Q: French marigolds have been shown to control root knot nematodes in small areas. Marigold root extract is available from China. Have any studies been done using it for nematode control ?

A: In my view, the research is inconclusive. Yes, marigold roots produce a chemical which can aid in the reduction of root-knot nematodes and other organisms, such as fungi, bacteria, insects, and some viruses. African and French marigolds are the most commonly used species. But I could not find any research that proves that root extracts are definitively effective. A better method of controlling nematodes is to plant a block of marigolds two months before planting nematode-susceptible vegetables like beans, okra and tomatoes. Till the marigolds into the soil before planting. Research is clearer that living marigolds planted en masse can suppress nematodes.