Guides
Companion Planting Chart
A useful chart of the classic pairings - plus an honest note on which ones the research actually supports and which are just tradition.
Companion planting is the idea that some plants grow better next to certain neighbors and worse next to others. It is one of the most repeated pieces of garden advice on the internet, and also one of the least tested. Before the chart, the honest part: most specific pairings are traditional knowledge, not settled science.
The honest bit first
University extension horticulturists are blunt about this.Illinois Extensionnotes that "there hasn't been a lot of research done, especially on a garden scale, on companion planting," and that even the famous Three Sisters (corn, beans, squash) has no compelling published evidence that it improves plant productivity or soil quality. Oregon State and University of Maryland Extension programs say much the same: there are centuries of anecdotal support but limited controlled studies.Washington State University Extensiongoes further and actively separates the science-based practices from the pseudoscience that fills popular gardening books.
None of that means companion planting is useless. It means you should treat the pairings as cheap, low-risk experiments rather than rules. The good news: the mechanisms that do have evidence are the ones that matter most.
The four things that genuinely help
- Nitrogen fixation. Peas and beans host bacteria that pull nitrogen from the air into the soil. Real, well-documented, and free fertilizer for the crops that follow.
- Attracting beneficial insects. Flowers such as dill, alyssum, and cosmos draw in ladybugs, hoverflies, and parasitoid wasps that eat aphids and caterpillars. This one has good research support.
- Trap crops. A sacrificial plant lures pests away from your vegetables - for example, Blue Hubbard squash pulls squash bugs off your other squash. Peer-reviewed and effective when done right.
- Living mulch and shade. Dense, low plants shade the soil, cut weeds, and hold moisture. This is the real engine behind square foot gardening's tight spacing.
Companion planting chart
The table below lists the widely-cited pairings. The evidence column is the part most charts leave out. "Supported" means there is real research or a clear physical mechanism. "Traditional" means it is popular lore worth trying but not trusting.
| Crop | Popular companions | The claim | Evidence |
|---|---|---|---|
| Tomato | Basil, marigold, carrots | Basil repels pests and improves flavor; marigold deters nematodes | Marigold-nematode: supported (as a prior cover). Basil-flavor: traditional |
| Carrot | Onion, leeks, lettuce | Onion smell confuses the carrot rust fly | Mixed - some trials help, many do not. Treat as traditional |
| Corn, beans, squash | Each other (Three Sisters) | Beans feed corn, corn supports beans, squash shades weeds | Partly supported - good space use; yield/soil benefit unproven |
| Any vegetable | Dill, alyssum, cosmos | Flowers pull in predators of aphids and caterpillars | Supported (beneficial-insect attraction) |
| Squash, cabbage | Nasturtium | Trap crop for aphids and flea beetles | Supported as a trap crop |
| Beans, peas | Leafy greens, corn | Legumes add nitrogen for hungry neighbors | Supported (nitrogen fixation) |
| Lettuce, spinach | Taller crops (tomato, beans) | Tall plants shade greens and slow bolting in heat | Supported (physical shade) |
| Beans | Avoid onions and garlic | Alliums stunt legumes | Traditional - little scientific support |
How to use companion planting in a square foot bed
In a 4x4 grid, companion planting mostly comes down to smart neighbors per square. Tuck a square of marigolds or dill into a corner to feed pollinators and predators. Put nitrogen-fixing beans near heavy feeders. Use tall crops on the north side so they shade heat-sensitive lettuce instead of your sun-lovers. Rotate plant families between seasons so soil pests do not settle in. Those moves are low-effort and backed by real mechanisms.
For the pairings in the "traditional" column, plant them if you like - they will not hurt - but do not build your whole garden around them. TheUtah State University Extension companion planting overviewis a good, balanced deeper read. When you sketch your own bed, ourbed planner flags traditional companion suggestions as you drop plants in, theplant spacing chart tells you how many of each crop fit per square, and the 4x4 planting plans already arrange friendly neighbors for you.
Frequently asked questions
Does companion planting actually work?
Some of it does and a lot of it is folklore. Four mechanisms have real research behind them: legumes fixing nitrogen, flowers attracting beneficial insects, trap crops pulling pests off your vegetables, and dense planting shading out weeds. Most specific "plant X next to Y" pairings, though, come from tradition and have little or no controlled evidence. Treat the chart as low-risk experiments, not guarantees.
Do marigolds keep pests away from tomatoes?
Partly. There is solid evidence that certain marigolds (especially French marigolds) suppress root-knot nematodes in the soil when grown as a cover the season before. The popular claim that a marigold next to a tomato repels flying pests like whiteflies is much weaker and largely anecdotal. Grow them for color and pollinators and treat pest control as a bonus.
What plants should not be grown together?
The strongest real reasons to separate plants are practical, not mystical: do not crowd tall crops where they shade sun-lovers, keep heavy feeders from competing in the same square, and rotate plant families so soil pests and diseases do not build up. Traditional "enemies" lists (like beans and onions) have little scientific support, so use them as gentle guidance at most.
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