By Joel M. Lerner
Saturday, November 29, 2008

A new crop of questions sprouts as fall changes the dynamics in our gardens.

Q My pachysandra does not grow well under our ash tree, but seems to grow fine under our oaks and maples. Is there a reason for this? -- John McCrea

A Pachysandra competes well under trees in limited sun, with the types of nutrients and water usually found in wooded areas. It obviously does not have the same environmental conditions under your ash tree as it does where it is growing well. Pachysandra prefers moist, well-drained, acidic soil high in organic material, as do many shade-tolerant ground covers. Add iron sulfate according to labeled directions to acidify the soil and mulch area with 1 to 1 1/2 inch of compost so it can begin to work its way into the root systems of both your tree and the pachysandra. In large areas without plants, lightly till the compost into soil without disturbing the root system of the tree.

While visiting Charleston, S.C., I discovered the delightful fragrance of tea olive and bought two. I lost them, purchased two on another trip and planted those against a south-facing wall. They have barely survived. I transplanted them into large containers to keep in my hothouse during winter. Is it possible to plant these in the garden and have them do well? -- Susan Pennington

The tea olive in Charleston could easily have been a variety that is not hardy to Washington winters, such as fragrant tea olive (Osmanthus fragrans). This means you will have to continue to take yours into the hothouse in winter. The species I have used successfully in protected sun and partial shade is holly tea olive (O. heterophyllus), hardy to Zone 6, which should be fine here. It's a zone tougher than its counterpart but also sports fragrant flowers in fall.

We planted several elaeagnus on our property. We love the fragrance, durability and deer resistance, but have become concerned about whether or not this shrub can be invasive. Can you provide any reassurance? -- Freda Cameron

Depending on where you have it sited, this fall-flowering, fragrant, evergreen shrub could, at some point, be fruitful, multiply and contribute to disrupting the balance of nature. When planted in partial shade it will stay smaller than if in full sun, with fewer tendencies to invade. There will still be enough flowers to produce fragrance.

According to several invasive plant lists, there are two elaeagnus considered a threat in the mid-Atlantic: Russian olive (E. angustifolium) and autumn olive (E. umbellata). Thorny elaeagnus (E. pungens), which I think is the one you have, is not listed as invasive in the Washington region. Unfortunately, this evergreen shrub is considered invasive in the southern United States (http://www.ncwildflower.org). That means it could pose a threat to native plant populations in the Washington region in the future.

I sprayed my roses weekly for black spot. They did well this summer and didn't develop a problem this season until early September. Why? -- Peggy Reaves

One reason might be that your roses have developed immunity to the fungicide you use, especially if you have always sprayed with the same material. You should alternate several fungicides to avoid the chance of roses developing immunity to a particular product. Use a systemic fungicide such as Funginex for a couple of weeks and then use a contact substance such as Mancozeb for the next couple of weeks. After that, switch to a different systemic such as Immunox for several weeks. Following this regimen might help avoid black spot for the entire season. I would only recommend this type of treatment for roses where funguses are a severe problem.

Last year we visited San Antonio. There was a small bush-type plant that gave off a minty fragrance when the leaves were rubbed. It was called a copper canyon daisy. Can you tell us more about this plant? -- Tom and Sherry Lum

Copper canyon daisy (Tagetes lemmonii), also called lemon marigold, Mount Lemmon marigold or Mexican bush marigold, is an evergreen sub-shrub that grows about two feet tall and spreads easily because ends of branches that touch the soil will root. It freezes back to the ground when temperatures reach about 25 degrees. It grows in full sun, is drought tolerant and deer resistant. Not related to daisies, it is in the same genus as common garden marigolds and a native of the Huachuca Mountains in Arizona. Copper canyon daisy blooms in late fall, producing a mass of bright yellow blossoms when there is enough precipitation.

A close cousin that does well in the Washington region as an annual is sweet marigold (Tagetes lucida). It is one of the few marigolds that emit a pleasant aroma, similar to tarragon, present in flowers and foliage. It is used as a tarragon substitute. Look for seeds now since it's not commonly found at garden centers in spring.

My daughter planted a long-needled pine seedling in my yard 24 years ago. Over the past several months some of the branches and needles have developed a black, powdery-looking, somewhat sticky substance on them, which is affecting a good bit of the tree. Some branches and needles are dying. How do I figure out what the problem is and how to treat it? -- Bill Beavers III

Your long-leaf or long-needle pine (Pinus palustris) is a southern pine growing farther north than its typical limit, southeastern Virginia. This means it isn't going to be a vigorous tree in the Washington region and must have been sited in moist, well-drained soil and full sun for it to have done so well for so long. This ordinarily problem-free southern pine will be susceptible to more insect and disease problems in our cooler climate. If the foliage appears to be covered with white flecks, pine needle scale is a likely suspect. With its long narrow shape, it is present primarily on the needles and feeds on all needle-bearing evergreen conifers. Pine bark adelgid also coats needles, stems and branches with white cottony masses. Insects that feed on the sap of a plant through its leaves and stems are aphids, adelgids and scale. They excrete excess sap (honeydew) they can't use. The sooty mold grows on this honeydew. Some twigs and branches may die.

Do a cleanup pruning (or have it done depending on the size of your tree). Prune dead wood and branches that have matted together because of webbing or wax from the scale insects or adelgids. Hose off sooty mold. Before growth begins in spring, spray tree with insecticidal soap and horticultural oil, such as Volck Oil Spray. This will begin to control insects that are weakening the pine. Never spray oil if temperatures are expected to be lower than 40 degrees within 24 hours of treatment. Apply oil and soap again when scale insects or adelgids are active. Two systemic insecticides that can be sprayed when you see the minute insects crawling are Isotox or Orthene. My preference is to use insecticidal soap and horticultural oil. Follow all labeled instructions and never spray on a windy day.

My goal is to continue producing enough food for my needs. I have used carpeting as mulch for 50 years. It has worked for me, keeping weeds out and holding moisture in the soil. Recently I stopped using carpeting. What can I put in its place? -- Max Sturman

Commercial landscape fabric is another permeable fabric that allows moisture and air in and prevents weeds from germinating. I prefer a two-to-three ply non-woven fiber that is spun-bonded and tough enough to be lifted when necessary. If aesthetics is one of the considerations, you might cover the fabric with mulch. Design the garden in a way that the rows are planted with a cool-season grass, such as rye that can be mowed in season and tilled into the soil when it's fallow. Use landscape fabric to cover only the rows of produce where you won't be mowing.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, http://www.gardenlerner.com.