Companion Planting Guide — What to Plant Together (and What to Keep Apart)

Companion planting has accumulated more folklore than almost any other topic in gardening. Plant basil next to tomatoes and they'll taste better. Put marigolds everywhere and nothing will ever bother your garden again. Grow garlic near roses and the aphids will leave. Some of these claims are backed by real evidence. Many are not.

The honest version: companion planting is primarily about pest management and space efficiency, with some real benefits around pollinator attraction and soil nitrogen. It is not magic. It will not replace good soil, adequate water, or disease management. But done thoughtfully, it can meaningfully reduce pest pressure, make better use of your square footage, and simplify your season.

This guide covers what the evidence actually supports, including the most reliable companion combinations, the pairings you should genuinely avoid, and the mechanisms behind why they work.

The Three Sisters — The Original Companion Planting System

Long before Western horticulture documented companion planting, Indigenous peoples across North America were growing the Three Sisters: corn, beans, and squash together in the same mounds. This is the most thoroughly validated companion system in existence, and understanding why it works reveals the logic behind all good companion planting.

How the Three Sisters work together

Corn grows tall and provides a living trellis. Beans — which are nitrogen-fixers — climb the corn stalks while their root nodules pull atmospheric nitrogen into the soil, feeding both the corn and squash around them. Squash spreads wide along the ground, its large leaves shading out weeds and keeping soil moisture from evaporating. The spiky texture of squash leaves and stems also deters animals like raccoons and deer from trampling through.

Each plant solves a problem the others create. Corn depletes nitrogen; beans replenish it. Bare soil loses moisture; squash covers it. Climbing plants need support; corn provides it. The system is self-reinforcing, which is the hallmark of a genuinely good companion planting combination.

Planting the Three Sisters

Plant corn first and let it get 6–8 inches tall. Then plant beans 6 inches from the corn base. Plant squash at the outer edge of each mound, about 18 inches from the corn. Give the mounds 4–5 feet of space between them — squash takes up more room than most gardeners expect. This system works best in full sun with fertile, well-drained soil; the nitrogen-fixing benefit from beans is most valuable in poor or depleted soil.

Tomato Companions — What Helps and What Hurts

Tomatoes are the most-planted vegetable in home gardens and have attracted more companion planting claims than any other crop. Here is what actually holds up:

Good companions for tomatoes

Basil is the most cited tomato companion, and it earns its reputation — not because it improves flavor (that claim lacks solid evidence) but because basil's volatile oils appear to repel thrips and aphids at close range. Plant it 12–18 inches from tomato stems. It also attracts pollinators and is useful to harvest alongside tomatoes in the kitchen.

Marigolds (French marigolds specifically, Tagetes patula) are among the most research-supported pest deterrents in the garden. Their roots exude alpha-terthienyl, a compound toxic to nematodes — the soil-dwelling pests that attack tomato roots. Plant them densely around the base of tomato plants, not just at the garden border. They also attract hoverflies, whose larvae eat aphids.

Carrots can be interplanted with tomatoes because they occupy different root zones — carrots deep, tomato feeder roots shallow and spreading. As carrots are harvested, they also aerate the soil around tomato roots. Just keep in mind that tomato shade will slow carrot growth somewhat.

Parsley attracts beneficial predatory wasps that parasitize tomato hornworm eggs. Let some parsley bolt and flower; the flat-topped flowers are particularly attractive to these wasps.

What to keep away from tomatoes

Fennel is allelopathic to almost everything — it releases chemicals through its roots that inhibit growth in neighboring plants. Keep fennel isolated in its own container or a dedicated bed far from tomatoes, peppers, and most other vegetables.

Brassicas (cabbage, broccoli, kale) compete strongly with tomatoes and are susceptible to some of the same soil diseases. Keeping them separate also makes crop rotation easier, which is the more important disease-management strategy.

Other nightshades planted too closely — peppers, eggplant, potatoes — share diseases with tomatoes (early blight, late blight, verticillium wilt). Crowding them together creates a disease reservoir. Give each nightshade family member its own section of the garden with good airflow.

Companion Planting Chart

Trap Cropping — Sacrificial Plants That Protect the Rest

Trap cropping is one of the most underused and most effective pest management strategies in the home garden. The idea is simple: plant something pests prefer even more than your main crops, placed where pests will encounter it first. The trap crop absorbs the pest pressure, protecting the plants that matter.

Nasturtiums for aphids

Nasturtiums are the textbook trap crop. Aphids — particularly black aphids and cabbage aphids — are strongly attracted to nasturtium foliage over most vegetables. Plant nasturtiums at the border of your garden, upwind of aphid-vulnerable crops like brassicas, beans, and peppers. Check the nasturtiums regularly; when aphid colonies build up, you can remove and dispose of the heavily infested stems, taking the pest population with them.

An added benefit: nasturtiums are edible (flowers and young leaves have a peppery bite), they attract predatory insects like hoverflies and lacewings, and they require almost no care. They are one of the highest-value plants you can add to a vegetable garden.

Blue Hubbard squash for squash vine borers

If squash vine borers have destroyed your zucchini before, plant a Blue Hubbard squash at the garden perimeter. Vine borers find Blue Hubbard irresistible compared to other squash varieties. They will preferentially attack it, sparing your summer squash and butternut. Once the trap crop shows heavy infestation, remove and destroy the infested vines — do not compost them, as eggs and larvae will survive.

Dill and fennel as trap crops for caterpillars

Black swallowtail caterpillars, which feed heavily on parsley, carrots, and dill, will preferentially colonize dill and fennel when those are available. If you grow dill as a companion crop anyway — and it benefits cucumbers, cabbages, and onions — you may find it naturally concentrates these caterpillars away from your carrot patch. Note that black swallowtails are beneficial pollinators as adults; the practical approach is to grow enough dill and fennel that both the caterpillars and your carrots can coexist.

Alliums as Pest Deterrents Throughout the Garden

Garlic, onions, chives, and leeks are among the most versatile pest-deterrent plants in any garden toolkit. The sulfur compounds in allium foliage and roots are volatile enough to mask the scent of nearby plants from insects that hunt by smell — and many of the worst garden pests, including aphids, carrot fly, codling moth, and Japanese beetle, navigate primarily by scent.

Interplanting alliums throughout the garden rather than growing them in a single dedicated bed distributes this protective effect more broadly. Chives are particularly useful because they are perennial, low-growing, require almost no maintenance, and produce attractive purple flowers that attract pollinators. Tuck them at the base of roses (where they genuinely reduce aphid pressure), at the edges of carrot beds, and between brassica plants.

Garlic planted in fall around fruit trees provides a first line of defense against aphids and various borers the following spring. The timing works well because garlic's foliage emerges early, just as overwintering pest populations are becoming active.

One practical note on garlic as a spray: some gardeners make garlic water (steep crushed garlic in water, strain, and spray foliage) as an aphid and fungal deterrent. The evidence for this is modest but the cost is near zero. It is unlikely to cause harm and may provide some benefit on soft-bodied insects at close contact. It is not, however, a replacement for the structural benefits of companion planting — a properly planned garden bed does more than any spray.

A Note on Scale and Realistic Expectations

Companion planting at home-garden scale works best when thought of as one layer of an integrated pest management system, not the whole system. A few nasturtiums at the garden edge will not stop a severe aphid outbreak during a heat wave. Marigolds will reduce — not eliminate — nematode pressure. The combinations in this guide reflect what the research and broad practitioner experience support; the folklore claims that get repeated without evidence (basil making tomatoes taste better, planting by moon phases) are not included here because they distract from what actually works.

What companion planting does reliably: reduce pest colonization early in the season, attract beneficial insects, make use of vertical and ground space more efficiently, and contribute to soil health through nitrogen fixation and root diversity. That is genuinely valuable. Build your planting plan around it, track what works in your specific conditions, and adjust year over year.