Yard and Garden: Autumn care of evergreen trees

AMES — We expect oaks, maples and sycamores to change colors and shed leaves each autumn, but many are surprised and concerned when evergreens begin to shed needles. Most conifers stay green all year, but they do drop needles in the fall — just not all of them. Iowa State University Extension and Outreach horticulturists answer questions about evergreens, shedding light on what's natural and what needs attention. To have additional questions answered, contact Hortline at 515-294-3108 or hortline@iastate.edu.

In recent days, some of the needles on my white pines have turned yellow and begun dropping to the ground. Is this a problem?

The yellowing and needle loss 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.

There are several bands of holes around the trunk of my pine tree. Is the tree infested with borers?

The holes were likely created by sapsuckers. Sapsuckers, members of the woodpecker family, damage trees by drilling holes in the trunk or large branches. Sapsucker damage is very distinctive. They drill uniform, quarter-inch holes in distinct rows. (In contrast, the holes created by insects are random.) Sap that flows from these wounds is eaten by the sapsuckers. They also feed on insects, such as ants, beetles and wasps, which are attracted to the sap. The damage caused by sapsuckers is usually not serious.

Sapsuckers can destroy trees if they drill several rows of holes around the trunk within a small area. The bands of holes effectively girdle the tree trunk. To discourage additional damage to trees, home gardeners can wrap a piece of burlap around the damaged area. Another option would be to spread a sticky substance, such as Tanglefoot, around the affected area. Trees most commonly attacked by sapsuckers include apple, crabapple, sugar maple, mountain ash, birch and pine.

My Scotch pine suddenly turned brown and died this past summer. Why?

Pine wilt is likely responsible for the death of the Scotch pine. Infected trees turn brown and die within a few months. Pine wilt mainly attacks Scotch pines. However, it may also infect Austrian, red and other non-native pines.

Pine wilt is caused by the pinewood nematode, a microscopic worm. The nematodes clog the water-conducting vessels of the pine tree, causing the tree to wilt and die. It is spread from tree to tree by pine sawyer beetles. Infected, dying trees are often attacked by secondary bark beetles, which leave small holes in the trunk and carry a fungus that causes blue staining of the wood, often visible as blue radial wedges when the trunk is cut in cross-section.

Pine wilt cannot be controlled with chemical treatments. Dead trees should be removed during the winter months when pine sawyer beetles are inactive. The dead trees should be promptly chipped, burned, or buried to reduce the risk of the beetles spreading the disease to healthy trees.