University of Maryland Extension

June Tree and Shrub Tips

black spot on rose leaves
Black spot on roses

(More tips from HGIC)

  • If you planted trees or shrubs this spring, be sure to keep them well watered through dry weather this summer. Initially, thoroughly soak the root ball every few days then decrease to about once every 1-2 weeks depending on the weather. Keep watering into the fall. A 2-3 inch layer of mulch is helpful. Keep mulch away from the trunk or stem.
  • Roses are especially hard hit by the leaf disease called black spot.  Use labeled fungicides to manage black spot throughout the growing season.
  • Monitor plants that are prone to pest problems for new signs or symptoms of insect damage or disease. Be sure you have accurately identified the problem before taking action. Cultural and environmental factors cause at least half of all the observed plant problems in home landscapes. If an insect pest or disease is the culprit, always select the best management solution.
  • Healthy container and burlapped and balled trees can be safely planted throughout the summer as long as they are watered during dry periods. Thoroughly soak the root ball every few days until the roots become established.
  • Insects, both beneficial and harmful species are now in “full swing” this month. One of the smallest yet very destructive is the ambrosia beetle. It is a tiny beetle that can bore into heartwood and cause dieback even in young, healthy trees. The beetle’s activity and damage to trees is favored by drought. Small holes can be observed that spiral up the trunk. The beetles push out short gray-colored tubes of frass from these holes. Badly infested trees may have to be removed. There is no chemical control once the tree shows signs of dieback from an infestation. Prevent this problem by keeping trees, especially young ones, well watered during droughty periods.
  • Aphids are actively feeding on birch, elm, poplar, and other trees. Aphids suck the plant sap from a wide variety of plants. Native populations of ladybird beetles, syrphid flies, green lacewings and wasp parasites build up quickly to keep aphids in check. Aphids excrete plant sap, known as honeydew when they feed. This leads to the growth of a black fungus on leaves, known as sooty mold.
  • Bagworm larvae are hatching out this month and constructing new bags. Look for the little bags walking around on evergreen trees and shrubs and be prepared to spray infested trees with the microbial insecticide, Bt between now and mid-July. An application of Bt is recommended for evergreen shrubs or trees that were damaged by this pest last year. Spray after you’ve observed the small larvae. (PDF HG 32 Bagworms and Their Control)
  • A variety of galls may be observed at this time on the leaves and twigs of oak, maple, hickory, and other trees. Galls are tumor-like growths of woody and leaf tissue. These galls pose no threat to the tree. (PDF HG 31 Insect and Mite Galls on Plants)
  • The hemlock woolly adelgid is an aphid-like sucking pest that can debilitate hemlock trees, particularly when they are stressed. Adelgid crawlers are feeding now and can be sprayed with ultra-fine horticultural oil throughout the summer or use a registered insecticide drench around the base of the tree in March. Check the label for rates and instructions and if registered for use in your area. (PDF HG 2 Hemlock Woolly Adelgid)
  • Honey locust plant bug and honey locust leafhopper frequently are observed feeding on honey locust leaves, causing yellowing, deformity, or stunting.
  • Lace bug feeding can be observed on azaleas, Japanese Andromeda, hawthorn, serviceberry, oak, and sycamore throughout the summer. They feed from the underside of the leaves, sucking the plant sap causing a white stippling to appear on the upper leaf surface. Monitor the feeding damage. Insecticides are only necessary when plants are heavily infested. 
  • Spider mites feed on a wide variety of plants.  Damage is greater during hot, dry weather.The mites feed on the leaf undersides and can be seen with a magnifying glass. The upper leaf surface will look yellow or scorched. Upon close examination you will see fine, yellow dots, known as stipples.
  • You may notice small, white bits of fluff floating down from the sky. These may be woolly apple aphids (photo) or woolly alder aphids (photo). They are harmless. 
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