Elms (Ulmus spp.) provide hours of cooling shade with their arching branches. Depending on the variety, these shallow-rooted trees can grow 50 to 125 feet high and 40 to 120 feet wide. Hardy in U.S. Department of Agriculture plant hardiness zones 2 through 10, most elms lose their leaves in the fall. They prefer moist, well-drained soil and full sun to partial shade. The elm that provides cool summer shade for your family, however, can become a hazard if diseases or pests weaken the tree or its branches. Keep the kids away from diseased branches and pruning tools, and hire a professional tree service to remove high branches.
1. Dutch Elm Diseae
Dutch elm disease causes curled, yellowed and browned leaves, and dark brown streaks under the bark. It has killed many American elms (Ulmus americana), which grow in USDA zones 2 through 9. The tiny black or reddish-brown elm bark beetle spreads the fungus through tree wounds. A professional tree service can remove severely diseased trees. To prevent the disease, have the limbs pruned only in the fall when bark beetles are not active, and bury freshly cut elm wood. Resistant varieties include frontier elm (Ulmus parvifolia x Ulmus carpinifolia), which grows in USDA zones 5 through 8, and “Prospector” elm (U. wilsoniana "Prospector"), which grows in zones 4 through 10.
2. Other Diseases
Other elm diseases include anthracnose and wood decay. The fungus disease anthracnose causes dark leaf spots and stunted or twisted branches in crowded, damp conditions. Severely infected trees may lose all leaves, but the disease usually is not severe, according to the University of California Integrated Pest Management Program. In USDA plant hardiness zones 7 through 10, you can plant anthracnose-resistant “Drake” elm (Ulmus parviflora “Drake”). If you see shelf-like growths on the lower trunk of your elm, it's a sign of wood decay, which infects through wounds and causes white rot in the inner trunk and roots. Take care not to injure trees to prevent this infection. Hire a professional tree removal service to trim or remove hazardous, weakened branches or trees.
3. Elm Leaf Beetle
The elm leaf beetle can deprive you and your family of those cool, shaded afternoons in the yard. This pest reduces elm leaves to skeletons, causes premature leaf drop and weakens the trees. To prevent elm leaf beetle problems, make sure your elm trees have enough water, and avoid pruning in the spring and summer when the beetles are active. Remove dead branches in the fall, and prevent injuries to the trunk and roots so there is no point of entry for the beetles.
4. Other Elm Pests
Other insects that bother elm trees include fruit tree leafrollers and woolly apple aphids. The larva of the tortricid moth, the fruit tree leafroller has a 3/4- to 1-inch-long green body and brown head. Symptoms include ragged, chewed leaves or leaves rolled in silk thread. Birds, beetles and other natural enemies usually control this pest, notes the University of California Integrated Pest Management Program. At less than 1/8 inch long, the woolly apple aphid has a fuzzy wax covering and causes wilting and dieback as it sucks sap from elm leaves. Gray sooty mold may grow on the aphids’ sticky waste or honeydew. You can remove aphids with a forceful stream of water from a hose.
- Iowa State University Plant and Insect Diagnostic Clinic: American Elm
- Wilson Brothers Landscape: Elm “Drake”
- Pennsylvania State University Extension: Elm Diseases
- University of California Integrated Pest Management Program: Bark Beetles
- University of California Integrated Pest Management Program: Resistance to Dutch Elm DIsease
- Colorado Tree Coalition: Frontier Elm
- U.S. National Arboretum: “Prospector” Wilson Elm
- University of California Integrated Pest Management Program: Anthracnose
- University of California Integrated Pest Management Program: Wood Decay Fungi in Landscape Trees
- University of California Integrated Pest Management Program: Elm Leaf Beetle
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