University of Maryland Extension

May Tree and Shrub Tips

carolina silverbell tree in bloom
Carolina silverbell, Halesia carolina. Photo: Chris Evans, Illinois Wildlife Action Plan,

(More tips from HGIC)

  • When selecting a new shade tree for your landscape select one with a single, straight trunk.  Contrary to popular belief it is unnecessary to prune or top a newly planted tree. Obviously damaged branches, however, should be removed. Common planting mistakes include planting in compacted or poorly drained soil, planting too deep and buying damaged trees with poor root systems. Good choices would include white oak, male ginkgo, dawn redwood, bald cypress, American holly, American hornbeam, Carolina silverbell (see photo above), fringe tree, Katsura tree, Kentucky coffee tree, Korean fir, white fir, paperbark maple, Persian parrotia, Serbian spruce.  (PDF HG 24)
  • If your azaleas, rhododendrons and other spring flowering shrubs are growing too large you can prune them after they bloom.
  • Mulches should be applied only 2-3 inches deep and kept away from tree trunks. Mature trees do not benefit from being mulched except that the mulch ring keeps lawn mowers and string trimmers from damaging your trees.
  • At this time in the spring the older leaves of holly and magnolia will begin to yellow and drop. This is a natural process of regeneration and does not indicate a problem with the trees.
  • The three major insect and mite pests of boxwood can be observed now. Boxwood mites produce small white spots, known as stippling, on leaves. The boxwood psyllid causes new leaves to cup (photo) and the boxwood leaf miner (photo) produces blister and blotch mines on boxwood leaves. Ultra-fine horticultural oil can be applied to control psyllids and mites. Leaf miner damage observed now occurred last year. Consider applying a systemic insecticide this summer if you observe numerous fresh mines. (PDF HG 52)
  • Lace bug feeding may be seen on rhododendrons, azaleas, andromeda, and mountain laurel. You’ll notice small white or yellow spots on the upper sides of leaves and small black fecal spots will appear on the undersides. (photo) Lace bugs are more of a problem on stressed plants on exposed hot sunny sites. (PDF HG 95)
  • Cankerworms look like inchworms and are yellow to gray in color and are feeding now on the foliage of a wide range of shade and forest trees. The damage first appears as shot holes in leaves. They feed between leaf veins causing foliage to have a tattered appearance. No treatment is necessary for established trees.   
  • Cottony camellia scale, also known as cottony taxus scale, (photo) can be found now on yews, camellias, holly, euonymous, hydrangea and beautyberry. You’ll notice white waxy egg masses laid on leaf undersides. You may also notice honeydew and black sooty mold that results from scale feeding. Immatures will hatch in June. It’s best to spray in June with insecticidal soap or horticultural oil.
  • Gypsy moth caterpillars are feeding at this time.(PDF HG 44)
  • Hemlock woolly adelgids (photo) are aphid-like sucking pests that appear as white, waxy masses on the underside of needles. Heavy infestations can debilitate trees, particularly when they are stressed. Another  One control option is to consider a systemic insecticide treatment. (PDF HG 2
  • You may notice large numbers of 1-inch long sawfly larvae (photo) feeding on pine trees. Handpick to control small infestations.
  • Eastern tent caterpillars are actively feeding. Tents can be removed with a long stick. Larvae are too large to be effectively controlled with Bt Mature Eastern tent caterpillars are still moving out of nests searching for a place to pupate.  (PDF HG 21)
  • Honey locust plant bug and honey locust leafhopper frequently feed on honey locust leaves, causing yellowing, deformity, or stunting. If infestations are severe, apply a summer rate application of horticultural oil.
  • Locust leaf miner adults are beginning to feed on black locusts. They feed between the leaf veins, causing leaves to look skeletonized. Although unsightly, controls are not necessary. This pest does not pose a threat to black locust trees. (photo)
  • Maintain good boxwood health by thinning out interior branches to promote air circulation.
  • Dense boxwoods may develop disease problems such as volutella canker, and macrophoma leaf spot.  Improved air circulation will help prevent these diseases.  Read PDF HG 52 for more information on these diseases.  Also be aware of the new boxwood blight disease that has been found in Maryland.  
  • Dogwood anthracnose is a serious dogwood disease. Early symptoms (photo) begin in mid- to late-May as leaf spots with tan or purple borders. These spots may enlarge in wet weather and kill entire leaves. The disease can then spread to twigs and branches and result in dieback of large limbs and even entire trees. Both cultural and chemical control strategies are necessary. Prune out all dead or dying twigs and limbs during dry weather. All water sprouts or suckers on trunks and branches should also be removed at this time. In the fall, leaves should be raked up and removed. Remove any dead leaves still attached to the branches. In areas with severe disease, consider planting resistant dogwood species such as the flowering dogwood cultivar ‘Appalachian Spring’, and cultivars of Cornus kousa. 
  • The blooms of many plants, including dogwood and peony, can be infected with botrytis blight, also known as gray mold. Flower petals will appear spotted and water-soaked and then wither and turn brown. Azaleas suffer a similar petal blight disease. Simply remove damaged blooms. 
  • A common fungal disease known as anthracnose occurs on oaks, maples, sycamores, ash, and beeches. Irregular shaped, brown spots appear on the upper leaf surface. Leaves may wither, die and drop as the disease spreads. Healthy, mature trees can tolerate these symptoms and will put out new foliage. Newly planted  Young trees that are badly infected can be sprayed next spring, with a labeled fungicide. Sprays will not help once leaf spotting symptoms are observed. Rake up and dispose of fallen, infected leaves to reduce the incidence of the disease next year. 
  • Ovulinia blight causes small water-soaked spots on azalea and rhododendron blooms that enlarge, causing the petals to turn slimy and brown. The petals remain attached to the plant. This fungal disease is more severe during cool, wet weather. 
  • Exobasidium gall can be seen on azaleas and rhododendrons. Leaves develop puffy swellings that turn white in color. Pick off infected leaves.



Maintained by the IET Department of the College of Agriculture and Natural Resources. © 2018. Web Accessibility