and local watering restrictions. Young trees, planted within the last two years, need water more often (University of Florida IFAS). Water before the tree starts dropping leaves if you can.

Mulch. Spread 2 to 4 inches of mulch over the root zone, keeping it at least 4 inches from the trunk (University of Illinois Extension).

Hold off on fertilizer and heavy pruning. Don’t fertilize a drought-stressed tree, avoid pruning live branches, and don’t dig under the canopy (University of Florida IFAS).