Yard and Garden: Fall Trees

AMES, Iowa — In early autumn, in response to the shortening days and declining intensity of sunlight, tree leaves become vivid reds, oranges, golds and browns — signaling their oncoming fall to the ground. Horticulturalists with Iowa State University Extension and Outreach explain it’s more than just leaves that fall from trees during autumn. To have additional plant and garden questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

The inner growth in my arborvitae is turning brown. Is this a problem?

The browning of the inner foliage is probably due to seasonal needle drop. It’s normal for evergreens (pine, spruce, fir, juniper, arborvitae, etc.) to shed their oldest (innermost) needles in fall. The innermost needles gradually turn yellow or brown and drop to the ground. Environmental stresses, such as drought, can cause greater than normal loss of needles. The needles on the outer growth remain green on healthy evergreens.

This fall some of the needles on my white pines have turned yellow and begun dropping to the ground. Is this a problem?

The loss of needles is probably due to seasonal needle drop. Deciduous trees, such as maple and ash, drop all of their leaves in fall. Though it largely goes unnoticed, evergreens also lose a portion of their foliage (needles) on a yearly basis. Seasonal needle drop on most evergreens occurs in fall. Needle loss is most noticeable on white pines. As much as one-half of the needles on white pines may drop in early fall. Seasonal needle loss is less noticeable on spruces, firs and other pines as they retain a higher percentage of their needles. Seasonal needle drop is uniformly distributed throughout the inner part of the evergreen. It is the oldest needles which are shed. The needles turn uniformly yellow or brown and drop to the ground.

Why does fall leaf color on trees vary from year to year?

The weather in early fall largely determines the intensity and duration of leaf color in those tree species capable of producing good fall color. Best fall color typically develops when days in early fall are sunny and mild and nights are cool (but remain above freezing). Long periods of cloudy, rainy weather or a hard freeze in early fall will mute fall colors.

My oak tree produced just a few acorns this year. Why?

It’s common for the acorn crop on oak trees to vary from year to year. Most oak species produce a good crop of acorns once every two or three years.  However, the white oak tends to produce a good acorn crop once every four to six years.

Weather and other factors can affect flowering and fruiting. For example, freezing temperatures in spring (when trees are flowering) can damage or destroy the flowers, drastically reducing the fruit crop.

There are gray-green patches on the trunk of my tree. What are they? Are they harming the tree?

The gray-green patches are probably lichens. Lichens are unusual organisms. They consist of two unrelated organisms, an alga and a fungus. These two components exist together and behave as a single organism. The agla provides food via photosynthesis. The fungus obtains water and minerals for itself and the alga. 

Lichens are common on trees because the bark provides a suitable place to gather sunlight and grow. They grow especially well on dead branches because they receive more sunlight. In addition to growing on the trunks and branches of trees, lichens can be found on exposed soil surfaces, rocks, wooden fence posts, shingles, gravestones, stone walls and other sunny surfaces. Lichens may be flat, leafy, or branched and hair-like. The lichens on trees are often gray-green. Other species may be orange, yellow, slate blue or black. 

Lichens are fascinating, unique organisms. They do not harm trees.