Norway maples (top leaves) are often planted as street trees. Other maple species, not all of which are street-suited, include bigleaf (middle), red (bottom left) and silver (bottom right).




Photograph by Shari Kaplan







Leaf loss and discoloration are simply signs of autumn

By Tony Tomeo

At about this time every year, I begin to get telephone calls from garden enthusiasts concerned about seemingly unhealthy trees that are dropping leaves very quickly. They just cannot understand what they are doing wrong! They want to know why trees that have been installed during the past year are suddenly becoming chlorotic and defoliating. Many even report very unusual "chlorosis" in shades of orange, red or even purple!

As much as I would like to inspect the trees, compose reports and charge my typical consultation fee, I ask a few questions instead, such as: "What is the species of the tree?" "How long have you been acquainted with the tree?" "Does the defoliation coincide with cooler weather?" Almost always, the tree is a deciduous species that is a new acquaintance and which started experiencing problems as the seasons began to change. (People may also become newly acquainted with established trees when they move into a new home.)

One of the primary horticultural issues that garden enthusiasts must learn about is the difference between evergreen and deciduous. "Evergreen" describes more than conifers, which produce cones. It designates any species that holds foliage through cold winter weather. Although some "deciduous" species hold abscised, necrotic foliage through winter until it is replaced by new growth in spring, most defoliate in autumn or winter.

This is very important when selecting tree species for particular functions. For example, deciduous trees that might seem to be a good screen during the summer would be ineffective during the winter. Likewise, evergreen shade trees that are very functional during the summer will shade adjacent buildings from radiant solar warmth in winter. I should get my mother to write about this--she is from Pennsylvania and knows the meaning of "winter."

As a native Californian, I thoroughly enjoy evergreen species. Our many types of palm trees can be enjoyed in any season. The many deciduous species, however, can also be enjoyed as they change with the seasons. Aside from being the most functional of shade trees near residences, deciduous trees may exhibit colors comparable to those of species grown for floral display in spring.

Many species of maple, birch, ash, pear, poplar, cherry and oak, as well as beech, ginkgo, honeylocust, sweetgum, tulip tree and pistache are among those that produce the most impressive of autumn foliar color. Of course, they all have distinct personalities and specialize in their own range of colors. For example, red oak and red maple excel at red; ginkgo and poplar produce excellent yellow; but sweetgum and pistache color as they please.

Deciduous trees are generally cleaner than evergreen trees because they defoliate rapidly in one season rather than dropping their foliage in small quantities throughout the year. Some deciduous trees, however, will hold their foliage into the winter, or even into spring until it is replaced by new foliage. For example, the pin oak will turn a simple brown, feigning death, until new growth appears in spring and the brown foliage completes abscission and falls from the tree. Likewise, the valley oak will turn brown and may drop foliage rapidly during unusually cold winters, but may otherwise drop foliage slowly through mild winters.

Tree of the Week: Norway maple

This species earned a bad reputation a few decades after it was one of the most popular street trees to service the vast urban sprawl of the late 1950s in the Santa Clara Valley. Back then, the Norway maple, Acer platanoides, was grown on its own roots which can eventually become greedy and invasive. More contemporary cultivars, however, are now grafted onto more complaisant rootstocks.

Although they can become quite large in the northwest, they rarely get taller or broader than 40 feet here. The somewhat densely foliated canopy is composed of five-lobed leaves approximately 5 inches wide. Foliage turns golden yellow in autumn. Some of the many cultivars available exhibit summer color as well, ranging from dark purplish to bronze. "Schwedleri," which has given the species the bad color reputation, has since been replaced by more intensely colored and better formed cultivars.

Because Norway maples are still not as common as some of the more trendy species, they are not available at all nurseries and may need to be specially ordered.

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Horticulturist Tony Tomeo can be contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.

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