Companion Planting: What Has Evidence and What Doesn't

Some of it works, most of the charts don't. Mixing plant species can reduce pests and attract natural enemies, and trap crops have solid trial data behind them. But the familiar charts pairing specific vegetables as "friends" and "enemies" are mostly untested tradition. A few popular claims, like marigolds protecting tomatoes from nematodes, work only under conditions very different from how gardeners use them.

Where the charts come from

Washington State University horticulturist Linda Chalker-Scott traces companion planting back to older traditions that grouped plants by assigned human-like traits and affinities, not by measured effects. Her conclusion is that "the phrase 'companion plant' is too vague to be useful" and that traditional charts have "entertainment, not scientific, value." She notes that research on traditional aromatic companions, including mints, found "little or no disruptive effect on insect behavior."

That does not mean plants never affect each other. They do, through shade, root competition, nitrogen fixation, soil moisture, and the insects they attract. Chalker-Scott suggests the more precise terms "intercropping" and "plant associations" for interactions that can actually be tested.

What has evidence

Diversity in general

The strongest evidence is for diversity itself, not specific pairings. A synthesis of 552 experiments by Letourneau et al. (2011) found diversified crops had stronger herbivore suppression, more natural enemies, and less crop damage than monocultures. It also found a small but significant reduction in crop yield. A more recent global meta-analysis of 609 studies in Nature Ecology & Evolution (Wan et al.) found plant diversity suppressed pests across farms, grasslands, and forests, and that crop diversification such as intercropping indirectly supported crop production through that pest suppression.

So the two large reviews agree on pest suppression and are not in full agreement on yield. That is typical: the answer depends on which crops, how they are arranged, and how much space the extra plants take.

Trap crops

Trap cropping is the best-documented form of companion planting. You plant something a pest prefers so it gathers there instead of on your main crop. The University of Connecticut's work on perimeter trap cropping surrounds summer squash with a border of Blue Hubbard squash, which cucumber beetles and squash vine borers prefer. Preliminary Connecticut research found the method lowered cucumber beetle and squash vine borer infestations on summer squash by up to 95% and 88%, respectively.

Two caveats matter for home gardeners. First, the commercial version usually sprays the trap crop border to kill the pests it concentrates; an unsprayed border is suggested only when beetle numbers are low. Second, UConn says the method works in garden-size plots of about 20 by 20 feet but becomes more cost-effective as plantings get larger. A trap crop that is not managed can become a pest nursery.

The Three Sisters

The corn, bean, and squash system is cited by both Illinois Extension and WSU as a legitimate plant association: corn supports the beans, the beans fix nitrogen, and squash shades the soil. It is a sound structural and soil system. It is not primarily a pest control method.

Flowers for natural enemies

Flowering plants that feed predatory and parasitic insects are a real mechanism, covered in detail in attracting beneficial insects. This is probably the most useful thing most companion planting advice gets partly right.

Mixed or conditional evidence

Marigolds and nematodes

This is the classic half-truth. Marigolds can suppress root-knot and some other nematodes. University of Florida researchers Krueger, Dover, McSorley, and Wang explain that the effect requires a dense planting grown as a cover crop at least two months before the vegetable crop, in the same spot. French marigold cultivars tend to be effective against the widest range of nematodes, but some cultivars are susceptible to certain nematodes and can make things worse.

Their verdict on the way gardeners usually use marigolds: "Intercropping marigold with other crops to reduce plant-parasitic nematodes does not appear to be effective." A row of marigolds around the tomatoes does not protect them. They also note that marigolds "cannot eradicate nematodes."

Aromatic herbs and mixed borders

Some studies do show pest reductions from specific combinations. Mississippi State University Extension cites research where basil and marigolds together reduced thrips in tomatoes, and where marigold, nasturtium, and onion reduced cabbage looper populations in cabbage. MSU's summary of the overall evidence is "Sometimes yes, sometimes no; and other times maybe."

That is a fair reading. Individual trials exist, but results vary by site and season, and they rarely translate into a reliable rule you can apply in your own garden.

Summary

What to do with this

• Grow a mix of crops and flowers rather than long single-crop blocks, and accept that it may cost a little yield.
• If cucumber beetles or squash vine borers are a yearly problem, try a Blue Hubbard border, and plan how you will deal with the pests it collects.
• If nematodes are confirmed, grow a dense French marigold cover crop in that bed for a season rather than interplanting.
• Do not reorganize your garden around a chart. Where it conflicts with spacing, sunlight, or rotation, follow the basics.

For pests that trap crops and diversity do not handle well, physical barriers are more dependable; see row covers for pest control.

Related

Companion planting is one piece of No-Spray Pest and Disease Management, which covers what has the strongest evidence for keeping pests below damaging levels without spraying.