Question
I have a sunset maple in my backyard that I planted about 7 years ago. The leaves on the tree have never turned the red color in fall, which was the specific reason that I planted this tree. In the fall, some leaves on the very end of the branches turn red, but that is all. Could it be related to a soil ph problem?

Answer
Anthocyanin pigments are responsible for the pink, red, and purple leaves of sugar and red maple, sassafras, sumac, white and scarlet oak, winged euonymus, and many other woody plants. They are formed in the cell sap inside the vacuole when sugars combine with complex compounds called anthocyanidins. The variety of pink to purple colors is due to many, slightly different compounds that can be formed. Their color is influenced by cell pH. These pigments usually are red in acid solution and may become purplish to blue if the pH of the cell sap is increased.

Anthocyanins are usually not present in leaves until they are produced during autumn coloration. A few trees, however, such as 'Crimson King' Norway maple and purple European beech have reddish leaves throughout the growing season due to anthocyanins in them. Not all trees have the genes required for production of anthocyanin pigments, and hence those trees only develop yellow and brown shades of autumn color.

Formation of Abscission Layer
Shorter days and cooler temperatures in fall initiate leaf senescence involving complex biochemical processes, which increase the enzymes in leaves that promote the breakdown of cells. One of the results is the collapse of cells in the living phloem to form an abscission layer at the base of leaf petioles where they attach to twigs. This is the point at which a leaf is eventually shed. The nonliving xylem tissues remain intact for awhile and still carry water to the leaves, but the phloem pathway for conduction of sugars out of leaves is severed. Sugars, derived from continued photosynthesis as well as from the conversion of stored starch to soluble sugars in the leaves, are trapped and are available for anthocyanin production. Trees of the same species growing together often differ in color. The colors vary because of differences in amounts of soluble sugars in the leaves for anthocyanin production. Trees exposed to the sun may continue photosynthesis and turn red while others in the shade may be yellow. A single tree may even have branches with different colored leaves due to one leaf shading another.
It could be the combination of the weather, sunlight and the age of the tree. As the tree gets older it will show the color better. I would fertilize the tree next spring to encourage foliage growth. Use 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered under the tree and water in good.