Amid the heat wave, gardeners still have questions

By Joel M. Lerner
Special to The Washington Post
Saturday, July 31, 2010; E04

Even though the heat has been record-breaking, you are still working in your gardens and have lots of questions. It's time to address some of them.

Q. I am having a problem with my hydrangeas. They were beautiful in the beginning of spring but then started to wilt and turn brown. This happened before the hot weather. We have been watering them, but it doesn't seem to help. Do you have any ideas about what is causing this? -- Ann Distler

A. Hydrangeas are susceptible to sunburn because they are a shade plant. Their leaves are sensitive to desiccation from heat and sunlight, and it's common for the foliage to turn yellow or brown in bright sunshine. Although the outer leaves look unsightly, the damage will not injure the plant. The hydrangea will continue to grow and flower. The damage becomes most unsightly when the soil in which they are planted is dry. The leaves will also scorch in the heat of the summer. The recommended care is to provide them with a soaking two or three times a week during dry, sunny periods and move them to a shadier site or provide more shade where they are growing. They will be much happier as temperatures fall later in the season.

Q. Do you have any information about allergy-causing shrubs or trees? -- Peter Prakke

A. Research has been done on allergy-causing plants. A system was developed to rate the amount of allergen created by certain plants. "Allergy-Free Gardening" by Thomas Leo Ogren (Ten Speed Press, 2000) and his sequel "Safe Sex in the Garden" (Ten Speed Press, 2003) address this subject. Although some plants can cause skin rashes, the greatest concern is over allergies to pollen.

In 1812, Konrad Sprengel first wrote about dioecious (separate-sexed) plants that produce pollen abundantly. Male plants that depend on wind dispersal for pollen to reach females produce massive amounts of pollen -- the reason for the chartreuse patina from the oaks and hickories flowering in the D.C. region in spring. The more we are exposed to inhaling these irritating pollens from trees and grasses, the more sensitive we become. Our eyes water, and our noses run. When I moved here 28 years ago, I was not an allergic person, but I am now sensitive to tree pollen. My eyes itch, I sneeze and my nose runs when pollen is in the air. According to Ogren, it doesn't have to be that way. Male plants are the culprits, producing pollen but no fruit. During the 1950s and 1960s, the fact that the males left no fruit or nuts to clean up was considered a plus because they required less maintenance. Dioecious trees were selected and bred to be primarily males and planted along city streets -- producing pollen that we are exposed to daily. Low fruit production on medium-to-large shade trees was a plus. In fact, the fruit of the female ginkgo was considered so malodorous that females were removed from commercial nursery stock. However, we failed to see the real problem: an urban forest of heavy pollen-producing plants that have created severe allergies for many people.

Q. We have a medium-size container against a brick outside wall in a semi-sunny part of our driveway. We are thinking of using the container for Cheyenne sky red switch grass, as it stays small and seems suitable for containers. Would this be a good choice, or would you recommend something else? -- Peggy Reaves

A. This switch grass will grow well in a container in full sun. If you want a taller grass that can be seen by drivers, there is a blue variety (Panicum virgatum "Heavy Metal") that grows four to five feet tall with blue-green grass blades. It has a strict upright habit that keeps its form into winter. Most grasses used as container plants will need to be watered often, especially during summers such as this. If grasses are being planted where you enter a thoroughfare, it is most important for you to make sure you allow for good visibility.

Q. I have a colony of thriving terrestrial orchids (Bletilla striata). This year I forgot to cut off the stems after they bloomed, and now the plants have gone to seed. Is it too late to cut the seedpods off? Should I allow them go to seed? -- Marianne Lymn

A. Hyacinth bletilla (B. striata) is a hardy terrestrial orchid that grows quite well in the D.C. region. It should be deadheaded as blooms fade, but since you missed that time period, let the seedpods mature. You can let the seeds lay where they fall and see what returns in spring. It isn't absolutely necessary to deadhead bletilla. This vigorous plant is easily divided from clumps of small tubers (bulbetts) that form on its roots.

Q. I found a viburnum that looked interesting (Cardinal Candy) but don't know if the berries are messy or whether it will grow in this area. What are your suggestions? -- Joyce V. Hoyle

A. Cardinal Candy is an interesting-sounding, hardy, small viburnum. The persistent berries shouldn't be a problem unless you plant it over a walkway. The fall color is a showy reddish-orange with ornamental berries that might persist through winter, since birds like other berries better. I don't know if the same is true for deer, because deer are eating native viburnums, even though they are considered deer-resistant. Try one and see how it performs. It likes moist soil and would have loved last winter. I'm not so sure about this summer.

Q. We have a 13-foot margin between our deck and rear fence and would like to screen out a neighbor's house. An easement prevents planting within five feet of the fence. Our deck is 30 feet long, about six feet above ground that slopes away from the deck. Can we put 12-foot emerald-green arborvitae spaced two feet apart close to the deck rail? -- Jane Paliokas

A. The emerald-green arborvitae can be planted on three-foot centers. Eleven would be a good number to use along a 30-foot deck. About 12 feet tall is a good size to begin with. They should probably be sheared annually to keep the branches stiff enough to avoid a massive snow from pushing the arborvitae over. Or you could consider lattice and a vine, such as cross-vine (Bignonia capreolata) instead.

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

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