Companion Planting Guide

What Is Companion Planting?

Companion planting is the practice of growing certain plants near each other for mutual benefit. Some combinations improve growth, deter pests, attract pollinators, or make better use of space. Other combinations should be avoided because one plant suppresses or competes with the other.

The practice is ancient. Indigenous peoples across the Americas have grown the Three Sisters (corn, beans, and squash) together for thousands of years. Each plant genuinely benefits from the others' presence.

However, companion planting is also a field full of folklore, anecdote, and unsubstantiated claims. Some widely repeated companion planting advice is based on solid observation and consistent results. Other advice has been passed down through gardening books without ever being tested rigorously.

The goal of this guide is to separate what works from what might work, and to give you the principles to evaluate companion planting claims for yourself.

How Companion Planting Works

There are several distinct mechanisms by which one plant can benefit another:

Nitrogen Fixation

This is the most well-documented companion planting mechanism. Legumes (beans, peas, clover, vetch) host bacteria in their root nodules that convert atmospheric nitrogen into plant-available forms. When legume roots die and decompose, this nitrogen becomes available to neighboring plants.

This isn't instant. A bean plant growing next to a tomato doesn't feed the tomato nitrogen in real time. The benefit comes over time as root material decomposes. The effect is strongest when legumes are used as cover crops or when they've been growing in a bed for at least one season before planting the main crop.

Pest Confusion and Deterrence

Many insects find their target plants by smell. A monoculture of cabbage releases a strong, uniform scent signal that cabbage moths follow easily. Interplanting cabbage with strongly aromatic herbs (thyme, sage, rosemary) disrupts this scent signal, making it harder for pests to locate their target.

This is sometimes called the "confusion effect" and it works for flying insects that use volatile plant chemicals for navigation. It doesn't work for all pests, and it doesn't eliminate pest pressure entirely, but it can reduce pest landing rates meaningfully.

Trap Cropping

Some plants attract pests more strongly than the crop you're protecting. Nasturtiums attract aphids so powerfully that aphids colonize them preferentially, leaving nearby brassicas less affected. Sunflowers can serve as a trap crop for stink bugs.

The catch: you need to manage the trap crop. If you let aphids build up indefinitely on nasturtiums, the population eventually overflows onto everything else. Either remove the infested trap crop periodically or use it to concentrate pests for easier targeted control.

Physical Interactions

Some companion planting is simply smart spatial design:

• Shade provision: Tall plants shade heat-sensitive companions. Corn or sunflowers can shade lettuce in summer.
• Wind protection: Dense plantings shelter more vulnerable neighbors.
• Trellising: Corn provides a living trellis for climbing beans.
• Ground covering: Sprawling plants like squash shade the soil, suppressing weeds and retaining moisture for upright companions.

Root Zone Differentiation

Plants with different root depths access different soil layers. Deep-rooted plants (comfrey, burdock, chicory) draw water and minerals from subsoil that shallow-rooted plants can't reach. The deep-rooted plants bring these resources to the surface (through leaf drop and decomposition), making them available to their shallower neighbors.

Combinations That Work

Tomato and Basil

This is the most popular companion planting claim, and there's reasonable evidence for it. Basil may help repel some tomato pests (particularly whiteflies and aphids) through its volatile oils. Both plants enjoy similar growing conditions (warm, well-drained, full sun). And they taste great together, which means you harvest them from the same area.

The pest-deterrent effect is modest. Don't expect basil to solve a serious pest problem. But there's no downside to growing them together, and the spatial efficiency is excellent.

Three Sisters (Corn, Beans, Squash)

The classic polyculture combination. Corn provides a trellis for pole beans. Beans fix nitrogen that feeds corn (a heavy nitrogen feeder). Squash covers the ground, suppressing weeds and retaining soil moisture for all three. The broad squash leaves also discourage raccoons from reaching the corn (they don't like walking on prickly squash leaves).

This combination genuinely works, but timing matters. Plant corn first and let it reach about 15cm before sowing beans. Plant squash last, when corn is about 30cm tall.

Carrots and Onions

Onions repel carrot fly. Carrots repel onion fly. Each plant's aromatic compounds confuse the other's primary pest. This interplanting combination has consistent observational support from gardeners across many climates.

Brassicas and Aromatic Herbs

Planting thyme, sage, rosemary, or dill near cabbage, broccoli, and kale reduces cabbage moth damage through scent confusion. The aromatic herbs also attract predatory wasps that parasitize caterpillars.

Marigolds as Universal Companions

French marigolds (Tagetes patula) produce root exudates that suppress soil nematodes. Planting marigolds in vegetable beds, particularly where you've had root-knot nematode problems, can reduce nematode populations over a growing season. This effect is well-documented.

Marigold flowers also attract hoverflies and other beneficial insects. Their strong scent may confuse some above-ground pests, though this effect is less well-documented than the nematode suppression.

Combinations to Avoid

Allelopathic Plants

Some plants produce chemicals that suppress the growth of neighbors:

• Black walnut trees produce juglone, which is toxic to many plants (tomatoes, peppers, potatoes, and many others). Don't plant vegetable gardens within the drip line of black walnut trees.
• Sunflower residues contain compounds that can inhibit germination of some seeds. Avoid direct-seeding into beds where sunflower stalks were recently incorporated.
• Fennel produces compounds that suppress many garden plants. Grow fennel separately from your main vegetable beds.

Heavy Feeders Together

Planting two heavy nitrogen feeders next to each other (corn and tomatoes, for example) forces them to compete for the same resource. Pair heavy feeders with nitrogen fixers instead.

Same-Family Groupings

Growing related plants together concentrates the pests and diseases that specialize in that family. Tomatoes, peppers, and eggplant are all Solanaceae; growing them together makes it easier for their shared pests to find them.

How to Evaluate Companion Planting Claims

The companion planting literature is full of claims that have never been tested:

Ask: What's the mechanism? If someone says "plant X helps plant Y grow," ask how. If the explanation involves nitrogen fixation, pest confusion through volatile oils, or physical interactions (shade, trellising), there's a plausible mechanism. If the explanation is vague or involves unmeasurable energies, be skeptical.

Ask: Who tested it? Many companion planting claims trace back to one gardener's observation in one garden in one year. That's an anecdote, not evidence. Look for claims that have been observed consistently across multiple gardeners, climates, and years.

Ask: How big is the effect? Even well-supported companion planting interactions are usually modest improvements, not dramatic transformations. Basil near tomatoes might reduce whitefly visits by 20-30%. That's helpful, but it's not a substitute for other pest management strategies.

Test in your own garden. The best way to evaluate any companion planting claim is to try it yourself with a simple comparison: plant one row with the companion and one without. Observe the difference. Your specific climate, soil, and pest pressure may produce different results than someone else's garden.

Practical Tips

Interplant, don't isolate. Mixing plant families within a bed is almost always better than monoculture blocks, regardless of specific companion claims. Diversity itself confuses pests, improves pollination, and reduces disease spread.

Consider timing. Many companion planting combinations work only if both plants are growing actively at the same time. Marigolds don't suppress nematodes if they're still seedlings when the crop is already infested.

Focus on the big wins. Nitrogen fixation, ground covering, and pollinator attraction are the companion planting mechanisms with the strongest evidence. Prioritize these over more speculative interactions.

Observe your own garden. The best companion planting knowledge comes from paying attention to what happens in your specific conditions over multiple seasons.

In PatternBase, the companion planting explorer shows you which plants work well together and which should be kept apart, based on documented interactions and functional relationships.