Visitors to my garden frequently comment, upon seeing a few marigold plants growing in my vegetable beds, that I must have planted them for pest control.
After all, marigolds are supposed to be one of the workhorses of biological pest control. Plant them and plant pests will be killed or — if they are lucky — merely repelled, right? It's an appealing concept: sunny plants that thwart pestilence and blight even as they brighten your garden with blossoms.
How marigolds are pest unfriendly
Marigolds' greatest claim to pest control fame is their effect, documented in numerous studies, on nematodes, which are a kind of worm that in some cases is destructive to plants.
Like other members of the daisy family, marigolds also do their share in feeding nectar to beneficial insects, such as syrphid flies, who prey on aphids and other insects that attack garden plants. Members of the daisy family do not yield nearly as much nectar as flowers of the parsley family — dill, for instance — but daisy family flowers keep the nectar flowing longer.
Other beneficial effects of marigolds are less dramatic or useful. They have been shown to have some slight effect in repelling cabbage worms from cabbage and their kin. And some marigolds, especially a variety called Stinking Roger, repel flies, except that the flies are the kind that bother cows and other domestic animals, not plants.
Read and listen to claims made for marigolds, and you also could press it into service as a fungus killer, an insect killer, even a selective weed killer.