In the Garden with Pat Rubin

Bee garden writer Pat Rubin writes about everything that grows, from flowers and trees to vegetables and lawns. Pat volunteered for several years as a Placer County Master Gardener and has written about gardening for many national and regional publications. In addition to gardening, she spends time raising and showing miniature horses and miniature donkeys.

In the Garden will include news, events, advice and other gardening tidbits. Pat will also answer reader questions.

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April 23, 2007

Marigolds and Nematodes

For years I’ve heard that planting marigolds in the vegetable garden keeps harmful nematodes from using the vegetable garden as their private cafeteria. The reality is, not exactly.

French marigolds can deter nematodes, but the catch is you have to turn the marigolds into the soil to derive any benefit for the following year. Marigolds work in two ways. Fist they act as a trap crop, luring the nematodes to their roots then essentially keeping them captive. They also release a nematode suppressing substance.

Nematodes are microscopic eel-like worms that feed on other insects, plants or animals. Many of them love the same vegetable plants we do. Tomatoes and peppers seem to be among their favorites. They essentially suck the life out of the plants through the roots. Affected plants don’t grow properly. They can look wilted, stunted, and yellow. The culprit is likely the root knot nematode, although there are scores of others that prefer other plants in the vegetable garden and orchard.

“When you pull the plant up, you’ll see a swelling at the base of the roots,” said Scott Meyer, editor of Organic Gardening. "The nematodes feed and develop within the swellings. They’re more prevalent in some soils than others. They’re definitely more of a problem in sandy soils than in clay soils, but just because you have clay soil doesn’t men you can’t have harmful nematodes.”

Some tomato varieties are nematode resistant - you’ll recognize them by the letter “N” on the plant tag.

Michelle Le Strange, UC Master Gardener adviser, writes in “Maybe Nematodes are Nibbling at Your Roots," “Marigolds have been proven to reduce nematode populations, but all marigolds are NOT alike. Some marigold varieties suppress nematode activity and other marigold varieties foster nematode activity. French marigolds 'Nemagold', 'Petite Blanc', 'Queen Sophia', and 'Tangerine' are effective. Signet marigolds (Tagetes signata or tennifolia) should be avoided. No marigold works well against the northern root knot nematode.

“The effect of marigolds is greatest when grown as a solid planting (like a lawn) for an entire season. When planted with annual vegetables with a row here and a row there, nematode control is not good. Marigold plantings should be mowed before flowers open, so their seed does not become a weed in next year's garden.”

If you sprinkle marigolds in the garden like I do, there's still some benefit. Marigolds also attract beneficial insects to the garden. If you mix marigolds in among the peppers and tomatoes, chances are if you do have pests attacking the plants, they won't get all of them. Plants are less vulnerable if they are spread out among other types of vegetables rather than planted as a monoculture.

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