Nematodes are tiny, microscopic organisms that have a needle-like appendage that burrows into the roots of plants. This interrupts the plants' ability to take up water and nutrients through their roots and the plants develop knobs or swellings on their roots. There is no way for the home gardener to eradicate them using chemical preparations. There are, however, a number of ways to control their population. The greatest injury caused to plants by nematodes occurs in sandy soil.
Rotate the garden to a new area each year, if you have the space, using two plots for the rotation. Plant fescue grass in the fallow garden bed each year as it will reduce the population of nematodes in the bed.
Use care when purchasing transplants. Only those certified as nematode-free or grown in sterilized soil should be planted. Check the roots of purchased transplants for the telltale knobs nematodes leave behind.
Dig or plow up roots to expose them as soon as you finish harvesting crops. The exposed roots will dry out in the wind and sun and reduce the population of nematodes that overwinter in your garden.
Apply organic soil amendments such as peat moss, manure, compost, shredded autumn leaves or chopped straw. These amendments stimulate micro-organisms in the soil that are nematodes' natural predators.
Grow green manure crops to till into the soil as green manure when they are just a few weeks old. These green manure crops of clover, alfalfa or annual ryegrass stimulate nematodes' natural predators.
Plant French dwarf marigolds. They exude a toxic compound from their roots which kills some types of nematodes and traps other types in their roots. Plant a row of marigolds around the perimeter of your garden and interplant them among your vegetables for best results.
Plant resistant or tolerant varieties of vegetables. Seed catalogs list some seeds as "VFN." The "N" indicates that the variety is nematode-resistant or tolerant.
Solarize your garden soil. Just before mid-summer, till the ground in an unplanted vegetable plot and water well. Cover the bed by laying down thick, clear plastic, making sure that it makes contact with the soil. Cover the edges with 6-inches of sand or secure with rocks or bricks. Leave the plastic on the soil for at least 6- to 8-weeks. The sun will heat up the soil and kill nematodes, weed seeds and other soil pests and diseases to a depth of 8-inches. Remove the plastic after eight weeks and plant a green manure crop for the most effective control. This method should give you and your plants at least two growing seasons of relatively nematode-free growing.