Q: I read in a book that tansy is good for deterring Japanese beetles. I have roses and other flowers that the beetles have a good lunch on for a few months. I don’t like to use sprays due to my pets and grandkids.
A: For hundreds of years, gardeners have looked for plant combinations that would reduce pests. Despite numerous anecdotes from gardeners, little scientific research has been done to determine the specific combinations in which companion plants really work.
We do recognize that it’s best to avoid planting a mono-culture of a single plant because a diverse environment confuses damaging insects. We do know that some plants attract beneficial insects.
But we don’t know scientifically if tansy repels Japanese beetles or if chives repel aphids or if mint deters cabbage moths.
Knowing how voracious Japanese beetles are, I’m doubtful if any plant scent could keep them off roses. You are welcome to experiment with tansy but don’t get your hopes up.
Better, if you have vegetables, plant flowers among them. Plants like salvia, zinnia, marigold, agastache and cosmos attract pollinators and predators and increase the bio-diversity of your garden.
While I don’t want to discourage you from trying traditional companion planting, don’t expect miraculous results.