Marigolds in The Vegetable Garden: Controlling Root Knot Nematodes

Root Knot Nematodes

Root knot nematode galls.

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Once a nematode population establishes, it can be very difficult to eradicate. The larvae of the nematode attach to the roots of the plant, and form galls or root knots. These larvae suck the nutrients out of the plant. This can be fatal for new plants; older plants will have reduced yields. Root knot nematodes attack over 2,000 types of plants and are responsible for crop losses globally.

Tagetes erecta – common marigold.

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The root of the marigold contains a compound that kills nematodes, making the plant an important bio-control method for the home gardener. Intercropping and rotating vegetable crops with marigolds are effective methods that provide an alternative to using chemical nematicides in the vegetable garden. Use either Tagetes erecta or T. patula varieties. According to the North Carolina Department of Agriculture and Consumer services, the best cultivars for nematode control are Crackerjack, Flor de Muerto, Bonita Mixed, Gypsy Sunshine, Scarlet Sophia, Single Gold and Tangerine marigolds.

Intercropping is simply growing marigolds in the same beds as the vegetables and fruits. This method is common with home gardeners, and more practical for small space gardens than crop rotation. Crop rotation would rotate the vegetable and marigold crops between two planting areas each year or growing season.

 Intercropping and Rotating Marigolds and Vegetables

1. Have the soil tested before planting and fertilize according to the recommendations. It is important for the soil to be well-balanced. Inadequate or improper fertilization can worsen nematode problems.
2. Plant half the garden with marigolds and half with vegetables. Rows or square foot gardening are the easiest ways to accomplish this. Space the marigolds seven inches apart. A 100 square-foot area requires 300 marigold plants for effective nematode control. For cost reasons, it makes sense to sow marigold seeds.
3. Cultivate around the marigolds early in the growing season to control weeds.
4. At the end of the growing season, harvest as many seed-bearing flower heads as possible for the following year.
5. Till the remaining marigold plants into the soil in the fall.
6. During the winter months, turn the garden soil often to expose nematodes and reduce the number of safe places they can hide during the winter.
7. Repeat the process the following year, except rotate the location of vegetable and marigolds from the previous year.

References:

Root-knot Nematodes: Biocontrol with French Marigold