Woody Ornamental Landscape Plants - May 2008
Please Note: All recommendations are for central Maryland. Time frames shift two weeks earlier in Southern Maryland and the Eastern Shore and two weeks later for Western Maryland. Check publications for further information.
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Culture
Keep young and obviously stressed trees watered during dry periods. Stressed evergreens and shade trees will attract bark beetles and borers. Furthermore, you may notice or hear woodpeckers drilling into trees that are infested with insect pests brought on by drought. Canker diseases can also be expected to show up on drought stressed trees.
Prune spring flowering shrubs like azalea, after bloom period but before they set next year’s flower buds. Shrubs such as holly, boxwood and euonymous can be pruned to a desired shape. Maintain good boxwood health by thinning out interior branches to promote air circulation.
Healthy container and burlapped and balled trees can be safely planted throughout the summer as long as they are continuously watered during dry periods and the soil is not saturated with water.
- Carry newly purchased trees by the root ball or container and not by the trunk. Handle the root ball gently. Shade trees should have 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.
- Avoid planting trees that are popular because they grow fast such as silver maple, Lombardy poplar, willow, and common cultivars of Bradford pear. These trees have weak, brittle wood that is subject to splitting.
Mulches should be applied only 2-3 inches deep and kept away from tree trunks. Mature trees do not benefit from being mulched. They should only be mulched to keep lawn mowers and string trimmers away from your trees.
Older holly and magnolia leaves will begin to yellow and drop. This is a natural process of regeneration and does not indicate a problem with the trees.
Rhododendrons and azaleas can be fertilized with a balanced fertilizer after the bloom period. 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. Gardeners often mistake this disease for natural petal senescence. However, petals that die naturally, will fall off the plant. This fungal disease is more severe during cool, wet weather. Botrytis, or gray mold, can infect older blooms causing them to turn brown. Damaged blooms can be removed. Exobasidium gall can be seen on azaleas and rhododendrons. Leaves develop puffy swellings that turn white in color. Pick off affected leaves.
Roses are available now in nurseries and garden centers as bare-rooted or potted plants. Select plants with 4-5 large canes coming from the base. Choose a sunny site, at least 6 hours of direct sun, dig a large hole, mix a starter fertilizer into the planting soil and plant so that the crown is one inch above the soil grade. Keep newly planted roses well watered. It’s time to begin spraying roses with a fungicide if they are susceptible to black spot disease; spray when leaves are fully expanded. There are new organic fungicides available, including a mixture of neem and oil, that are labeled for black spot. Rose bushes that suffered considerable dieback from winter weather should be pruned back to live, green wood. This may mean pruning back to the crown. Rose slugs can be hand-picked or controlled with an application of pyrethrum and soap. Rose plants should also be fertilized at this time with a balanced fertilizer.
Boxwoods that have become overgrown and weak can be thinned out now. The principal boxwood disease is volutella canker, which produces salmon-colored spores on infected stems and trunks and is encouraged by wet weather.
Clematis requires a soil with neutral to slightly basic (sweet) soil. Test and amend your soil with lime, if necessary, before planting clematis. For best results, mulch the root system to keep it cool.
Prune wisteria and early flowering clematis after it is finished blooming.
Trim back English ivy that is invading walkways, turf and garden beds.
Inspect ‘wintercreeper’ and Japanese ‘euonymus’ foliage for scale problems and apply a summer rate of horticultural oil where these insects are high in population or plants are showing symptoms of heavy feeding.
Cut poison ivy vines back to the ground to weaken plants. Poison ivy foliage can be sprayed with glyphosate. Continue to spray or cut back plants through the growing season to eradicate this noxious weed. Be aware that the offending oil of poison ivy, urushiol, is active and can produce allergic reaction symptoms during any time of the year. Do not handle the hairy poison ivy vines wrapped around trees.
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Insects
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 and the boxwood leafminer produces blister and blotch mines on boxwood leaves. Ultra-fine horticultural oil can be applied to control psyllids and mites. Leafminer damage observed now occurred last year. Consider applying a foliar absorbed systemic insecticide this summer if you observe numerous fresh mines.
Monitor your conifers for spruce mite by taping a branch and holding a piece of paper below. If present, you’ll see small dots walking around on the paper. Spruce mite feeding produces fine flecking or stippling on the needles. They can be controlled now through June with a spray of water or insecticidal soap or horticultural oil. Oil sprays should be applied to dry foliage when temperatures are below 85°F. Oil sprays will discolor Colorado blue spruces trees. Consider rchasing and releasing predatory mites if you observe a severe infestation in your landscape. Blue color will not regenerate on old needles.
Lace bug feeding can be seen now 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. Lacebugs are more of a problem on stressed plants on exposed sites. You can spray with an ultra-fine horticultural oil on underside of leaves or, if faced with a severe infestation, with a systemic insecticide.
Cankerworms look like inch worms 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.
Cottony camelia scale can be found now on yews, camelias, 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.
Remove bagworm bags from evergreen trees. The bags contain hundreds of eggs, which will hatch out in late-May through June. Discard or destroy the bags- don’t just leave them on the ground. When they hatch, you’ll see the tiny ags, with the caterpillars inside, walking around. The best time to spray to control baqworm is June 15-July 15 using the microbial insecticide, B.t.
Gypsy moth caterpillars can be out feeding at this time. The Maryland Department of Agriculture’s spray program began the first week of May. You should have already been notified if you live in an area designated for spraying. B.t. sprays are effective against these caterpillars when they are young.
Hemlock woolly adelgids 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. Spray trees with ultra-fine horticultural oil when the crawlers are out in late May or early June. Be sure to spray underside f needles.
Scale insects, like pine needle scale and hemlock scale, can be controlled with a summer rate application of horticultural oil.
You may notice large numbers of 1-inch long sawfly larvae with dark heads feeding on pine trees. Handpick or control young larvae with ultra-fine horticultural oil.
Young Eastern tent caterpillars are actively feeding. Tents can be removed with a long stick or broom. Do not attempt to burn the tents as this may damage your tree. Larvae are now too large to be effectively controlled with B.t. Mature Eastern tent caterpillars are still moving out of nests searching for a place to pupate. These are harmless caterpillars, although they appeared in large numbers this spring. If they are a nuisance around your home, sweep them up and dispose of them in a bucket of soapy water.
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 leafminer adults are beginning to feed on 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 locust trees.
Aphids actively feed in May. Aphids are small, pear-shaped soft-bodied insects that come in many different colors and suck the plant sap from a wide variety of plant leaves. They may cause leaves to pucker, twist or curl downwards. No insecticide treatments are usually necessary because they are controlled effectively by resident populations of beneficial predators like lady bird beetles, hover fly larvae and green lace wing larvae as well as by parasitic wasps. Sooty mold is a fungus that appears as a black coating on leaves. Sucking pests, like aphids, feed on leaves and excrete honeydew, a substance on which the sooty mold grows. Look for pear shaped, green aphids on the growing tips and prune out damaged shoots. You can control heavy infestations with horticultural oil. If beneficial insects are observed, do not use pesticides as it will be harmful to the good guys.
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Disease
Powdery mildew can be observed on viburnum, dogwood and lilac. The symptom is a powdery white coating on the upper leaf surface. It can be severe under humid conditions. If you have a shrub with significant damage, consider applying a labeled fungicide, like horticultural oil.
Dogwood anthracnose, also known as discula, is a serious dogwood disease. Early symptoms 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. Avoid over application of fertilizer that results in succulent new growth with greater susceptibility to disease. Avoid digging native trees from the woods and transplanting them into landscapes. This practice can introduce the disease into a neighborhood community that was previously disease free. Fungicides can be utilized on trees in landscapes in the spring at bud break, followed by additional sprays every 10-14 days until leaves are fully expanded. Trees should also be sprayed once in the fall after the leaves have changed color, but before leaf drop. In areas with severe disease, consider planting resistant dogwood species such as the flowering dogwood cultivar ‘Appalachian Spring’, kousa dogwood cultivars, (Cornus kousa), or hybrids between kousa and flowering dogwood such as ‘Celestial’, ‘Milky Way’, ‘Stardust’, ‘Steeple’, ‘Stellar Pink’ and others.
The large, gelatinous, orange-colored spore horns of the cedar-apple and cedar-quince rust fungi can be seen on cedars and junipers. The disease will move to the foliage of crabapples, fruiting apples and hawthorns where it appears as yellow-orange dots on the upper leaf surface. These will enlarge through the season depending on weather conditions. These lesions will then move to the leaf undersides by mid-summer. If you plan to plant these trees, investigate resistant cultivars.
The blooms of many plants, including dogwood and peony, are 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.
Sclerotinia, a fungal disease of forsythia, causes flagging of terminals followed by dieback. Prune out affected stems at the crown.
A common fungal disease known as anthracnose can show up on oaks, maples, sycamores, ash, beeches and apples. 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. 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.
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