French Marigolds Work Against Nematodes
Some, but not all, marigolds suppress root knot nematodes and lesion nematodes. (Don’t use Signet marigolds [Tagetes tenuifolia], for example, since these have the opposite effect and are a host to nematodes, allowing them to proliferate and increase their numbers.)
Varieties of French marigolds (Tagetes patula), that suppress nematodes include ‘Nema-Gone’, ‘Golden Guardian’, ‘Petite Blanc’, ‘Queen Sophia’, and ‘Tangerine’. These varieties have been tested and found effective. However, the way this works best is to plant a whole bed with the marigolds and withhold planting anything else in the bed for one whole summer season. This will rid the bed of nematodes but unfortunately they will come back the following summer when you begin planting susceptible plants once again. (For more on how to use marigolds to rid a garden bed of nematodes, please refer to my organic book on page 296.)
Interplanting crops with marigolds doesn’t work as well. Nonetheless, I always used to plant a row of marigolds surrounding my vegetable terraces in summer. Marigolds are a warm-season, summer flower and need warmth in order to grow. For best results plant them in April or May.
Another way to reduce the depredations caused by nematodes is to add organic matter, such as manure and compost, to the soil. Nematodes are most active in dry sandy soils. Manure and compost help soils to retain water. There will be just as many nematodes in the soil, but plants won’t be so susceptible to them.
Additionally, in the case of tomatoes you might consider purchasing heirloom tomatoes that have been grafted onto nematode-resistant roots. Some traveling purveyors of tomatoes carry grafted ones. Some catalogues carry them also. Resistant rootstocks are also resistant to tomato diseases that are more devastating to heirloom varieties than to disease-resistant ones.