Digging In - Advice on Nematodes in the Garden

By Scott Aker
Special to The Washington Post
Thursday, April 2, 2009

Q: In a recent column, a reader asked about 50-year-old azaleas dying, as well as their replacements. One common cause of the symptoms described is the presence of parasitic nematodes. Do you think the reader should have the soil and roots tested for nematodes?

A: Nematodes afflict a wide variety of plants, and often they are diagnosed only after other causes have been eliminated. They are tiny wormlike creatures that can seriously sicken plants by feeding on their roots, limiting a plant's ability to take up nutrients. Suspect nematodes if the whole plant is weak and has yellow, chlorotic leaves. If some branches are affected and others are healthy, nematodes are unlikely to be the problem.

Azaleas are susceptible to the stunt nematode in particular, but other species may damage them as well. The only way to confirm or rule out a nematode problem is to have a sample of the roots and the soil around them tested. You can contact your local extension service to find out how to do this.

If nematodes are present, you may have to allow the affected section of your garden to lie fallow for a year to reduce the population enough to avoid future problems. Keep the area free of vegetation, including weeds.

With time, the lack of a plant host will drastically reduce the number of nematodes in the soil. No pesticides available to homeowners are effective against nematodes. Contrary to popular belief, marigolds aren't terribly effective in controlling the pests unless planted very thickly and plowed under like a cover crop. Part of their effect is probably because they are not suitable hosts, and growing them as a cover crop is the same as allowing the area to lie fallow. If the azaleas are appropriately sited in partial shade, the marigolds won't grow well enough to function as a cover crop anyway.

Q: What is a floating row cover, and where can I buy it?

A: Floating row cover is a gossamer-thin fabric made of non-woven strands of polypropylene. The fabric is so thin that it allows about 85 percent of sunlight to pass through, but it serves as an effective barrier to insects. Heavier grades are used to give frost protection to seedlings and more-tender plants. It is extremely light and needs to be firmly anchored to the ground with metal pins. Floating row cover really doesn't float above the plants, but it is picked up by even a slight breeze, so it often appears to float above vegetation.

Check your local garden center for row covers and landscape staples. Catalogue sources include Johnny's Selected Seeds (http://www.johnnyseeds.com), Gardener's Supply Co. (http://www.gardeners.com) and Burpee (http://www.burpee.com).

Scott Aker is a horticulturist at the U.S. National Arboretum.

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