D.C. area plants and trees: How to deal with hot weather problems

August 12, 2011

As we enter into yet another hot-weather month, many readers have written to me about problems that appear to be tied to the area’s scorching heat. Here’s are some of the issues they’ve raised, as well as other problems they’ve been struggling with:

Q. On the north side of our house, we planted skimmias. Until recently, they looked great; now, the leaves are turning yellow and dropping off. The north side of the house gets lots of heat and sun in the summer. I want to transplant them to a shady area in the back before they die. Can I transplant now?

A. Skimmias, slow-growing evergreens that stay low to the ground, must be planted in shade with moist acidic soil that is high in organic material (compost). It sounds like the heat and sun may be taking a toll on your skimmias. In this climate, skimmias are susceptible to mites that suck nutrients out of the leaves and discolor them. Sunlight also bleaches the leaves.You should transplant your skimmias to a shady part of your yard. But before you dig them up, soak the plants with a hose until the roots are moist. The leaf discoloration can be treated with an insecticidal soap available in most garden centers. You could also use liquid dishwashing soap, but do not use detergents made for dishwasher use.

Q. Do you have any suggestions as to why my lilies of the valley do not bloom? They were healthy transplants, planted in the shade three seasons ago. They are spreading but have only put out three or four flower stalks. I have grown them in other locations with success. What can I do to promote blooming next spring?

A. Lily of the valley (Convallaria majalis) is a cool-weather perennial, native to most of the Northern Hemisphere. It is happiest in the northern United States and Canada (USDA hardiness zones 2 to 5). That is where its flowers tend to be the showiest. The Washington area is probably the southern-most point where these plants can thrive. The area’s sizzling summer heat has probably worked against your lilies, and, unfortunately, there’s not much you can do about it at this point.

Q. The lovely crape myrtle planted in my front yard about eight years ago is now about 18 feet tall. It started to lose strips of bark from the trunk and has peeled off in areas up to about six feet. The tree looks healthy except for the peeling bark. What should I do?

A. There is nothing wrong with your tree. The sloughing of the bark is a normal part of the crape myrtle’s habit. As the outside layer of bark exfoliates, the trunk becomes more ornamental.

Q. Carpenter ants have invaded my North American red oak. I discovered their presence when I was showered with sawdust while standing under the tree. They have established a nest where a branch was removed several years ago. Since then, I’ve been spraying their nest with soapy water. I think I’ve reduced their population. Could you please provide some guidance?

A. There is little doubt that you have ants working on moist, decaying wood. I don’t know what kind they are, because there are many types of wood-damaging ants. Some of them bite, so be careful.

If you want to continue using soap, you might try making a mixture that’s more heavily concentrated with soap. But do not use the type of detergent made for the dishwasher, which can harm the tree. If the ants persist, try using Victor Poison-Free Ant and Roach Killer. The active ingredient is mint oil, which is effective and offers some residual value. But be sure to follow the directions on the label. If you are willing to use more toxic material, hire a pest control professional trained in baiting techniques, which basically spread toxic chemicals near the nest. The ants then eat the chemicals and take them to their colonies to feed their pals. That should destroy the colony.

But keep in mind that carpenter ants do not damage trees. They are eating wood that’s dead, and they don’t cause any further harm, according to the University of Minnesota. But if they get into your house, then they can do some serious termite-type damage.

Q. Our scarlet oak is in distress! This tree is at least 75 years old with a 36-inch-diameter trunk. This summer, the bark at the base on the north side pulled away from the wood in a section 2 feet by 3 feet leaving a hollow space. Fungi sprouted in this area. There is a small amount of dead wood in the canopy. The other two old-growth oaks on either side are thriving. Do you have any suggestions?

A. Given what you’ve described, it sounds like a large area of the cambium (live wood) is gone. Only time will tell how the scarlet oak responds to the decay. For safety reasons, you should have a certified consulting arborist evaluate the tree. An oak of that size could destroy any structures below it or hurt a person if a branch fell. If the tree is dying, replacing it with several younger ones that will grow more vigorously without posing any threat to nearby structures or people would be the best idea.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md.

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