Green Scene

As season changes, gardeners get ready

This Carol Mackie daphne gave us fragrance in early spring for 15 years.
This Carol Mackie daphne gave us fragrance in early spring for 15 years. (Sandra Leavitt Lerner For The Washington Post)
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By Joel M. Lerner
Saturday, September 11, 2010

Gardeners are outdoors taking advantage of this perfect season preparing for fall, and your questions are signaling that the season is right around the corner.

Q: I have a bed of sedum sexangular. It came through the snowy winter just fine. After flowering last spring, portions of the plants turned brown and look awful. Should I shear it? When? -- A. Roudabush

A: Sedum sexangular (S. sexanulare), also commonly called tasteless stonecrop, is extremely tolerant of cold conditions. This European native is also quite tolerant of drought but will not endure with poor drainage. In wet situations it's quite susceptible to root rot, and your browned plants might have experienced too much wetness in spring. Improve drainage around your plants by adding a fine-textured gravel [chicken grit] into the soil in the area. Shear back browned and dead stems before growth begins in spring. Clean your shears with bleach, and oil them lightly before using them on any other plants.

Q: I have a thick lawn that my neighbors say is the best in the neighborhood. I have done all of the things you recommend except adding compost, but have that scheduled for this fall. After aerating I am planning to spread a few bags of gypsum to try and break up the Prince William red clay soil, hard as concrete, that is under the sod. Do you recommend using gypsum? -- M.E. Dunard

A: Without breaking down the fine-textured clay into a more pervious surface, what you have is a layer of impermeable soil with a lawn that you are supporting from above with fertilizer and irrigation. To encourage deep rooting and create conditions where water can percolate through the clay layer, use a plug aerator to make numerous and deep aeration holes. Soil that is high in sodium and clay will often be impermeable to water. It takes many years of aeration every spring and fall for compost to enrich a clay-based soil so that it is permeable to water. Gypsum in your soil or lawn preparation regimen is an excellent idea. Spread it in a broadcast or drop spreader. According to the text "Soils" (Prentice Hall, 1983), applications of gypsum (calcium sulfate) for three years increased percolation to 19 inches, from only 10 inches in soil that did not receive gypsum. On the basis of this study, and recommendations from the late plant scientist (and my friend) H. Marc Cathay, gypsum will always improve soil. The recommended rate of gypsum application is about 40 to 80 pounds per thousand square feet depending on whether you are turning it into the soil or broadcasting it over existing vegetation.

Q: I need a suggestion for a viburnum about 8 to 10 feet tall and 6 to 8 feet wide. I would like something that has a moderate growth rate and is shade-tolerant. -- J.V. Hoyle

A: Allegheny viburnum (V. rhytidophylloides) will give you flowers and is still deer-resistant as of today's writing. It has a leathery leaf, is semi-evergreen and grows moderately fast. After a decade of growth, it will mature at about 10 feet by 10 feet.

Q: I am trying to have a chemical-free lawn and have been working on it for the last three years with considerable success. I am about to start my fall treatment and am curious what kind of seed you usually use. -- A. Talen

A: I consider cool-season, dwarf, turf-type tall fescue to be the best-performing grass for lawns in the D.C. region. It needs at least six hours of sun per day to become lush. There are hundreds of hybrids; choose one that is named and contains at least three varieties. The recommended seeding rate for new lawns is approximately eightto 10 pounds per 1,000 square feet; for established lawns, use about 4 to 5 pounds for the same area. Water is the key to seed germination and growth.

Q: After reading your interesting article, I would like to amplify some points about soil testing: The Fairfax County Extension Office provides test kits. Kits are also available at garden centers. The kit is free, and test results are mailed to your home. But it will not tell you what the nitrogen requirements are. -- A. Gouge

A: A test will yield recommendations for your soil's need for potassium and phosphorous and indicate whether your pH (acidity or alkalinity) requires adjusting to create a healthy lawn. Get a test kit from one of the sources mentioned (most county extension offices, in addition to Fairfax, have them); the kit includes instructions on how to collect a soil sample and send it, for a fee, to a lab at Virginia Tech for testing. The test does not cover nitrogen in the soil because nitrogen levels do not hold steady enough for the test to be accurate. I generally count on sprinkling of compost into the aeration holes you make in the soil and leaving lawn trimmings to fall through the blades of grass and onto the soil where the lawn is growing. The trimmings decay very quickly. They are often referred to as "green manure" for their high nitrogen content.

Q: I'm a little confused as to the timing of the lawn tasks you recommend -- aerate, compost, overseed, fertilize and weed control? Can I do some of these on the same day? -- M. Gutierrez

A: You cannot seed a lawn and spread weed control material at the same time; the latter will keep grass seed from germinating. Do pre-emergent weed control in late winter, early spring when the soil is moist but not soggy. All other tasks can be done on the same day in the following order: aerate, sprinkle fine textured compost into holes, overseed and fertilize. (Fertilizing is optional pending a soil test.) Keep the soil moist as long as grass is growing. Remove accumulations of leaves from young grass.

Q: I have some variegated winter daphne on the north side of my home within the shadow of the building most of the day. They have been there for at least 40 years. I would like to plant two new plants I recently purchased in a location where we can enjoy their lovely odor -- either near an outside deck or preferably in the front of the house. How should I proceed? -- J. Thompson

A: A variegated daphne that has thrived in its location for 40 years is located in the perfect microclimate and soil. Try to recreate the conditions of these shrubs, and find out their botanical name to be sure you use a similar plant. We grew a variegated daphne (Daphne X burkwoodii "Carol Mackie") for about 15 years before it succumbed to last winter's snow. Carol Mackie is one of the hardier daphnes. A protected, lightly shaded location in soil that is low in organic material seems to be the best environment.

Thanks for writing and for reading Green Scene.

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

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