Fall leaf color

Horticulture Hints

Bright, sunny fall days and cooler night temperatures help trees to develop their glorious autumn leaf colors. But, if future days of fall arrive with many gray, overcast days, the leaf color in trees will not develop as well.

Cooler weather of fall brings out carotenoid pigments, the yellow and orange colors which are hidden under the green layer of chlorophyll in tree leaves. These color pigments, although present all season, do not appear until the green chlorophyll that is covering them breaks down in the fall.

Although hidden throughout the growing season, hardwood trees contain many different color pigments. The green pigment, chlorophyll, found in the top layer of the leaf is most plentiful and is apparent throughout the growing season. Chlorophyll is responsible for photosynthesis, the process which produces food energy for all plants, but disappears in the fall.

In the autumn, leaf sugars also create a pigment called anthocyanin, which produces red, pink and purple leaf colors. As trees can produce different amounts of sugar in individual leaves, the amount of anthocyanin in a leaf causes different colors to appear on the tree. Not all trees species have the anthocyanin gene, and thus, some trees do not produce color pigments and have leaves that turn brown in the fall. This brown color is due to the presence of tannins, produced as the waste products found in cell walls after green chlorophyll color is no longer produced.

Did you know? Emerald ash borer still looms as a threat to northern Iowa ash trees, and one should consider planting a replacement tree for any existing ash trees. While chemical treatments are available to help prevent emerald ash borer attack, costs of these treatment injections can prove prohibitive. For further information, see the ISU publication: “Emerald Ash Borer Management Options” at https://store.extension.iastate.edu/product/13114 for recommendations to homeowners within the 15 mile radius of a confirmed EAB presence.

Are your ash trees considered at risk? See the map of reported emerald ash borer infestations in Iowa at: https://hortnews.extension.iastate.edu/2017/06/emerald-ash-borer-confirmed-5-more-iowa-counties to download.

Gardening questions? Contact McCormick at yvonne@iastate.edu for information or advice.