In the realm of gardening, marigolds have a great reputation. Whether or not they have officially received these honours, they are well known for warding off many insects and pests. It’s common to spot their chirpy heads nodding in the warm summer sun next to tomatoes, squash, and broccoli. Root-knot nematodes are very successfully repelled by Marigolds. Their roots release alpha-terthienyl, a substance that prevents nematode eggs from developing.