Plant Winter Rye - If your bed is particularly infested you may want to try a corrective cover crop during the winter. Plant a cover crop of winter rye in the fall and till it under in early spring to reduce root-knot nematode populations in the soil. Nematodes enter the roots and die because they cannot develop further. When the rye is tilled under, it decomposes, releasing an organic acid toxic to the pests. Rye also provides a cover for the soil in winter, adds green matter when tilled in, and conditions the soil.
Soil Solarization - Solarization has been effective in controlling nematodes in the warmer southern and western states. This is a matter of plowing or tilling deeply during hot weather, super-soaking the soil, covering the area with clear plastic, and leaving it on until the soil temperature heats up enough to destroy the nematodes, as well as many weed seeds. (See chapter 7 for more information on solarization.)
Intercropping or Companions - You can grow repellent plants to discourage nematodes. Certain French marigold (Tagetes patula) cultivars are effective in controlling harmful root-knot nematodes. The marigolds act as a trap crop, allowing the nematodes to enter their roots, where the nematodes appear to be trapped without being able to reproduce. Marigolds effectively suppress nematodes by releasing a nematocidal substance from their roots. French marigold varieties, such as Tangerine and Park's Nemagold, work best.
Plant the nematode-infested area of the garden with nothing but marigolds. Let them grow, and then turn them under at season's end. The roots will decay in the soil. Once the plant matter has broken down, you can plant the usual crops again. The larvae that do penetrate the roots can't lay eggs. Keep weeds under control so the impact of the marigolds is not diluted.
Roots of plants such as some mustards and grasses, rattlebox, smartweed, and wild chicory are also toxic to some nematodes. Some species of chrysanthemum (in particular, Chrysanthemum coccineum and C. pyrethrum) reduce harmful nematodes in the soil.
Insect Predators and Parasites Found in the Garden - Predatory mites are important controllers of harmful nematodes. Springtails are also excellent nematode controllers. Tardigrades and Earthworms do their share as well.