When my guest Don Burnett spoke the words “abscission layer” on my radio show ‘Art Knapp’s Garden Talk’ last Saturday morning the look on my co-host Terry’s face was priceless. We had been discussing how few leaves had fallen off trees and shrubs in mid-November because of the warm autumn we’d been experiencing and how the sudden arrival of Arctic air had freeze-dried the foliage. Don’s comment that the abscission layer had now formed and the leaves would now be quickly falling baffled Terry, and perhaps a few of our listeners as well!
The process of plants shutting down to prepare for winter is a fascinating one. There is a constant flow of hormones and proteins running through roots, stems and leaves during the growing season; turning sunshine into food and keeping the plant growing.
As the leaves begin to grow in spring photosynthesis increases and continues to do so until the leaves reach maximum size. Shortly after they reach their maximum the growth slows down and a process known as senescence begins.
I’m convinced that we humans go through the same type of change every fall when light levels become reduced and it gets colder. We slow down, we know winter is approaching and we naturally cocoon in our living rooms at night. Senescence for me this autumn consisted of ordering two more tiers on my cable so I could watch more hockey games!
For plants the progression is somewhat different. Respiration rates in the leaves drop, leaf protein levels fall sharply and carbohydrates begin to break down. I have heard plantsmen with far more experience than I talk about “plants taking their starch to the roots.” The most visible sign of senescence is when the leaves turn colour on trees and shrubs as the green pigment, chlorophyll, breaks down and the orange and yellow pigments take over, giving us wonderful fall colour.
The end result of all this slowing down and decay is abscission. An abscission layer forms at the end of the stem; cells inside the leaf expand as they break down and finally the pressure is so great within the cells that the leaf detaches and falls to the ground.
Senescence and abscission are two vital processes for plants. Without senescence, that time to prepare for winter in our cold climate, the frost would kill the leaves before the materials inside them could be salvaged for the rest of the plant to use. Without abscission to get rid of the old leaves they would still be there next spring to shade the new growth.
The killing frosts that arrived ten days ago caught many plants with leaves still attached. The fact that some of them are still attached has everything to do with the fact that the cold came so suddenly that the abscission layer hadn’t yet fully formed. But it’s there now. All it will take is a good stiff breeze or a warming trend to bring those leaves raining down.

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