Answer: I don't have any scientific data on hand to prove that marigolds are effective against pests, but there is certainly anecdotal evidence. Scientific studies have been done that show that certain harmful root nematodes are deterred by certain marigolds; Burpee sells a marigold named Nema-gone which has been bred for just this purpose.
I've found that if I monitor plants regularly and see aphids when they first appear, that I can control them with blasts of water from the hose. Spray underneath the leaves, and all those hard-to-reach crannies they seem to get into.
Also consider planting flowers and herbs that attract beneficial insects, such as lady beetles, which feed on aphids. Plants from the Umbelliferae (carrot) family, such as dill, parsley and fennel, are lady beetle favorites.
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