You have no doubt heard of companion planting as a method of reducing pest insects on vegetable plants. For years, it has been thought that certain aromatic plants repelled insects and should be planted around susceptible plants. British researchers recently put these beliefs to the test, with results that those who hate to weed will love.
Cauliflower plants, susceptible to egg laying by cabbage root flies, and onion plants, susceptible to egg laying by onion flies, were closely surrounded by plants with various attributes such as grayish leaves, strong aromas, etc.
The researchers found that nearly all surrounding plant species reduced egg laying on cauliflower and onion plants, compared to plants surrounded only by bare soil. Two of the best at reducing egg laying on cauliflower were weeds – Chenopodium album (lambsquarter) had 18% of the eggs compared to plants surrounded by bare soil; and Chamaenerion angustifolium (fireweed) had 29% compared with plants on bare soil. Carrots were also good companions, with 25% of the eggs compared to plants on bare soil. On onions, the best companions were Pelargoniums; however, more than half of the surrounding plants tested significantly reduced egg laying.
Overall, researchers concluded that egg laying was reduced most by taller species and by species with green (rather than red or grayish) leaves. Also, there was no indication that the pests were affected by chemicals produced by the plants. Rather, the flies spent more time on the surrounding plants than on the susceptible plants, leaving less time for egg laying on them. Allowing cauliflower and onion plants to be surrounded by taller plants appears to be a good strategy for diverting cabbage root flies and onion flies.
(Hort Ideas, January 2004, 750 Black Lick Rd., Gravel Switch, KY 40328; email: gwill@mis.net. subscriptions are $25 a year printed, $15 by email.)