The gardening season is nearing its end, but your questions keep coming.
QWe 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?
-- Anne Wilson
AAll 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.
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? -- Terri Schaffer
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.
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? -- Barbara Bowers
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.
How well do trees tolerate overwatering? Could you advise on the symptoms of overwatering? -- Tony Sharkey
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.
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? -- Pauline Sterin
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 there aren't many trees here that flower 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.
I have a never-ending problem: rose bushes with black spots on almost all of them. I've tried spraying, but this doesn't eliminate the spots. What would you recommend? -- Bob Heina
Black spot (Diplocarpon rosae) on roses is one of the most prevalent diseases in our region. Whenever I'm asked about it, I am reminded of the advice that my nurseryman friend Carl Orndorff would give listeners years ago on my radio show. He would say, "If you want to control black spot, take a vacuum into the garden; reverse the hose, and blow the leaves dry after a rain or morning dew."
Roses would rather be growing in a field with lots of air circulation than where most are grown -- in a walled courtyard garden or an area lined with boxwood hedges, where air stays still and fungal spores grow unchecked. Couple this with our humidity and 18 months of record rainfall, and you can understand why black spot is such a problem.
The key to solving it is keeping the foliage dry. Following Orndorff's reasoning for a natural control, you also want to avoid overhead watering, remove infected canes and clean up and destroy fallen leaves. Orndorff never used fungicide; he believed that if you controlled the cultural conditions, fungicide was not necessary.
With that said, a dormant application of lime-sulfur after pruning plants and before growth begins will go a long way to controlling the spores that cause leaf spots.
If you are going to spray fungicide to keep your roses free of black spot, you have to know the disease is coming. Once they show on the foliage, black spots cannot be erased. Spray Funginex Rose and Shrub Disease Control, Orthenex Insect & Disease Control, Daconil 2787 Plant Disease Control or other fungicide labeled for roses. Treat just as the shrubs begin to grow leaves. This will kill the spores before they can cause spots.
For roses with a bad infestation, you will have to apply it every seven to 10 days as long as weather is wet. It is not needed during dry periods. Vary fungicides so spores won't develop an immunity to a single control.
Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, www.gardenlerner.com.