Companion planting is one of those gardening topics where folklore and legitimate science have been so thoroughly mixed together that it's hard to know what to believe. Some combinations have real evidence behind them. Others have been repeated in gardening books for fifty years without anyone checking whether they actually work.
Here's an honest breakdown.
The Combinations That Have Real Evidence
The Three Sisters
Corn, beans, and squash planted together is one of the best-documented companion planting systems in North American agriculture. It was developed by Indigenous farmers over thousands of years and it works for specific reasons: the corn provides a trellis for the beans, the beans fix nitrogen in the soil that feeds the corn, and the squash leaves shade the ground, retaining moisture and suppressing weeds. Each plant provides something the others need.
For New England growers, the timing is tight. Corn needs warm soil and a long season. You're planting it around Memorial Day and hoping for a dry September. Stick to shorter-season corn varieties and plant beans two weeks after the corn has a head start.
Nasturtiums as Trap Crops
Nasturtiums attract aphids. That sounds like a reason to avoid them, but the logic is that aphids will colonize the nasturtiums instead of your vegetables. You plant them as a sacrifice crop at the edges of your garden, let the aphids have them, and your brassicas and tomatoes stay cleaner. It works reasonably well if you actually remove the nasturtiums once they're heavily infested rather than leaving them as a permanent aphid nursery next to your food.
Dill and Fennel Attracting Beneficial Insects
Umbellifers — the family that includes dill, fennel, cilantro, and parsley — have flat-topped flower clusters that attract predatory wasps and other beneficial insects that eat pest larvae. Letting some of your dill bolt and flower near pest-susceptible plants is a legitimate strategy. The evidence here is solid. The caveat is that fennel is allelopathic — it inhibits the growth of many plants nearby — so keep it away from your
Tomatoes and Carrots
This one has more going for it than most. Carrots are a root crop that physically breaks up and aerates the soil around tomato plants as they grow. Looser soil around the root zone means better drainage and easier penetration for tomato roots going deep. The carrot tops also provide low ground cover that helps retain moisture around the base of the plant.
The mechanism works in the other direction too. Tomato plants produce solanine, a compound that some research suggests suppresses certain soil pests that would otherwise attack carrot roots. The evidence here is preliminary but more credible than most companion planting claims because there's a plausible biological reason for it.
The practical caveat is timing. Carrots need to go in earlier than tomatoes and take 70 to 80 days to mature. Direct sow them around your tomato transplant locations two to three weeks before your last frost date. By the time your tomatoes are established and spreading, the carrots are filling in underneath. Harvest the carrots before the tomato canopy gets dense enough to shade them out completely.
One thing to watch: don't use this combination if you're applying heavy nitrogen fertilizer around your tomatoes. High nitrogen pushes carrots toward lush tops and poor root development. If your tomatoes need feeding, keep it targeted at the base rather than broadcast across the bed.
The Combinations That Are Mostly Folklore
Marigolds Repelling Everything
Marigolds do have a legitimate use: Tagetes species produce a compound called alpha-terthienyl that suppresses nematodes in the soil — but only after being grown as a cover crop for a full season, turned in, and allowed to break down. The idea that planting a marigold border around your vegetable garden repels pests above ground is not well supported. They're beautiful and worth growing. Just don't rely on them as pest control.
The Honest Bottom Line
Companion planting works best when you understand the mechanism behind the combination rather than following a chart. Ask what each plant is actually doing for the other. If you can answer that question with something specific — fixing nitrogen, providing shade, attracting predators, drawing aphids away — the combination probably has value. If the answer is "they're friends," skip it.
Whatever you're growing this season — grow it well.