October 1, 2016

Fall cleanup should include removing all dropped leaves and stems from around garden plants. It is possible that this debris could harbor various fungal species that may cause powdery mildew or other diseases come next season. It is best not to compost this debris, as the pathogens on any infected tissue will not be killed, as most compost piles will not reach a high enough temperature. Should any plants appear to have diseased stems or leaves, these should be cut off at this time and discarded into trash bags for removal from the property.

Most plants under normal circumstances are not prone to disease, especially non-woody varieties. However, should any pruning be required for infected branches or stems, cuts should be made in dry weather at least a foot below tissue showing symptoms. All pruning tools should be cleaned between cuts to prevent spreading any pathogens. To sanitize tools, use a bleach solution (5%), an alcohol spray or Lysol disinfectant. Disinfecting your tools is a good idea in any case throughout the summer and it should be done at the very end of the season. I buy a box of individual alcohol “wipes” sealed in plastic in the spring from any local pharmacy and keep them on hand with my tools for use throughout the year.

As the month of October passes, the change in colors of foliage becomes more and more apparent. With many of the larger oaks and maples these colors can be most dramatic, especially here in the Northeast. Normally, in our gardens, the change in leaf color is a bit more subtle but still lovely and dramatic.

These color changes result from changes in the leaf pigments, natural substances produced by the leaf cells. The most common is the green pigment, chlorophyll. The chlorophyll absorbs red and blue light from the sun so that the light reflected from the leaves looks green.

Another pigment in the leaves of most plants is carotene. Carotene absorbs blue-green and blue light. The light reflected from carotene appears yellow. During the growing season, the carotene pigment absorbs light and transfers it to the chlorophyll to assist in the process of photosynthesis. Chlorophyll and carotene are in leaf cells all through the summer. The chlorophyll covers the carotene so that summer leaves appear green, not yellow or orange. The carotene persists in leaves, even when the chlorophyll has disappeared. Once the chlorophyll disappears from the leaf, the remaining carotene causes the leaf to appear yellow, orange or brown.

Another group of pigments occurring in leaves of some plants are anthocyanins. These compounds are only produced in the fall. Anthocyanins absorb blue, blue-green and green light so the light reflected by the leaves appears red. Anthocyanins exist in cell sap and are sensitive to the sap’s pH: bright red for acidic and purple for less acidic. They are formed by a reaction with sugars and certain proteins in the sap along with sunlight.

Shorter days and cooler nights trigger changes in plants. One of these changes is the growth of a membrane between branches and leaf stems that slows down the summer nutrient flow, preventing chlorophyll to be produced so that the green coloring fades. Low temperatures also destroy chlorophyll and promote the formation of anthocyanins. These anthocynanins are also enhanced by bright sunshine and dry weather. Thus, bright reds are produced in dry, sunny days followed by cool, dry nights. Not all plants can make anthocyanins. Note that red leaves occurring early in the season could be a sign of stress on the plant and not normal fall activity.

As the days become shorter, sunlight diminishes and the temperature slowly drops. Plants respond to the decreasing sunlight by producing less chlorophyll. Once the chlorophyll is no longer produced, the carotene that is also in the leaves now shows through.

The rainfall during the year will also affect the autumn leaf coloring. A dry summer can delay autumn colors and a wet fall will lower the intensity of the colors. The best combination for bright autumn colors is a warm, wet spring; a summer that’s not too hot or dry and a warm, sunny fall with cool (not freezing) nights.

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