Companion Planting Guide

What it is

Some plants help each other. Some plants poison each other. Companion planting is the practice of putting the helpers together on purpose.

The mechanisms are real and most are measurable. Pest scent confusion. Nitrogen sharing through root networks. Physical shade and structure. Allelopathic warfare.

Memorising pair lists is the beginner trap. Learn the mechanisms and you can design your own combinations for your soil and your pests.

Why it works

Scent confusion. Many pests find their host by smell, locking onto specific leaf volatiles. Interplant aromatic species like basil, dill, or marigold and the scent profile turns to noise. Trials on brassicas show 30 to 60% drops in cabbage aphid and diamondback moth when crops are interplanted with non-host species. The effect collapses if you only border the bed. Mix them through the rows. This is one of the cheapest forms of integrated pest management.

Nitrogen sharing. Beans and peas fix atmospheric nitrogen in root nodules. Some of that nitrogen leaks to neighbours through mycorrhizal networks during the season, and the rest releases when the legume dies. Plant a legume in every guild.

Allelopathy. The dark side. Black walnut roots and leaves leach juglone, which kills tomatoes, peppers, and blueberries within the root zone. Fennel produces its own broad-spectrum inhibitors. One wrong neighbour ruins a bed.

Classic pairings

Three Sisters. Corn, climbing beans, squash. Corn gives the pole. Beans feed the corn. Squash shades the soil and bristles against raccoons. Plant corn first. Wait until it hits 15 cm before adding beans and squash so the stalk is strong enough to hold a vine. See three sisters for the full mound.

Tomato and basil. Basil releases linalool and estragole. Aphids, whiteflies, and thrips dislike both. Basil also fills the lower canopy that tomatoes leave open, so you get two crops from one footprint. The flavour-enhancement claim is folklore. The pest and space benefits are not.

Carrot and onion. Carrot fly hates onion. Onion fly gets lost in carrot. Alternate rows and both pests give up.

Marigold. The single most useful companion flower. French marigold (Tagetes patula) roots release alpha-terthienyl, which kills root-knot nematodes. Best result comes from growing them as a cover crop and turning them in at season's end. Interplanting still helps. The flowers feed hoverflies and parasitic wasps that hunt aphids and caterpillars. Plant them through the garden, not around it.

Plants that fight

Fennel. The garden bully. It suppresses beans, tomatoes, and most brassicas. Grow it in its own bed or a pot. Dill tolerates fennel but will cross-pollinate with it once both flower, so keep them apart at seed time.

Black walnut. Juglone persists in soil for years after the tree is gone. Tomatoes, peppers, eggplants, potatoes, blueberries, and azaleas all collapse near walnut roots. The affected zone reaches past the drip line. If you must garden there, stick to beans, beets, carrots, corn, and squash.

Brassicas next to strawberries. Brassicas are heavy feeders. They strip the bed and pull in cabbage white butterflies whose larvae also chew on strawberries.

Alliums next to legumes. Onion, garlic, and leek release sulfur compounds that suppress the nitrogen-fixing bacteria in bean and pea nodules. You lose the whole point of the legume.

Build it as a guild

Stop thinking in pairs. Think in groups of three to five plants that cover complementary roles. A working guild has:

A primary crop you want to harvest
An aromatic pest repellent
A nitrogen fixer
A ground cover
A pollinator attractor

Not every guild needs all five. The point is functional diversity instead of a checklist.

Lay out the garden as a polyculture, not as monoculture rows. A tomato-basil-marigold-bean guild this year becomes a brassica-dill-nasturtium-clover guild next year. You keep the diversity and you still rotate.

Tend it

Keep a journal. What you planted, where, how it did, what ate it. Two seasons of your own notes beat any printed pairing chart, because they reflect your soil and your pests.

Layer in succession planting. Sow radishes or lettuce between newly transplanted tomatoes. The quick crop shades the soil during the vulnerable weeks and clears before the tomatoes need the room. After the tomato harvest, sow crimson clover or field peas. The bed feeds itself through winter.

The first season you watch a marigold patch fill with hoverflies while the bed beside it gets stripped by aphids, the whole system clicks. You stop following lists. You start designing.