We are having a spectacular fall. Just enough frost to turn the leaves. No wind or rain to knock them off yet and bright sunny days to enjoy all the color. But what makes the trees turn color?
After the autumnal equinox, light levels have dropped so much that the trees can no longer carry on efficient photosynthesis. Cooler temperatures and a couple of good frosts slow the tree’s metabolism even further, triggering chemical changes that pull sugars stored in the leaves down into the roots for winter storage. With the removal of the sugars, green chlorophyll is no longer needed by the tree, and it breaks down exposing the underlying yellow, red and orange pigments that become the beautiful fall colors we get to enjoy.
At the same time, the stems holding the leaves receive a chemical signal to harden off a layer of cells where the leaf attaches to the tree. This layer seals off the tree, and the leaf falls off. It will take another month or so for the tree to finish pulling sugars and sap from the tree’s cells down into the roots at which the tree will be fully dormant. This protects the tree cells from breaking when it is below freezing.
In the spring the process is reversed. As the weather warms, the sap and sugars rise into the tree in preparation for bud break and new spring leaves. A good example of this is the harvesting of maple sap in the Northeast that is then boiled down to make maple syrup. The sugar harvesters capture this rising flow of sap and slowly boil out the excess water to concentrate the sugar.
The warm, dry fall has also brought out the annual clouds of blue ash aphids. Millions of these tiny aphids are moving between their summer host plant and their winter host. These tiny insects make it hard to walk outdoors in the midafternoon this time of year without either getting them in your eyes or inhaling a few. They go by several different names including conifer root aphid, blue ash aphid, Oregon ash aphid or smoky-winged ash aphid.
During the summer the aphids feed on sap pulled from the roots of fir trees. As the first frosts come in the fall, the aphids begin moving from the fir trees to ash trees, their winter host. The aphids mate and the females lay eggs in bark crevices. In the spring, the eggs hatch and the aphids move back to the fir trees. Their flight to the ash trees takes two to three weeks each fall so they will be gone shortly.
Like most aphids, heavy infestations of these bugs can cause damage to the fir trees they inhabit in the summer. Yellowing needles are usually the most common sign of damage. Fortunately, such damage is not very common. If you need to control them, the best way is to apply dormant oil to the ash bark in late March just before their spring hatch.
Pat Munts has gardened in the Spokane Valley for over 40 years. She is co-author of “Northwest Gardener’s Handbook” with Susan Mulvihill. She can be reached at pat@ inlandnwgardening.com.
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