Green Scene

A time to take a look back and plan ahead

Fast-growing, disease-resistant river birches give a driveway an informal edge.
Fast-growing, disease-resistant river birches give a driveway an informal edge. (Sandra Leavitt Lerner For The Washington Post)

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By Joel M. Lerner
Saturday, November 14, 2009

Gardening tasks don't stop with the end of the growing season. You can still plant trees, do landscape design work, inspect insect damage, and evaluate what performed well in the garden this year and what needs more attention. Here are answers to some of the questions that have been sent in recently.

Q: We have property in Leesburg on which we would like to plant trees to line a 400-foot driveway. The ground is fairly rocky. Our ultimate plans are to build a vineyard or an events center, so a dramatic effect is important. What trees do you suggest? -- Anna-Marie Termini

A: You have a wide variety of choices. Is the lane straight? Do you want a formal or informal appearance? What will be there, other than trees? With rocky soil, maybe rock outcroppings would be appropriate. Here are some ideas to get you started, assuming there is good sun and well-drained improved soil:

Fastigiate hornbeam (Carpinus betulus 'Fastigiata') is a formal deciduous tree branched full to the ground, pyramidal in form. It would be quite impressive lining both sides of the drive on 10- to 15-foot centers. Hophornbeam (Ostrya) is a small tree with fruiting bodies that look like hops and a rounded canopy, giving the appearance of an aerial hedge as used in a French grand-style garden. Plant on 10- to 15-foot centers. River birch (Betula nigra) is a fast-growing, disease-resistant tree that will fit the natural landscape and line the drive in less formal fashion, planted 15 to 20 feet apart. Or you could line the lane with an evergreen that has red berries in winter using American holly (Ilex opaca), or a striking blue spruce, both generally deer resistant.

Q: I have two artichoke plants that have not yielded any artichokes this year. What should I do to winterize them and help them produce fruit next year? -- Anna Maslowicz

A: I assume you are inquiring about globe artichokes (Cynara scolymus). They grow large, ball-shaped flower buds. The bracts that never formed on your plants are the parts that are eaten. They prefer cooler summers and warmer, longer winters than this region experiences, so you need to take special measures to grow them here. Plant them in full sun and in soil that's rich in leaf compost. Cut back now. Protect crowns (tubers), and leave them in the ground. Cover with No. 1-size plant pots to leave air space and keep them from smothering. Dump a thick layer of mulch over the pot. This is an easy approach with a fairly good survival rate for the plant.

Q: I have a hydrangea that's over five years old that I received from a florist. I've left it out each winter on a somewhat protected patio, and it's survived with new green branches each year. I'd like to plant it. I have shade and sun in various parts of my yard, with the usual Montgomery County clayish soil. Where is a good location to plant it? -- Liz Gould-Leger

A: Bigleaf or florist hydrangeas (H. macrophylla) will thrive outside year-round in the Washington region. They should be protected from drying winter winds that can cause freeze damage to the tips of the plants, where flower buds formed during the previous growing season. Grow them in the native "clayish" soil you have, but amend it with lots of compost and keep the soil moist. They flower well in sun to partial sun. Locate the plants four to six feet from an east-facing house wall with bright northern exposure or another sunny location protected from the heat of the day. The flower color of florist hydrangeas depends, to some extent, on soil pH. Pink blooms are commonly found on plants in alkaline soil, and the intense blue color is most commonly found when there is good availability of aluminum in soil. You can change the color to blue by adding aluminum sulfate, following instructions on the label. Pulverized, ground limestone sprinkled over roots in spring and fall generally turns flowers purple to pink within a couple of years. Do not allow the soil to become fully dry.

Q: I have two 15-year-old spruces that stand about 12 feet tall. I noticed that one of the trees looks sparse at the top. After closer examination, I saw many two-inch cocoons hanging on the tree. The top of the tree is especially infested with cocoons. What should I do to get rid of them? -- Jim Gordon

A: The spruces sound like they have bagworms. The bags are full of eggs that hatch in May. Larvae feed on needles. They are especially fond of conifers. The best control is by hand-picking the bags from trees and placing in the trash now. Don't squish them on the ground near the host plant because surviving larvae that hatch in May will feed on the same tree. If you can't hand-pick bags, treat the larvae in May and June with Bacillus thuringiensis (BT), available at nurseries and garden centers. Follow all instructions on the label.

Q: I have cedar siding, and carpenter bees seem determined to bore holes in every inch. How do I find almond oil, and how do I get rid of the bees with it? Is it a spray, or does it have to be painted on? -- Lydia Elliott

A: Almond oil is commonly sold at health food stores. Several I know of are Whole Foods, MOM's and the Takoma Park-Silver Spring Food Co-Op. It is also available in bulk where massage therapy supplies are sold. Check garden centers and general grocery stores, as well. Brushing or spraying it onto your wood should be equally effective. Good coverage in places where carpenter bees are boring is the most important part of process.

Q: Our scarlet oak is in distress. It is at least 75 years old, with a 36-inch radius. It is in full sun on the east side of our property. This summer, the bark at the base pulled away from the wood in a two-by-three-foot section, leaving a hollow space. Also, fungi sprouted in this area. There is a small amount of dead wood in the canopy but not more than the annual amount. Two old-growth oaks on either side are thriving. -- Julie Hawley

A: Oaks, among many other hardwoods and conifers, are very susceptible to Armillaria root disease, which has hit this area rather hard in the last decade or two. The oak you describe sounds very much like it is succumbing to this fungal disease. There is no treatment for it. The sloughing bark and fungi are signs there is dead wood inside the tree. The vascular system of the tree that runs just underneath the bark might continue to carry nutrients to the canopy for another several years, or the oak might not leaf well next year. It is crucial that you have the tree evaluated by a certified arborist this season, especially if it is in a location where it can threaten structures below.

Thanks for writing and for reading Green Scene.

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


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