Marigolds—a flowering plant in the daisy family—are common companions for tomatoes. In the long lore of gardeners, the marigold is thought to do something to help tomatoes avoid pests. New research finds that it isn’t just lore.
Researchers from Newcastle University tested marigolds to see exactly what kinds of gaseous chemicals were released by the plants. The most notable of those compounds was limonene, also the major component of the oil in citrus peels.
In a couple of greenhouse trials, they discovered that pots of marigolds do in fact deter whiteflies, small insects that are usually found on the underside of plant leaves. Whiteflies are significant pests of fruits and vegetables, along with ornamental plants, but when marigolds were nearby, the whiteflies significantly reduces their number.
But that first experiment only applies to preventive measures. What if you’ve already got a whitefly problem? Can you just plop a potted marigold next to your tomato plant?
Another experiment, published in the same article, found that adding an “emergency” marigold has only a minimal effect. But the researchers also created a limonene dispenser, a little bit like an air freshener, that spat out nice citrusy limonene. That dispenser turned out to be much more capable of reducing whitefly populations, even when they’ve been allowed to build up.
The researchers note that actually planting marigolds alongside your tomatoes is a good idea, if you can; aside from looking nice, using another plant as a pest repellent is a far better solution than a pesticide, which kills beneficial insects as well as harmful ones.