By Joel M. Lerner
Saturday, September 16, 2006

As we head into autumn, it's time to answer some more of your garden questions.

Q I planted variegated and non-variegated liriope in my garden. It spread profusely, even into the lawn. I don't know which kind it is. How does one permanently get rid of it? -- Emilie Hoppes

A Your liriope (lilyturf) invasion is what has earned creeping lilyturf ( L. spicata ) a place on the invasive plants list. It spreads vigorously by rhizomes and bulblets. Clump-forming liriope ( L. muscari ) is not invasive and is a dependable garden plant.

The best control for lilyturf is to keep it contained. Edging placed at least five to six inches deep and an inch out of the ground will keep it from growing over the top into your lawn. Use vinyl, aluminum, steel, concrete or other strong barrier. You can spray the liriope not growing in the lawn with a non-selective herbicide. There has been success with products containing strong vinegar, such as BurnOut Weed & Grass Killer, or Roundup, a synthetic herbicide. Both of these will kill foliage, and Roundup will also kill the roots. Dig out plants manually from your lawn. Follow labeled instructions for any weed-killers you use.

I have been trying to rid my lawn of oxalis, trying many products without success. The yellow flowers continue unimpeded. Is there a pre-emergent herbicide that I can use in the spring? -- Harvey Long

Oxalis responds well to treatments at this time of the year. It cannot be fully controlled with a pre-emergent weed-killer in spring. Between late summer and early fall, when cool-season turf is at its best, treat the clover-looking foliage of this yellow-flowering lawn weed with a material that contains tryclopyr, such as Weed-B-Gon Lawn Weed Killer. It will not harm the turf. Follow labeled instructions. Applying a spray will allow you to localize treatment to problem areas instead of broadcasting over a large area. If you catch them when they're young, pulling them is the environmentally friendly option.

When is the best time to prune my butterfly bush? Any other tips on maintenance? -- Erika Cohen Derr

Cut shrub to 18 to 24 inches by March 1. The butterfly bush is completely root-hardy and will grow four to eight feet tall from a plant that has been cut to the ground. Flowers occur on the current year's growth, and the plant will return to bloom where you can see and appreciate the blossoms. It blooms best in full sun. Deadheading spent flowers regularly will keep them from forming thousands of seeds that could become weeds. Europe is beset with butterfly bushes growing into the nooks and crannies of walls, walks and neglected sites.

I have six peonies that have been healthy plants with lots of blooms. Recently some leaves of one plant looked withered. This has spread to other leaves and another plant. What should I do? -- Umesh C. Mullick

The withered appearance after a hot spell is no surprise, but if the problem gets worse and entire stems wither and die or the plant develops a gray or white appearance on the leaves, they could have one of two possible diseases -- gray mold ( Botrytis cinerea ) or downy mildew blight ( Phytophthora cactorum ). Gray mold is best controlled where your plants will receive good air circulation, and they must be in well-drained soil. Downy mildew blight can turn the plant brown and wilt it in short order. They should be treated with a copper-based fungicide according to labeled instructions. Bordeaux mix is fairly innocuous, made of copper sulfate and hydrated lime. It is available in solution or premixed as a water-soluble powder. Use at seven- to 10-day intervals when the symptoms show. Foliage must dry by night. Good practices are pruning damaged stems to the base, very little mulching and thoroughly cleaning up debris in the fall.

How should grass be cut when dethatching or power-aerating the lawn? Do you recommend any particular brand of fescue grass seed? -- Neil Lamb

The lowest you should mow any cool-season turf is 2 1/2 inches. Growth of healthy lawn and germination of grass seed are most important when you remove thatch and aerate. Mowing higher retains more leaf surface on existing turf, which also shades the seed, holding moisture and helping with germination. Water lightly every other day to keep seed moist. A deep watering once a week will get established lawn growing quickly during this cool season.

Plant a named, compact, turf-type tall fescue. There are disease-resistant and drought-tolerant varieties. Several names I have used are rebel, arid, mustang and falcon; there are hundreds on the market. New varieties I have read good reports about but not tested are compact tall fescues that have an exceptionally low-growth habit and are very wear tolerant. A couple of them are named Magellan and gazelle. Plant a blend of three varieties, based on the theory that even if one is lost the other two will succeed. Over-seed at a rate of three to four pounds per 1,000 square feet.

In summer the lawn behind my house is a field of dirt mounds thrown up by cicada killer wasps. Dozens of them swarming around the lawn from mid-July throughout August discourages us from using the area, to say nothing of the lawn damage. I tried insecticide. Any suggestions? -- Mike Ahern

The cicada killer wasp is one of a group of insects called solitary wasps. They are not social insects that cooperate with others of their species. They live alone, digging tunnels in the ground, using them as nests to feed their young. The cicada killer is considered a beneficial insect because it feeds on cicadas that can be crop pests. The wasp stings a cicada, paralyzing it, drags it into its nest and lays eggs on it. The larvae hatch and feed on the cicada.

The wasps usually inhabit a property only for several years, then move to other areas to find cicadas. They are not known to defend their nests, so they will not sting unless they are handled or highly provoked. Applying lawn insecticide to their nests will control an infestation of these wasps. I don't think it's necessary, because it will kill other beneficial predators, such as spiders, wasps, assassin bugs and damsel bugs that control a variety of insect pests. Try to wait out their cycle. It will be good for the environment.

What's the best way to control weeds in a small (four by eight foot) vegetable garden? Every year, we clear weeds by hand before planting tomatoes, cucumbers, peppers, lettuce and other summer vegetables. It's impossible to pull all new ones without disrupting newly planted seeds. We want to avoid chemical weed-killers. -- Jeanne Sweeney

The best way to control weeds in the vegetable garden is to use mulch. Composted leaf mold will keep weeds from getting the grip they need to germinate and grow. Leafgro is commercial compost that will serve the purpose. Lay it three to four inches thick, like a blanket over the vegetable garden. At the end of the season, the compost can be incorporated into the soil to condition it for next year. Straw laid four to six inches thick is excellent mulch. It will decompose during the growing season and create compost for next year. Landscape fabric will discourage the most invasive weeds by acting as a barrier for anything trying to grow from the ground. When you install your crops, you need to slice through only the material where you are planting. The fabric is porous and allows air, moisture and nutrients to reach the root zone. The only weeds you will need to control are from seeds that fall onto the mulch and germinate and grow.

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.