Why Leaves Change Color

Why Leaves Change Color

A green leaf is green because of the presence of a group of pigments known as Chlorophylls. When they are abundant in the leaf's cells, as they are during the growing season, the chlorophylls' green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus the leaves of summer are characteristically green.

The chlorophylls have a vital function: they capture some of the sun's energy and utilize it in the manufacture of the plant's food- simple sugars which are produced from water and carbon dioxide. These sugars are the basis of the plant's nourishment- the sole source of the carbohydrates needed for growth and development.

In their food-manufacturing process, the chlorophylls themselves break down and thus are being continually "used up." During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.

But as autumn approaches, certain influences both inside and outside the plant cause the chlorophylls to be replaced at a slower rate than they are being used up. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. Then other pigments that have been present (along with the chlorophylls) in the cells all during the leaf's life begin to show through. These are the carotenoids; they give us colorations of yellow, brown, orange, and the many hues in between.

The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. These pigments are not present in the leaf throughout the growing season as are the carotenoids. They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences- both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of a certain chemical (phosphate) in the leaf is reduced.

During the summer growing season, phosphate is at a high level. It has a vital role in the breakdown of the sugars manufactured by chlorophyll.

But in the fall, phosphate, along with the other chemicals and nutrients, moves out of the leaf into the stem of the plant. When this happens, the sugar-breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light during this period, the greater the production of anthocyanins and the more brilliant the resulting color display we see. When the days of autumn are bright and cool, and the nights chilly but not freezing, the brightest colorations usually develop.

Anothocyanins temporarily color the edges of some of the very young leaves as they unfold from the buds in early spring. They also give the familiar color to such common fruits as cranberries, red apples, purple grapes, blueberries, cherries, strawberries, and plums.

In our autumn forests they show up vividly in the maples, oaks, sourwood, sweetgum, dogwood, tupelo, black gum, and persimmon. These same pigments often combine with the carotenoids' colors to give us the deeper orange, fiery reds, and bronzes typical of many hardwood species.

The carotenoids occur, along with the chlorophyll pigments, in tiny structures - called plastids - within the cells of leaves. Sometimes they are in such abundance in the leaf that they give a plant a yellow-green color, even during the summer; but usually we become aware of their presence for the first time in autumn, when the leaves begin to lose their chlorophyll.

Carotenoids are common in many living things, giving characteristic color to carrots, corn, canaries and daffodils, as well as egg yolks, rutabagas, buttercups and bananas. Their brilliant yellow and oranges tint the leaves of such hardwood species as hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder.

Shop Wisely

As many of you know, I love to save a few pennies here and there. One of the best ways to save money is to shop nursery and garden centers “End of Season Sales”. Most nurseries and garden centers do not want to hold plant material over the winter, so they begin to discount the price towards the end of September in hopes of moving the plants. Prices can be reduced anywhere from 20 to 50% off of retail. Now even though this sounds like a tremendous bargain, you need to keep a few things in mind. First and foremost, don’t go nuts buying plants without having the idea where they are going to be planted on your property. Remember that Garden Journal we talked about in spring? That is the perfect guide to bring with you when purchasing plants, and hopefully you have been diligent with keeping it up to date.
When you get to the garden center and start to look at the plants, look carefully and inspect the bark and leaves. Are the leaves yellow, wilted, curled or brown? These all can point to several different problems. Yellow, wilted or brown leaves could be caused from inadequate watering (which is a definite possibility, especially with balled and burlapped plants), diseases (wilts or anthracnose such as on dogwoods) or insects (look for blackish-brown dots on the undersides of leaves). When looking at the bark, make sure that it is securely wrapped around the plant and there are no missing areas of bark. Remember the active growing area of the stem is right below the bark of the tree, so if the bark is missing, the active area will dry up and not transport the nutrients the top of the plant needs. Also look for holes in the bark from burrowing insects such as borers. A few yellow leaves should not stop you from getting a good deal, just understand what the cause of the yellowing is and take appropriate action when you get the plant home.
One place where people do not check the plant, even in growing season, is the root zone. Check to see if the root ball is solid and firm if the plant is balled and burlapped. If it is loose, the broken soil may include a broken major root which will hinder the plant from adapting and getting set for the upcoming winter. One other thing to check is how many layers of burlap the ball has wrapped around it. I have purchased plants before at end of year sales that looked fairly good, but when I got it home and went to plant it I pulled the new top piece of burlap off to reveal an older, semi-rotted piece underneath. This tells you that the plant was held over last winter and they put a new piece of burlap over the old to dress it up. If the plant is in a container, don’t be scared to give it turn it upside down and slide the pot off to get a good look at the root system. If the plant has been around for a while, you will notice a lot of roots and little soil (aka the plant is root bound). When planting, just tease the roots to allow them to spread out into the soil and to stop the circular habit.
If the plants pass these tests, remember to add compost to the planting hole, a good soaking right after they are planted, and watering once a week if we hit a dry period. The good news is that Fall is for Planting, and the plant will have the needed time to adapt to the new location through fall, winter and the early part of spring before the hot weather returns next year and stresses the plant. You will see that many plants will do better when planted in fall than spring and you will save a few dollars as well.