University of Maryland Extension

September Tree & Shrub Tips

fall color on a sweetgum

(More tips from HGIC)

  • Poison ivy leaves will begin to turn red this month. Don’t be fooled by their color change, the leaves are still very irritating. Do not handle or shred the leaves and do not burn the vines. To control poison ivy, spray with a systemic herbicide like glyphosate or triclopyr. The herbicide moves from the foliage down to the roots where it disrupts plant growth. Even when dead, handle the vines with care using gloves as the irritant oils can still cause a rash. Throw the gloves away after handling the vines.
  • Each year we see early fall coloration of many shade trees and the inner needle drop of white pines. You may notice older leaves dropping from rhododendrons and other evergreen shrubs. This is normal for this time of year.
  • Trees and shrubs should only be pruned at this time if they have dead, damaged or hazardous branches. Wait until after all the leaves have dropped for all other corrective and cosmetic pruning. To determine if a limb is still alive look for live green buds and scrape the bark and look for green tissue. When in doubt, or when the pruning job is too dangerous, consider hiring a certified arborist to evaluate your tree. Read HG 84.

Insect

  • You may have noticed the browning of black locust tree leaves caused by locust leafminer feeding. This is a perennial pest that does not kill trees. The damage you see now was caused earlier in the summer. Locust trees put out new growth throughout the growing season.
  • The large tents of the fall webworm may be seen at the ends of tree branches. The caterpillars are done feeding but the large nests on the ends of branches are still visible. It is unsightly but causes little damage. They can be removed with a stick or pruned out. Many other kinds of caterpillars are feeding on shade trees. No controls are necessary unless severe defoliation is observed.
  • Different kinds of wooly aphids have been observed on crabapple, beech and maple. They look like white cottony masses stuck to the stems. This insect can be controlled with insecticidal soap if the infestation is severe.
  • Scale insects can infect many woody plants. You may notice leaves coated with black sooty mold, a fungus that grows on excess plant sap (honeydew) excreted by the scales. Control them with a dormant oil spray in the fall. Cottony Taxus scale can be found now on yews, camellias, holly, euonymus, hydrangea, and beautyberry.
  • Cottony camellia scale can be found now on camellias, holly, euonymus, hydrangea, and beautyberry. You may also notice the honeydew and black sooty mold that results from the scales feeding. Dormant sprays of horticultural oil, applied in late fall, will control this pest.
  • You may also notice yellow jackets congregating around trees that have large quantities of honeydew produced by scale or aphid infestations. European hornets are large yellow and brown insects that you may see stripping bark from trees or shrubs to build nests. They are attracted to lights at night.
  • Lots of different leaf and stem galls may be observed on shade trees. They appear in many different colors and shapes and are mostly harmless to affected trees. Galls are produced when small wasps, midges, and mites feed on leaf tissue. Chemicals produced by them cause the swelling and deformation of leaf tissue. They are generally harmless and control is not necessary.
  • Bagworm infestations are heavy at this time on evergreens, especially spruces. The caterpillars inside the bags will soon pupate. After pupating into adults the flightless females will lay eggs inside the brown bags hanging from trees. The males can fly and look for females. Remove the bags where possible to prevent the overwintering eggs from hatching in the spring. If they cannot be removed by hand and are numerous, consider applying Bt next year between mid-June to mid-July. It is too late for biological control this year. Only apply a registered insecticide if they are still feeding.
  • Spruce spider mites will become active again on evergreen trees as the weather cools down. Monitor for this pest by tapping branches while holding a piece of white paper underneath. Look for moving specks, these will be the tiny mites. They can be controlled with ultra-fine horticultural oil. Spraying oil on Colorado Blue spruce will make them turn green. This color loss is harmless and the new growth next year will be the normal blue.

Disease

  • Powdery mildew is the common name for the disease and symptoms caused by a closely related group of fungi. It affects most shade trees such as oak, sycamore and tulip poplar as well as, crepe myrtle, lilac, euonymus, and many other popular plants. These fungi grow on the upper and lower leaf surfaces, young stems, and shoot tips of plants. Affected plants turn white or light blue-gray. The optimum conditions for powdery mildew development are warm days followed by cool, humid nights. Dry daytime weather allows spores to spread to other plants on air currents. Next year, powdery mildew can be prevented or reduced next year by spraying a summer rate of horticultural oil on the foliage before or as soon as mildew appears.
  • Rose diseases will continue to be a problem through most of the fall. Continue spraying black spot susceptible roses with a labeled fungicide. Powdery mildew is another common disease that is active through fall. It affects leaves but will also attack flower buds which can cause petal distortion next spring.
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