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Green Scene: Readers questions

By Joel M. Lerner,

Many of your questions have been very interesting this month. They covered queries from avid gardeners and people who simply noticed anomalies in the landscape.

What’s the name of the invasive shrub that is taking over hiking trails? It grows about 3 to 4 feet tall, spreads rapidly, and has four 4-by-5-inch leaves and bamboo-like central trunks. The flowers occur above the leaves, which grow on reddish, up to half-inch diameter, stems. —Bob Youker

You offered an excellent description of this shrubby invasive plant, commonly called Japanese knotweed. I know it as Polygonum cuspidatum, but taxonomists changed the name to Fallopia japonica. In a little more than 100 years this plant has become a threat to native habitats of the United States. Japanese knotweed was introduced from England for erosion control. It will grow new plants from its root system, making it a good choice for this purpose, however it’s too invasive. If you accept Ralph Waldo Emerson’s description that a weed is a plant whose virtues have not yet been discovered, it is not a weed.

Japanese knotweed is a medicinal herb in many cultures and has a long list of positive traits, starting with its phytochemicals. It is used as a form of painkiller, antipyretic, diuretic, remedy for cough, arthralgia, recurring bronchitis, jaundice, amenorrhea and hypertension in conventional Chinese medicine. It also is considered to have antioxidant properties, offer cardiovascular protection and strengthen the immune system. Unfortunately, its escape from cultivation has created less than a beneficial plant. Find more information at invasivespeciesinfo.gov/plants/knotweed.shtml.

Is it better for the lawn if grass clippings are bagged? I always thought it was better to leave the clippings on the lawn. —Bill Tracy

It is definitely better for your lawn if you do not catch clippings and instead allow them to decay and add organic material back to the soil. Grass trimmings are sometimes called green manure because they decay quickly and add nutrients to established lawns. Grass trimmings will decompose and add fertilizer to the lawn if cut at the proper height. A good rule of thumb is to cut no more than one-third of the height of the grass. Only remove clippings if the grass was mowed when it was too tall or too wet. Clumps of trimmings will shade lawns, keeping out sun and suffocating the grass below.

My one Camelia japonica, ‘April Remembered,’ has a number of branches affected with gall. I haven’t seen this disease in this area and have lived here since 1973. Is that the correct diagnosis? —Sue Hodapp

You do in fact have leaf gall. It is most common during periods of warm, moist weather. The leaf thickens from a fungus (Exobasidium camelliae). The galls can be pruned from the camellia, which is all that is necessary for the home garden. They need to be pruned before the thickened leaves turn white. If white spores form, your problem will worsen next year. Placing camellias in areas of good air circulation will help the plant dry before the spores can begin growing. Since spores need moisture to grow, gall will often occur in very shady conditions.

What would you recommend for someone who doesn’t like the look of ornamental mulch? Is there something I could use that would give me the benefits of moisture retention and weed control but retain a natural look? What mulch do you recommend for a small vegetable garden? —Don Ranard

You can use a material that would not only give the look of good rich soil, but also create it. Well-decayed compost provides the appearance you want. It must be decayed enough and be fine textured with a dark, rich color. Spread it two inches thick over the beds. It has a great appearance, good moisture-holding ability and it discourages weeds. Cultivate it into the soil where plants won’t be harmed. Water the area if there isn’t substantial rainfall after mulching. This is the same treatment to utilize in your vegetable garden.

I love poppies and am having a hard time growing them. I planted some in my garden five years ago. They return yearly without flowering. They never grow bigger than their original size (a few inches high). Any suggestions? —Joan April

Poppies love cool, even temperatures averaging about 75 degrees Fahrenheit. We are on the warm side of their optimum temperatures. They are happier in the northwestern part of the country and further north than Washington, D.C. Poppies are short-lived plants. Perennial poppies seldom return for more than two or three years. The best chance of having them in this region is to grow them as annuals. Annual, biennial and perennial poppies flower in spring to early summer. They require excellent drainage, and when the heat of summer begins, they fade. The mission poppy (Papaver californicum) and tulip poppy (P. glaucum) are annuals or biennials that should flower with good drainage. The most commonly grown poppy in this region that might return for two or three years is the Oriental poppy (P. Orientale). It has beautiful scarlet flowers in early summer and dies back to the ground with our first hot temperatures. Mail-order nursery and seed catalogs sell poppies; try local nurseries from late winter into early spring.

Our skip laurel just finished blooming and needs to be pruned. We live in New Jersey. The plant is about 8 feet high. Some leaves are yellow — is that a problem? We need more growth on the inside of the plant. New leaves are generating on many stems. How should I prune it? —Lynne Poag

Skip laurel (Prunus laurocerasus ‘Schipkaensis’) is an evergreen shrub that grows to about 8 feet in height. It should be selectively pruned after flowering by removing the tallest stems to a lower leaf bud that is at about the height you want the plant to grow. This will stimulate new growth in the lower part of the plant. Do not prune more than one-third of the limbs at this time of the year. Selectively prune this cherry-laurel early next spring before it flowers. You’ll sacrifice most flowers next spring, but the plant will produce more foliage. Cherry-laurels need some shade protection. Yours might be in too much sun, accounting for yellowing leaves. Plant a tree or large shrub to shade it during the hottest part of the day. It should be growing in moist well-drained soil. If it continues to defoliate, it might have a disease or insect problem that can be diagnosed by the Agricultural Extension Service in your county.

Thanks for writing and reading “Green Scene.”

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

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