WASHINGTON -- The gardening season is nearing its end, but your questions keep coming.

Q: How well do trees tolerate overwatering? Could you advise on the symptoms of overwatering?

A: It is difficult to tell if a plant is too dry or wet by looking at it, because symptoms are similar for both conditions -- wilted leaves, yellowing and defoliation. You must check the roots.

The way to monitor your soil is to use a dowel or long screwdriver. If you sink it into moist soil that you dug for the trees, it should press in easily, and you might be able to detect moisture on the end of the rod when you pull it up. Drought conditions usually make it difficult to sink a probe into the soil. When the soil is moist, do not water. When it's dry, sprinkle. An inch of water caught in a dish or tin can on the surface translates into percolation about four to eight inches deep.

Avoid placing an overabundance of compost into a small, poorly drained planting hole. It acts like a basin and holds water. The result will be a soggy, rotting root ball. Dig your planting hole as wide as possible and mix all the native soil with about one-third leaf mold or other composted organic material before replacing it. To ensure drainage, install the plants high; up to a third of the root ball on woody plants can stand above the existing grade. Cover the roots standing above ground with the soil and compost mix you made, and leave a ditch to catch rainwater around the outer edge of the planting.

For plants that are newly installed from containers, monitor the moisture in the area of the root ball. Bound-up roots that are growing in mixes can stay dry while the soil around them is moist.



Q: We are having a problem with Leyland cypress. The lower branches do not get much sun and are dying. We have them growing under a huge oak tree. What would you recommend?

A: All plants need sun to photosynthesize. Leyland cypress needs full sun. The Leylands should be replaced with shade-tolerant evergreens.

Several narrow evergreens that do well in low sunlight and can attain 15 to 20 feet in height are arborvitae (Thuja species), hinoki false cypress (Chamaecyparis obtusa), koehne holly (Ilex x koehneana), dragon lady holly and Nellie R. Stevens holly. They can take twice as long to grow tall in shade, so start with mature plants.

Here are suggestions for shade-tolerant evergreens to create screens that can grow five to 10 feet tall: Japanese aucuba (A. japonica), chindo viburnum (V. awabuki Chindo), hicks yew (Taxus x media Hicksii) and Manhattan or Siebold euonymus (E. kiautschovicus Manhattan or Sieboldiana).

Don't plant these shrubs in poorly drained soil. Use about a third compost in the soil.



Q: We have a native cherry tree that was sideswiped by a car. The tree is otherwise healthy, about 20 years old, 12 inches in diameter and 50 feet tall. The damage covers an area four-feet high by a few inches to one-foot wide. The bark in the damaged area is gone and there are no deep gouges in the tree. Can it heal on its own or should we do something to seal the damaged area?

A: You could lose the tree. Don't seal the wound. Instead, to try to help the tree heal, prune away bark that has separated and is not attached to fresh wood. Monitor the bare part of the wood as the tree grows. If the bare area becomes soft and pithy, scrape the decay to solid hard wood to keep it from spreading into the tree. Use a linoleum knife.



Q: Several months ago you mentioned that a vinegar solution is useful for spraying on weeds and works like Roundup. Could you publish a source for the vinegar in your next column?

A: Vinegar on grocery store shelves is about 5 percent and seems to work to kill weeds. Use it straight, undiluted, in a plastic spray bottle or tank. It is corrosive to metal. It is to be used when weeds are actively growing and generally works overnight.

There are vinegar-based weed killers at garden centers that can be more effective for heavier duty. Burnout Organic, for example, is promoted as environmentally friendly and safe for pets. It is effective but non-selective. It will injure or kill weeds and ornamental plants. For more information, see www.commonsensecare.com.



Q: Last spring, we noticed landscape contractors had pruned crape myrtles back to about four feet. This seemed drastic, but the shrubs bloomed nicely this year. What's the reason for this?

A: In spring, as growth begins, is the time to prune crape myrtle. The beauty of this plant is that it can be cut to any size, even to the ground, and will grow back and flower throughout the summer the same year. The ones you saw were probably cut back to train as shrubs. This can be done because new stems and foliage grow from almost anywhere you cut. I prefer crape myrtle grown in tree form, because it shows its handsome lacy, exfoliating bark on older trunks, and it flowers all summer long. Other than thinning the plant and cutting suckers, I would cut the plant back only if it was dead wood, had limbs that needed to be raised or was in the way of other plants.



Q: Do the crape myrtle varieties Hopi and Acoma, developed at the National Arboretum, need to be of a certain age to bloom? Mine have been planted for three summers, are about five feet high and get afternoon sun. They appear healthy but have never bloomed.

A: Like all crape myrtles, Hopi and Acoma are precocious bloomers. They should be blooming well for you.

The most common reason for failure to bloom is lack of light. Even a few hours of shade drastically reduces their tendency to flower, and your crape myrtles need additional, morning sunlight to flower. Move them to a sunnier location, and they should bloom well for you.



Q: Many of the river birches in my neighborhood have leaves that are turning yellow and dropping. It seems as if the trees believe it's fall. Is this a normal condition or a problem that requires action?

A: The yellow leaves are the hallmark of a wet summer. The above-average rainfall has encouraged fungal diseases on a range of garden plants this year. In the case of the birches, a fungus named Discula betulina@ causes the yellowing. If you look closely at the leaves, you will see brown spots that are surrounded by the yellow tissue. Often, the leaves drop while they are still partially green.

This leaf anthracnose disease is nothing serious. It won't harm the trees over the long term, and it may not even be present next year if the weather is dry.



Q: I have a four-year-old pear tree that has begun to produce fruit. Last year I had a crop of about 20. When I went to pick them, they had disappeared. Not a trace could be found either on the tree or on the ground.

This year, with almost 40 pears, I netted the tree. Again, I checked them and found every last one had disappeared, even the ones in the highest branches.

Below the tree there was no sign of disturbance or animal prints. I know a critter could have gone up the trunk and avoided the net; it was not tied securely around the trunk. We have squirrels in the neighborhood as well as deer, lots of birds, chipmunks and who knows what else.

A: I think your pears were eaten by squirrels, deer, raccoons, or all three. The netting, unless it was tied securely around the trunk of the tree, would do nothing to protect the pears from any of them, and they would not have to disturb the netting to get at the fruit.

Unlike most home orchard fruits, pears should not be allowed to ripen on the tree. If they remain on the tree for long after they have grown to full size and lighten in color, they will become mushy and lose flavor. Test a full grown pear by cutting into it -- if juice appears when you cut into the flesh, you can harvest all the pears on the tree. Harvest pears by twisting and pulling upward on the stem attaching the fruit to the tree. If it doesn't detach easily, wait and try a few weeks later. Pears can be stored for several months in the crisper section of your refrigerator. Bring them to room temperature for several days or a week to soften them and develop full flavor.