Companion Planting: Myth or Reality?
Companion vegetable planting for home and local markets is a historical concept, passed between generations and from neighbor to neighbor. The idea is that the presence of one plant species improves the growth of another. Many recommendations are based on folklore and tradition rather than scientific research. There are, however, valid mechanisms of certain plant associations that can lead to minor pest suppression and greater crop yields.
Documented benefits from plant associations include physical, chemical, and biological alterations that can improve the establishment and survival of desired plant species. The mechanisms of plant interactions are not completely understood, but there are several scientific conclusions that explain the process:
trap cropping occurs when one plant species or variety is attractive to a pest, and is not affected by yield loss, or is used as a “sacrificial” crop. The trap crop is either treated or removed after infestation, and pest pressure on the desired crop is reduced. An example is using Petunia ‘Carpet Blue’ to attract thrips away from tomatoes or peas.
nitrogen fixation by roots of legumes (peas, clover) adds nitrogen to the soil, reducing the need for nitrogen fertilizer, which is helpful for vegetables like corn, tomato, or cabbage.
nurse cropping by using tall or dense plants can protect tender plants through shading and wind breaking. Oats, for example, are used to help establish alfalfa by preventing weeds from establishing.
pest suppression can occur through a variety of mechanisms:
a) Chemical exudates from parts of certain plants may have pesticidal properties. In two studies examining the chemical compounds of marigold (Tagetes spp.), one found that naturally occurring compounds in flowers and foliage were lethal to Mexican bean weevil, and another found that root compounds were lethal to cabbage maggot larvae. Another example is the use of mow-killed rye grain as a mulch. The chemicals from the rye residue prevent weed germination and do not harm transplanted vegetables.
b) Mixing plant species may cause an interference with visual or olfactory orientation of pests to their host plants.
c) Mixing plant varieties of the same species circumvents insects’ ability to adapt to natural plant defenses. For example, mixing field-planted barley cultivars resulted in reduced aphid feeding as compared to a planting of a single cultivar.
d) Refugia (beneficial habitats) provide desirable environments of plant material and shelter for predator and parasitic insect species such as syrphid flies, parasitic wasps, lacewings, and robber flies. One research project showed that the addition of chicken wire plus bedding straw in tilled soybean fields reduced pest injury by 33% by providing habitat for spiders and predatory beetles.
The table shown below provides examples of plant associations that have traditionally been followed. Some home gardeners have reported benefits by mixing certain plant species while others see no difference. Understanding how plant associations work can improve their use and consistency of effects.
-Marion Murray, IPM Project Leader
Halaj, J. 2000. Modular habitat refugia enhance generalist predators and lowers plant damage in soybeans. Environmental Entomology. 29:2, pp 383-393.
Leger, Catalina. 2009. Evaluation of marigolds and entomopathogenic nematodes for control of the cabbage maggot (Delia radicum). Journal of Sustainable Agriculture. 33: 2, pp 128-141.
Ninkovic, Velemir, et al. 2002. Mixing barley cultivars affects aphid host plant acceptance in field experiments. Entomologia Experimentalis et Applicata. 102: 2, pp 177-182.
Weaver, D.K., et al. 1994. Insecticidal activity of floral, foliar, and root extracts of Tagetes minuta against adult Mexico bean weevils. Journal of Economic Entomology. 87: 6, pp 1718-1725.
Table of traditional recommendations of compatible plant associations. Note that these relationships are based on the mechanisms described on the previous page, such as trap cropping, disruption in host-finding, shading, chemical exudates, and habitats for beneficials, not on whether plants "love" or "hate" one another.