What Companion Planting Actually Is (and Isn't)
Companion planting is the practice of growing specific plants near each other because they benefit one another — through pest deterrence, pollination support, nitrogen fixation, or simple space efficiency. It is not magic. It is not a guarantee. But after six seasons of deliberate companion planting trials in our raised beds, we can tell you that the right pairings genuinely reduce pest pressure and improve yields.
The key word is "raised beds." Most companion planting charts are written for row gardens with 3-foot spacing between crops. In a raised bed, plants are 6–12 inches apart. That proximity magnifies both the benefits and the risks of your pairings. Choose well and your plants shelter each other. Choose poorly and they compete for the same nutrients and invite the same pests.
The Science: Why Companion Planting Works
Nitrogen Fixation
Legumes (beans, peas, clover) host Rhizobium bacteria on their roots. These bacteria convert atmospheric nitrogen into plant-available ammonium. When the legume dies or is cut, that nitrogen is released into the soil for neighboring plants. This is not folk wisdom — it is well-documented microbiology. A single pole bean plant can fix 50–100 pounds of nitrogen per acre per season.
Pest Confusion
Many garden pests find their host plants by smell. Cabbage moths locate brassicas by detecting glucosinolates in the air. When you interplant brassicas with strong-smelling herbs like dill, thyme, or sage, the overlapping scent profiles confuse the moths and reduce egg-laying on your kale and broccoli. Studies from the University of Reading showed a 30–60% reduction in cabbage aphid populations when brassicas were interplanted with clover.
Trap Cropping
Some plants attract pests away from your main crop. Nasturtiums are famous for drawing aphids to themselves and away from tomatoes and beans. Blue Hubbard squash lures squash vine borers away from your zucchini. You sacrifice the trap crop to protect the cash crop.
Pollination Support
Flowering herbs and companions attract pollinators to the area, which benefits your fruiting vegetables. Basil flowers, borage, and sweet alyssum bring bees directly to your tomatoes and peppers. More bee visits means better fruit set and larger harvests.
Good Companions: Pairings That Work
| Main Crop | Good Companions | Why It Works |
|---|---|---|
| Tomatoes | Basil, carrots, marigolds, borage | Basil may repel whiteflies and thrips. Marigolds deter root-knot nematodes. Borage attracts pollinators and predatory wasps. |
| Peppers | Basil, onions, spinach, carrots | Onions deter aphids. Spinach acts as a living mulch, shading soil and reducing moisture loss. |
| Beans (bush or pole) | Corn, squash, carrots, beets | The classic "Three Sisters" — beans fix nitrogen, corn provides a trellis, squash shades the soil. |
| Lettuce / greens | Radishes, chives, strawberries, tall crops | Tall crops like tomatoes and corn provide afternoon shade that prevents bolting in summer heat. |
| Carrots | Onions, leeks, rosemary, sage | Alliums confuse carrot rust fly. Rosemary and sage repel multiple flying pests. |
| Brassicas (kale, broccoli, cabbage) | Dill, thyme, chamomile, onions | Dill attracts predatory wasps that eat cabbage worms. Thyme confuses cabbage moths. |
| Cucumbers | Beans, peas, sunflowers, dill | Legumes provide nitrogen. Sunflowers act as a windbreak and attract pollinators. |
| Squash / zucchini | Nasturtiums, corn, beans, radishes | Nasturtiums trap aphids. Radishes repel squash vine borers when planted around the base. |
Bad Companions: Pairings to Avoid
| Keep Apart | Why |
|---|---|
| Tomatoes + fennel | Fennel releases compounds through its roots that inhibit tomato growth (allelopathy). Keep fennel in its own container, period. |
| Beans + onions/garlic | Alliums stunt the growth of beans and peas. The sulfur compounds interfere with Rhizobium bacteria on legume roots. |
| Tomatoes + brassicas | Both are heavy feeders competing for the same nutrients. Brassicas also attract different pests that complicate management. |
| Dill + carrots | They are in the same family (Apiaceae) and will cross-pollinate. Mature dill can also inhibit carrot growth. |
| Potatoes + tomatoes | Same family (Solanaceae), same diseases. Late blight will jump from potatoes to tomatoes overnight. Also compete for the same nutrients. |
| Cucumbers + aromatic herbs (sage, mint) | Strong aromatic oils from sage and mint can slow cucumber growth. Dill and basil are the exceptions. |
Companion Planting Layout for a 4×8 Raised Bed
Here is a sample layout we use every summer. It fits in a standard 4×8 bed at 12 inches deep and produces tomatoes, basil, peppers, carrots, and lettuce in a single bed.
Layout: Summer Production Bed
- Back row (north side): 3 indeterminate tomato plants (Juliet, Sun Gold, Cherokee Purple) at 24-inch spacing, staked or caged
- Second row: Genovese basil between each tomato, plus a borage plant at one end
- Third row: 3 pepper plants (Jimmy Nardello, Lunchbox Orange, Shishito) at 18-inch spacing
- Front row (south side): Alternating carrots (Nantes, Danvers) and lettuce (Buttercrunch, Red Sails) in 4-inch bands
The tomatoes shade the peppers from afternoon sun in hot climates. The basil flowers bring pollinators to the tomato blossoms. The low lettuce acts as a living mulch for the peppers. And the carrots fill every gap without competing with the heavier crops above them.
The Three Sisters in a Raised Bed
The traditional Haudenosaunee (Iroquois) Three Sisters planting — corn, beans, and squash — works beautifully in raised beds, but you need to adjust the spacing. In a 4×8 bed:
- Plant 6–8 corn stalks (Golden Bantam or Painted Mountain) in the center, spaced 12 inches apart in a block pattern (not a single row — corn is wind-pollinated and needs neighbors)
- Plant 2 pole bean seeds at the base of each corn stalk once the corn is 6 inches tall (Kentucky Wonder or Rattlesnake are reliable climbers)
- Plant 2 squash or pumpkin plants at opposite ends of the bed (Butternut, Delicata, or a small pumpkin like Sugar Pie)
The corn provides a natural trellis for the beans. The beans fix nitrogen for the corn and squash. The squash's large leaves shade the soil, suppress weeds, and reduce moisture loss. It is a 3,000-year-old polyculture system that still outperforms monoculture in raised beds.
Companion Planting with Flowers
Don't underestimate the power of a few flowers tucked into your vegetable bed:
- French marigolds (Tagetes patula): Release alpha-terthienyl from their roots, which kills root-knot nematodes. Plant them every season for cumulative effect.
- Sweet alyssum (Lobularia maritima): A low-growing flower that attracts hoverflies, whose larvae eat aphids. Plant it along the edges of any bed.
- Borage (Borago officinalis): Bees love it. One borage plant in a tomato bed noticeably improves fruit set. It also self-seeds reliably.
- Nasturtiums (Tropaeolum majus): The ultimate trap crop. Aphids flock to nasturtiums, leaving your beans and brassicas alone. Plus the flowers are edible.
Want to build a full pollinator-friendly bed? Our guide to pollinator gardens in raised beds covers everything from plant selection to bloom timing.
Getting the Chart as a Printable Reference
We have condensed all of the companion planting data above into a one-page printable PDF chart. It covers 25 common vegetables, their best companions, and which plants to keep separated. We will send it to you free when you join our newsletter — just reply to the welcome email with "companion chart" and we will send it over.
For specific variety recommendations, check out our guides on the best tomatoes for raised beds and growing herbs in small spaces. And if you need a bed to plant in, browse our pre-cut raised bed kits in white oak and cedar.
Frequently Asked Questions
Does companion planting really work or is it folklore?
Some of it is well-supported by science (nitrogen fixation, trap cropping, marigold nematode suppression). Some is observational tradition that has not been rigorously tested (like basil improving tomato flavor). We plant based on the science first and tradition second, and our pest pressure has measurably decreased since adopting companion planting in every bed.
Can I companion plant in containers?
Absolutely. A 5-gallon container with a tomato and two basil plants is one of the simplest and most effective companion pairings. Just make sure the container is large enough — companion planting does not override the basic need for adequate root space.
What if I planted bad companions together — should I move them?
If they are already established, leave them. Transplanting mid-season causes more stress than a bad pairing. Just note the combination and change it next year. The "bad companion" effect is usually reduced yield, not dead plants.
How close do companion plants need to be?
For pest confusion and pollination benefits, plants should be within 12–18 inches of each other. For nitrogen fixation, legume roots extend 6–12 inches from the stem. In a raised bed, nearly everything is close enough — that is one reason companion planting is more effective in raised beds than in row gardens.
Do marigolds really repel pests?
French marigolds (Tagetes patula) genuinely suppress root-knot nematodes — this is backed by decades of agricultural research. As for above-ground pests, the evidence is mixed. Marigolds attract hoverflies and parasitic wasps, which eat aphids and caterpillars. They do not magically repel all insects, but they do shift the predator-prey balance in your favor.