Defoliation

Defoliation refers to the widespread stripping away of leaves.

Defoliated tree branches (Pikist) Public Domain

Leaves are a plant’s food factory. This is where the majority of photosynthesis occurs. Without leaves, a plant cannot convert the sun’s energy into the sugar and carbohydrate molecules needed to survive and thrive. Leaves are also used to breathe, maintain moisture levels, filter pollutants, and stabilize soil temperatures. They are also part of the plant’s disease and pest resistance, and winter hardiness. Yes, leaves are all that. Losing all its leaves is perfectly normal behavior for deciduous trees in autumn, but defoliation can be a sign of serious problems, otherwise.

Typical fall foliage in the Adirondacks (DigbyDalton) CC BY-SA 4.0

Autumn leaves

Every autumn (okay, most autumns), the leaves of deciduous trees turn bright yellow, orange, red, and even purple, before falling to the ground. This occurs because the green pigment responsible for photosynthesis, chlorophyll, is in short supply. Without all that chlorophyll hanging around, you can actually see the other colors that were there all along, and some new colors that are produced especially for winter preparation. These senescent leaves fall naturally, in a behavior called abscission, and they make excellent additions to your compost pile. Other causes of defoliation are not as colorful or as beneficial. And trees aren’t the only plants that risk defoliation.

Chemicals

Chemicals, such as the dreaded Agent Orange, can defoliate entire regions. Overspray from herbicides, such as glyphosate, can also cause plants to lose their leaves unexpectedly.

​Commercial agriculture uses specific chemicals to stimulate defoliation of crops, such as cotton, to make harvesting more efficient. They also use chemicals to defoliate trees, such as almonds, that tend to carry potentially devastating diseases on their leaves. One good spray, all the leaves fall, and cleanup ensures. Since these trees are deciduous, the leaves were going to be lost anyway.

​Diseases

Mangrove forests (top) defoliated (bottom) by Agent Orange in the Vietnam War (Wikipedia)

​Molds, mildews, blisters, stem streaking, rusts, and leaf spots may indicate fungal, bacterial, and viral diseases that can cause defoliation. If you are able to identify the disease, you will then have the information you need to break the disease triangle and protect your plants.

​Grazers

Insects, slugs and snails, and caterpillars may be small, but there can be a lot of them, and they can eat a lot of leaves. Leaf-eating pests include katydids, Japanese beetles, grasshoppers and crickets, budworms, hornworm caterpillars, and western tussock moth larvae, just to name a few. Some pests, including psyllids, scale insects, aphids, treehoppers, leafhoppers, sharpshooters, melon flies, whiteflies, and thrips don’t cause defoliation by eating leaves. These pests weaken a plant by sucking out nutrient-rich sap, leaving the plant too weak to hold onto its leaves. Their feeding can be compounded by the diseases they carry. Maintaining an awareness of insect migrations can help you protect your plants.

Snail eating lettuce (Pikist) Public Domain

Herbivores, such as deer, rabbits, and voles can devour entire gardens, given the opportunity. While voles and rabbits generally do not have access to tree leaves, deer can reach just about anything. And those height-challenged rodents and lagomorphs can girdle a tree by gnawing through the bark, interrupting the supply of sap, water, and nutrients through the vascular tissue, causing defoliation.

Rabbit inspecting stone pine seedlings (Pikist) Public Domain

​Environmental factors

Environmental factors, such as water stress, pollution, and salinity can all lead to defoliation. Water stress can mean too much or not enough water. Whether it is caused by flooding or drought, water stress can result in partial or complete defoliation. Water stress, pollution, and injury together are causing severe defoliation in a condition called shade tree decline. Too much salt in the soil or in irrigation water can also cause plants to lose their leaves.

If you notice defoliation occurring, try to identify its cause. This is the only way to help your plants recover, and reducing the likelihood of it happening again. Frequent defoliation nearly always results in plant death.

If defoliation is a natural process, or if the plant survives whatever caused the leaf loss in the first place, the recovery process is called refoliation. Factors involved with a plant’s ability to recover from defoliation include its age and overall health, the time of year when defoliation occurs, the frequency of defoliation, any secondary pests or diseases that make have taken advantage of the plant’s weakened state, and the availability of irrigation and nitrogen.