Ask a Master Gardener: Companion planting

(Posted April 22, 2021)

By Shirley Kindrick, Madison County Master Gardener

Much has been learned over the years concerning which plants grow well together and which ones are bad neighbors. Through years of scientific study, this information has not only been used to assist big farms but the home gardener, too.

One of the biggest complaints of home gardeners is how to safely reduce pests. Pesticide chemicals often harm more than the pests they are intended to control, including humans, pets and pollinators. The use of companion planting for pest management is a real asset for those gardeners wanting to avoid pesticides.

Trap cropping is one method by which the desired crop is protected from pests. For this method to work, the trap crop must be planted ahead of the desired crop by a few  weeks. By allowing the trap crop to develop earlier, the pests will presumably find this crop, allowing the desired vegetable to grow with less pest problems. It is important to recognize that the desired crop may not be totally pest-free, but the pest problem should be significantly reduced.

For trap cropping to work well, a few guidelines need to be considered.

  1. For highly mobile pests (eg. Colorado potato beetles, squash bugs, harlequin bugs or lygus bugs) or pests that are the offspring of highly mobile insects (eg., cabbage worms or diamond back moth caterpillars), locate the trap crop on the perimeter of the garden, several feet from the desired crop.
  2. For pests of limited mobility (eg., aphids, mites, whiteflies and flea beetles), interplant the trap crop very close to the vegetable crop in the same row or alternating rows.
  3. For pests that have overwintered away from the garden, place the trap crop between the garden and overwintering area.
  4. When pests arrive on the trap crop, vacuum them with a wet dry vac or pick them off into soapy water.

Sometimes, one particular plant of a desired crop becomes the trap “plant” for a specific pest, but generally a specific crop is grown for this purpose.

Examples of plant companions include:

  • Collards protect cabbage from adult diamond back moths.
  • Blue Hubbard protects various squash from squash vine borers.
  • Trailing nasturtiums help deter squash bugs from zucchini.
  • Basil masks tomato plants from thrips
  • Calendula (pot marigold) helps deter aphids from cole crops.
  • The fragrance of sage, dill, chamomile and hyssop helps to deter the egg-laying behavior of adult cabbage worm butterflies on cole crops.

These companion plant partners are not guaranteed to eliminate 100 percent of garden pest problems. Each gardener needs to be actively involved in observing his own garden ecological habitat. There is much trial and error to determine what works best in each plot. By adding some of the suggested partners, pollinators will be drawn into the garden and possibly insect predators of the garden pests, and you will be improving the biodiversity of your piece of land.

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