Companion Planting Chart: Evidence for Each Pair

The Companion Planting Chart: Five Pairs, Rated by Evidence

Every pair below has a source behind it. The evidence level is a four-step scale. Tested means a field or garden trial measured the effect. Mixed means the results disagree or the effect depends on conditions. Reasoned means a mechanism is described but the sources show no trial. Lore means handed down and never measured.

No pair here sits at the lore level, because a pair needs a source to earn a row. An Illinois Extension article says much of the recommended companion advice has not been tested in a research study. A longer list is not a better list.

For the same chart across 19 vegetables, with the evidence level stamped on each pairing, The Companion Planting Plan prints it as a page you can mark up.

A companion plant is still a plant, and it still takes sun, water and space from its neighbor.

Tomato and Basil: Half the Tomato Plants, a Third More Fruit Each

Tomato and basil, a pair many charts list, is a good example of a mixed result. A Kentucky State University paper by M.K. Bomford reports two summers of garden beds in Morgantown, West Virginia. With basil, each tomato plant gave 27.7 fruits. Alone, it gave 20.6.

The difference is 7.1 fruits, or about 34 percent more per plant (7.1 divided by 20.6). That looks like a win. The paper adds a catch. The mixed beds held half as many tomato plants as the pure beds, and tomato yield did not differ significantly between them.

The author says the result fits two readings. Basil may have helped its neighbor, or the pure tomato beds may simply have been too crowded. Both readings are fair, and the trial cannot choose between them.

The same Illinois article puts the cost of being wrong plainly. If basil does not deter insects, you still have basil.

Squash and a Trap Crop: The One Tested Win, With a Catch

The cleanest result in the chart is a trap crop. A trap crop is a plant bugs prefer, grown to draw them away from the one you want to harvest.

Utah State University Extension ran this in 2021 at two research farms. Blue Hubbard squash went in at the ends of pumpkin rows two weeks before the pumpkins. Pumpkins in plots without the trap crop carried significantly more squash bug adults, nymphs and eggs.

The catch matters. The Utah team hand-picked bugs and removed egg masses from the Hubbard plants, and not from the pumpkins. Their note says the pest has to be treated on the trap crop for the system to work. A trap you never visit is just another squash plant.

Marigolds: A Rotation Crop, Not a Neighbor

Marigolds beside vegetables are a common pairing, and the evidence is thinner than the habit. A University of Florida Extension publication says marigolds are best known for suppressing plant-parasitic nematodes. It also says that planting marigolds between susceptible crops probably does not work, and may damage the crop.

What works is rotation. Florida Extension says marigolds should be planted at least two months before the vegetable crop, on the exact site where the vegetable will go. Some varieties are susceptible and can make the problem worse, so choose a resistant one.

For a home bed, that means a season of marigolds in a bed, then vegetables there. It does not mean a border of marigolds beside the tomatoes.

Corn, Beans and Squash: A Mound With a Plan

The Three Sisters is one of the oldest pairings in companion planting. West Virginia University Extension describes each plant's job. Corn supports the beans. Pole beans capture nitrogen the corn and squash can use. Squash vines shade the soil and slow moisture loss.

The same page gives the layout. Plant four to seven corn seeds 6 inches apart in the center of a mound. When the corn is 4 to 6 inches tall, plant three to four pole bean seeds in a circle about 6 inches from the corn. Plant four squash seeds around the outside ring, 3 to 4 feet apart.

All three are warm-season crops. WVU says to wait until the soil reaches 60 F, which is around mid-May in West Virginia. Illinois Extension adds that plants growing this close together may make harvesting harder.

Herbs and Flowers to Tuck Among Vegetables

One Oregon State University Extension list of herbs and flowers names what each plant is said to help. The list cites no trials, so treat every row as reasoned at best.

The same list says marigolds enhance all crops. The Florida publication above says marigolds in the same bed probably do not work against nematodes. When two sources disagree, plant the herb for its own sake and test the claim in one bed.

In a small raised bed, pick the herb you will cook with. Basil, chives and dill all earn a place on the plate even if the pest effect is small.

Pairs to Avoid: Tomato and Brussels Sprouts, and Cool With Warm

Two rules come from the Kentucky State trial. Tomatoes grown with Brussels sprouts produced 15.2 fruits per plant against 20.6 alone. That is 5.4 fewer fruits, about 26 percent less.

The author blames heat for the poor Brussels sprout crop in all beds, since the plant is a cool-season crop. Its conclusion is that cool-season crops should not be mixed with warm-season crops. That rule covers more pairs than the one he tested.

Beyond that pair, the sources linked here offer no trial for the other pairs found on avoid lists. Treat those lists as guesses until a source backs them.

Fitting Pairs Into a Bed You Actually Have

A pair on a chart still has to fit. The Kentucky State trial set mixed plants at the average of each crop's solo spacing. Tomato at 53 centimeters (21 inches) and basil at 30 centimeters (12 inches) averaged 41.5 centimeters, about 16 inches.

Bomford warns that this method tends to leave mixed beds less dense than pure ones. The table shows it for tomatoes.

Expect fewer plants of the big crop in any mixed bed. Fewer plants each gave more fruit with basil and fewer with Brussels sprout.

Match the big plant to the small one. Basil was the least competitive plant in the trial and took less than a third of the combined land-use measure. Brussels sprout took at least two thirds. A pair of one large and one small plant shares a bed more easily than two heavy feeders.

Keep one job per bed. Pick a pest, a support need or a yield question, then choose the pair that answers it. Pair for every reason at once and you cannot tell what worked.

When a Free Page Is Enough

The honest limit of this chart is its size. It holds five pairs, plus the herb and flower list, because only these had a source open to check. If you plant one or two pairs a year, a free extension page is enough and you do not need a printable.

If your bed holds more pairs, The Companion Planting Plan counts its timing charts from your own last-frost date, with a 108-station NOAA freeze table inside for any US zone. It arrives as a zip of US Letter and A4 PDFs through a download link sent by email, and you print it at 100 percent so a 4 by 8 foot bed plan keeps true scale. Fill in the dated plan before planting, the pair planting log as each pair goes in, the trap crop scouting log on each pest check, and the side-by-side test page and pair verdict at harvest.