The Chemistry of Fall

Describes the chemistry changes plants go through to prepare for winter.

Rhus aromatica 'Gro-Low'
Item # 82550 		Rhus aromatica 'Gro-Low'
Gro-Low Sumac

each $8.29
3 to 6 $7.99
7 or more $7.79

Item # 80404 		Prunus besseyi 'Select Spreader'
Sand Cherry

each $8.29
3 to 6 plants $7.99
7 or more $7.79
  • Topic: Fall Gardening
  • Author: Cindy Bellinger
  • Keywords: fall, winter, Fall Gardening, gardens, leaves,
  • Date: September 2004

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Fall is in the air, and it’s already touching a few leaves. Oh, so slowly, tufts of cottonwoods are turning yellow and the leaves of Virginia Creeper are turning red. But how does this work?

Put simply, fall colors are a result of diminishing daylight and declining night temperatures. Behind this, though, is a complex set of chemical reactions.

We all know the green of leaves is chlorophyll. But as things go, chlorophyll is not very stable. Bright sunlight causes it to decompose, and to maintain their green-ness, plants constantly need to produce chlorophyll.

One of the primary stimulants that triggers chlorophyll production is warm temperatures. When nights begin cooling, that’s when the green begins to disappear. But another layer of chemistry is also going on.

Think of it this way: Under the chlorophyll are other pigments that are much sturdier elements. Two are carotene and xanthophyll; another is anthocyanin.

If leaves contain carotene and xanthophyll when the chlorophyll disappears, they’ll appear yellow or orange as with the cottonwoods and aspens. If leaves contain anthocyanin when chlorophyll disappears, they will appear pink, red or purple as with maples, sumacs and Virginia Creeper.

It’s also the anthocyanin pigments that are responsible for red apple skins and purple grapes. Another pigment is tannin, which shows up as brown as with some oaks.

When summer begins to fade, the cooling nights trigger a growth of corky membrane between branches and the leaf stems. This interferes with the flow of nutrients to the leaves, which in turn slows the production of chlorophyll. This is when the green color begins to fade.

And if temperatures stay above freezing without any rain, the pigments underneath the chlorophyll really get an opportunity to develop their most intense colors. Weather has a very dramatic affect on fall colors causing the leaf display to vary from year-to-year. Ideally, if fall weather is such that dry, sunny days are followed by cool, dry nights, the kaleidoscope of bright fall colors will keep developing to their greatest intensity.