Tip of the Week: Trap Crops—Working with Nature

Posted in Gardening Tips on June 29 2009, by Sonia Uyterhoeven

Sonia Uyterhoeven is Gardener for Public Education. Join her each weekend for home gardening demonstrations on a variety of topics in the Home Gardening Center.

There are many ways to ward off pests in the vegetable garden. Spraying a crop with an approved, biologically friendly insecticide is one option, but generally that method is used as a last resort. On rare occasions I will use an insecticidal soap in the vegetable garden. More common is an application of commercial hot pepper sauce (to deter rabbits) and BT (Bacillus thuringiensis), a harmless, soil-dwelling bacterium that demoralizes cabbage worms.  

My preferable method of dealing with pests in the vegetable garden is to scout for pests on a weekly basis and to deal with any potential problems through more garden-friendly means, such as hand-picking pests off plants before they balloon into real trouble.

Preemptive measures such as row covers provide a physical barrier to prevent pests from reaching their target. I keep cutworms away from my young tomato plants by wrapping a small piece of a glossy magazine cover around the plant’s base; the cutworm will not gnaw through this protective shield.

Companion planting is a traditional practice of warding off pests. Everyone has their preferred combinations. Plants with strong fragrances are supposed to confuse many pests and prevent them from finding target crops. The classics are marigolds (Tagetes, pictured at right) and scented geraniums (Pelargonium).

My favorite marigold is the signet marigold (Tagetes tenuifolia) with its sweet lemony fragrance. While the odor of many marigolds deters pests, it is also overwhelming for the gardener. The ‘Gem Series’ has an inviting fragrance. My favorite scented geranium is the peppermint geranium (Pelargonium tomentosum) with its velvety leaves.

Similar to companion planting is the notion of trap crops. Instead of warding off potential pests, trap crops entice the pests and either destroy them or act as a sacrificial lamb and bear the brunt of the pests’ destruction. An example of this is the planting of four-o’ clocks (Mirabilis), scented geraniums (Pelargonium), or larkspur (Consolida) near roses. The trap crops purportedly act as decoys, attracting rose-loving Japanese beetles to eat their poisonous leaves.

In the commercial trade, vegetable growers often use the method of perimeter trap cropping as a means of protection. It is like building a fortress wall around the field as the trap crops either kill the pest or retain it within the border so that it doesn’t stray to the main crop.

Growers often surround their cabbages (Brassica oleracea [capitata group]) with collards (Brassica oleracea) to protect the cabbages from attack of a particular moth. A field of cabbages is completely surrounded with two rows of collards. The moth will congregate in the collards. If the population gets too large, then farmers introduce a parasitic wasp (a beneficial insect that preys on the pest) as additional help to prevent the moth from journeying to the cabbage.

Similarly, to prevent pepper maggot infestation, cherry peppers (Capsicum annuum) are used to surround bell peppers (Capsicum annuum). Cucumber beetles and squash vine borers are kept away from yellow summer squash (Curcurbita pepo) by border of Blue Hubbard squash (Curcurbita maxima).

Many insects don’t like to land on soil and will look for something nice and green to use as a landing strip. Tests have shown that if you plant clover (Trifolium, a good cover crop) around cabbage then cabbage root flies will have a difficult time finding the right space to lay their eggs. They will land on the clover, realize they are on the wrong plant, try again, and then finally get discouraged by not reaching their goal (the cabbage).

Mother Nature provides us with many wonderful means of tending our gardens.