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.
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.
Hold on a second, however, before you blanket your garden in marigolds. Some of these claims have been blown out of proportion.
Those marigolds that helped repel cabbage worms: They also stole water and nutrients from nearby cabbages. So which is better? Stunted cabbages, or those with some leaves lacy from caterpillar feeding?
Marigolds, especially the Gem varieties, also are a favorite food of slimy slugs and Japanese beetles.
As such, they have been used to stop Japanese beetle damage — by attracting the beetles away from other garden plants.
Of course, such schemes commonly backfire by attracting more pests to the area than would have been there otherwise.
If you really want their pest-controlling benefits, blanket your garden with oodles of marigolds.
British studies showed that African marigolds killed weeds such as ground ivy and bindweed, but the marigolds were planted densely and early in the season, then allowed to grow 5 feet tall.
Might not any tall, dense growth do the same?
Similarly, marigolds suppress nematodes only when the marigolds are grown as a cover crop, that is, planted thickly and allowed to grow for many weeks.
To sum up, marigolds seem to have little actual benefit in suppressing disease and aboveground insect pests, except perhaps to woo certain insects away from other plants. Be wary of such claims as, “I planted marigolds in my bean patch and did not have any beetles to speak of, while my neighbor’s bean plants were devoured by Mexican bean beetles.” Was this gardener growing the same bean variety as the neighbor? Were soil conditions the same? Did he or she perhaps forget about the insecticide also applied? It happens.
Below ground, marigolds do have some benefit — on nematodes, at least. However, you have to plant masses of marigolds to get this benefit and not every garden has nematode problems.