Ever wonder why some leaves don’t drop? - East Idaho News

Ever wonder why some leaves don’t drop?

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All trees and shrubs lose at least some of their leaves every year. Deciduous trees and shrubs drop all their leaves in the fall … usually. Marcescent (withered but not falling off) leaves are common in some species, such as oaks.

Some may argue that there were plenty of leaves on the ground last fall because they had to rake them up. Others have noticed and asked why so many trees have not dropped their brown leaves. Sometimes the answer is because of insect damage or a disease such as fire blight in apples and pears. This year the answer is a little more complex.

Leaf abscission is a gradual process brought on by lengthening nights and cooling temperatures.

Dying tissues

As fall temperatures cool and nights get longer the chloroplast organelles die and the green pigment (chlorophyll) fades. The chlorophyll masks most underlying pigments and when it fades the yellows, oranges, reds, and purples are evident. Eventually, the cell nucleus dies. As the leaves die the tree scavenges nitrogen, carbon and other materials from the dying leaves to store in the stems over the winter. This slow death of the leaves is sometimes referred to as “programmed cell death” (PCD) and is necessary for deciduous plants to survive our winter climate.

Young peach tree with total leaf abscission. | Ron Patterson, EastIdahoNews.com

Hormone interaction

During this seasonal change phytohormones (plant hormones) stimulate a double layer of cells to form at the base of the petiole (leaf stem) that weaken its attachment to the stem (abscission layer). The weight of the dead leaf or a breeze will separate the leaf from the tree.

If there is a killing frost before the abscission layer forms the phytohormones don’t get to complete their work and the leaf stays attached. It will eventually separate, but maybe not until new growth begins in the spring.

Cause for concern?

If it was a quick frost that disrupted these natural plant processes then the only cause for concern is if the leaves gather too much snow, which may result in broken limbs. Sometimes a sudden, hard freeze in the fall may not show any damage until the next growing season. Young trunks may split, or vascular tissue in the stem is damaged. In this case, you just have to wait to see how the plant responds.

If your tree does not show symptoms or signs of disease or insect attack, such as cankers on the branches or holes in the bark, then relax until pruning time. The best time to prune most trees and shrubs is March.

Horticulture Educator Ron Patterson can be reached at (208) 529-1390.