Tree troubles: Causes of early leaf drop

A single leaf sitting in the grassEven though it was officially the first day of autumn just a few days ago, leaves shouldn’t be falling en masse quite yet.

The region you live in will determine when peak fall color season will arrive, and tree leaves generally begin to fall a week after peak color.

According to The Davey Tree Expert Company, in the Northeast leaves will start falling either early or late October. In the Midwest, trees can be expected to lose their leaves in mid-to-late October while in the South, leaves won’t tend to fall until late October to mid-November. In the West, leaves can fall anywhere from early to late October.

Trees determine when the proper time to drop their leaves is based on temperature and day length. Tree leaves feature chemical light receptors, phytochrome and cryptochrome, which detect red and blue light, respectively. When day length changes, the tree will begin to stop producing chlorophyll for the process of photosynthesis.

While deciduous trees’ roots, branches and twigs can endure winter’s wrath, their tender leaves cannot, so the leaves must be sealed off and shed before the cold comes in full force.

A layer of cells, known as the abscission layer, will form at the base of the leaf stem and will start to seal off the flow of water and nutrients to the leaf. When the layer is completed, the leaf dies and falls from the tree.

Not all trees form the abscission layer at the same rate, which is why some species like ash will usually be the first to drop their leaves, while sycamore trees won’t drop theirs completely until midwinter.

If your customers are seeing early leaf drop, below are a handful of reasons that could be the possible cause behind this.

Environmental troubles

If your area is coming out of a hot, dry summer, remind customers not to be surprised to see leaves drop, as trees have had to shed foliage in order to preserve water. Sometimes a tree will have grown more leaves than it can truly support, so early leaf drop can help it conserve resources and thin out a crowded canopy.

While a lack of water makes sense as something that causes leaves to shrivel and die, an overabundance of water can cause leaf drop as well. Overwatered trees’ leaves can yellow and drop if a tree’s roots are being suffocated by water.

Trees can also lose their leaves early if they are not suited for that region’s hardiness zone.

Some of the other environmental factors that can affect the pattern of leaf loss include early or late frost, high winds and soil conditions. Trees in cities also have to deal with man-made stressors like air pollution and herbicides. Trees growing under street lights will have their cycle disturbed by the light.

Pests and diseases

Another cause behind early leaf drop can be pests or fungal leaf diseases. Fungal infections tend to appear after a wet growing season.

Oak wilt is a common cause for leaf drop in oaks. It will first turn leaves yellow, then brown before dropping from the top of the canopy. Tar leaf spot is a disease that can be found in maple trees. Leaves will turn yellow and then black before dropping off. Anthracnose is another tree fungus that can cause ash trees to lose their leaves early.

The fungi that cause anthracnose and tar spot can overwinter on infected leaves that fall and spread the next spring via spores carried by the wind.

The best management strategy is to rake and destroy the infected leaves in the fall.

Insects such as mites, scales, whiteflies and aphids can cause early defoliation. The size of the tree will determine how practical it is to treat with pesticides.

Old leaves

Like the cells in your body, leaves have a limited lifespan and are replaced. If the leaves that are dropping early are older leaves on the inside or lower parts of the tree, this is normal and nothing for your customer to worry about.

If the tree is dropping the newest leaves at the tips of the tree, there could be a sign that the tree is in serious trouble. Contact a certified arborist to inspect the tree if you do not have one in-house.

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