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Walter Arp cleans up fallen leaves.

While some leaves have already begun to trickle down from the sky, the annual flood of leaves is almost upon us. With our hands about to blister from the raking, while removing the fallen leaves, many of us will have a bit of time to contemplate the following lessons that falling leaves can teach us.

1. There is a biologically proper place for a leaf to separate from a tree, and although it may cause a “leaf scar,” the injury heals quickly and the plant remains healthy. Leaves separate from twigs at the “abscission point,” an ideal place for the old leaf tissue to divide from the remaining bark tissue. While this seems obvious, leaves do not sometimes separate leaving part of the petula (stem), or part of the leaf, or tearing away part of the bark with it when it falls. No, every time, without fail, the leaf separates at the abscission point. (Incidentally, the easy way to tell if a leaf is simple or compound is whether a single leaf or several leaflets cleanly separate at the abscission point. When a leaflet is torn from the petula, it does not tear cleanly).

As I was thinking about the abscission point of leaves recently, I decided to look closer at twig attachment. As I looked closely at several dead twigs on living trees, I could see that there is something like an abscission point for twigs. (It is natural for trees to shed twigs and branches as well as leaves). The “abscission point” for twigs, I found, is right near the place for proper pruning. This place, filled with meristem cells, is located just outside the swollen area called the branch bark collar.

Lesson 1: Since leaves always separate from the twig at the same biologically significant point, limbs, branches and twigs should always be removed at the same biologically significant point.

2. There is a biologically proper time for a tree to drop its leaves. Healthy trees do not randomly drop their leaves throughout the growing season. Leaves represent an investment by a tree. Maximum return for the investment comes when the tree holds all of its leaves for all of the season. Trees handle their resources with great efficiency. The change in color comes when the green chlorophyll recedes back into the tree. Only after this move of conservation does the tree drop its leaves.

Lesson 2: Since a tree naturally keeps its leaves all season, when pruning a tree, it is best either to wait until the leaves have already dropped to prune or to prune in such a way as to preserve the maximum amount of leaves on the tree.

Trees naturally do their business the healthy way and at the appropriate time.

In part 2, we will look at two leaf-drop lessons that apply to disease.

Reach me at josarhuap@aol.com.

Joshua Arp is an ISA-certified municipal specialist, Clarks Summit’s municipal arborist and an operator of an organic lawn and landscape maintenance business.