Throughout the growing season leaves on plants contain all of the chemical compounds that give the normal summer leaf coloration. Unfortunately, due to photosynthesis, the very process that makes plants thrive through the absorption of solar energy, the human eye is only able to see chlorophyll, the chemical which plants use in this process, and which for the main part is green.

	Fiery reds epitomize autumn

As the plant begins to go dormant for the season (this usually starts around the beginning of August), the amount of chlorophyll production is reduced and the other colors become more prominent in the leaves of trees and shrubs. In a nutshell, the leaf color is a chemical reaction, not unlike that of what humans experience when we suffer from a chemical imbalance. Fortunately, trees and shrubs know how to self-medicate!

The Colors Of Chemistry

Why do some plants turn different colors, you ask? It is because of the other chemicals which are present in a given plant's foliage. The intensity of the pigment of color is directly related to the interaction between the different chemicals present. Aside from chlorophyll, there are carotenoids and flavonoids present in plants. Each provides some of the various nutrients which that particular plant requires. These are essentially sugars, and depending on their makeup they determine the autumn color.

Yellow is a common fall color

Carotenoids are the most common and display the ever recognizable yellows and oranges which so many Northerners see in the fall. Flavonoides are a little more complicated; some display a yellow color when present. Others known as anthocyanins display pigments of reds, blues and purples. Anthocyanins are only present in about 10% of North American trees - hence their relative rarity in the northern landscape.

Whether or not you will get the color you expect is a little more complicated than just having these chemicals present, and this is where you the gardener and soil health come into play. The interaction between the chemicals is dependent upon the degree of acidity in the soil, which affects which and how many of these chemicals are able to be stored in the leaves of trees.

The intensity of the coloration is directly related to the interaction of sugars and proteins and is highly sensitive to the pH levels. When there is a high level of acidity, the fall color is revealed as a bright red. When acidity levels are low, then purple is more prominent. This is why often times the leaves are internally red and purple only on the edges - the acidity does not always make it to the edges of the leaves.

	Purple tones are exaggerated in alkaline soils

Soil Matters

Now anthocyanins are not always present in the volume required in a specific tree species to guarantee a more vibrant color than yellow, so adding copious amount of an acidifier will not do anything. In regions with clay or alkaline soils, the primary plants which display good red/purple fall colour are Amur maples, flowering crabs and serviceberries, to mention a few. Other typically red-coloring trees and shrubs may turn a deep purple under these conditions.

But alas, some plants do not provide much color change at all and the leaves go directly from green to brown. This results from a number of reasons. In some instances it is due to an overabundance of chlorophyll contained in the leaves. This result in rapid cellular breakdown when things get cold, thus bypassing the whole chemical change process which gives us the beautiful autumn displays. This is often the case with trees and shrubs that are not particularly hardy in the North. In other cases, such as with oak trees and walnuts, it is because there are tannins in the leaves which overpower the other chemicals.

So get out and enjoy the fall colors, now that you know a little more about what's going on beneath the surface!

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