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.
WEIGH PROS AND CONS
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.
AND NOW, FOR SOME MARIGOLD REALITY
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 anyway, not every garden has nematode problems.
So why are those marigolds in my vegetable beds? ‘Cause they
look pretty.
