The Friendship Between Plants
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Garden guide

The companion tables say who can sit next to whom. This guide adds the rest of garden design: where to put things, what job each plant does, and how a bed changes through the year. Only advice that holds up in university extension research is included.

Observe, then place

  • Watch the site before you plant: where the sun falls in summer and winter, where the wind comes from, where water runs or puddles, where frost lingers.
  • Put what you visit every day (salads, herbs, tomatoes) close to the kitchen door. Put what needs little care (potatoes, squash, perennials) further out.
  • Re-check the layout every season: plants grow, shade moves.

Shape of the bed

  • Keep beds narrow enough to reach the middle without stepping on the soil: about 1 to 1.2 m wide, with paths of about 50 cm.
  • Size the paths for mature plants: they shrink at the peak of the season.
  • No-dig beds (cardboard, compost, straw, planted through) leave the soil life undisturbed. They also suit perennials, which do badly when the soil is dug around them every year.

Layout inside a bed

  • Height: put tall plants (corn, sunflower, staked tomato, pole beans) on the north side so they do not shade the rest. Lettuce, spinach and celery do well in their shade in summer.
  • Roots: mix large taproots (carrot, radish, parsnip) with shallow, spreading roots (corn, onion, lettuce). They then use different layers of soil and compete less.
  • Mix, do not block: place plants of the same species as far apart as possible, so a pest cannot sweep through a whole block of one crop.
  • Pests find their host by landing on green plants and tasting them. Every landing on a leaf that is not their host wastes their time. So flowers and herbs planted inside the bed work; a border around the edge works less the further the crop is from it.
  • Sprawling plants (squash, melon, sweet potato) go on the edge. Climbers (cucumber, peas) go next to a trellis or a tall neighbour.

Stacking in time

  • Fast and slow: sow radish with carrot or parsnip. The radish marks the row and is gone before the slow crop needs the space. Lettuce or radish between young tomatoes and peppers is harvested before the canopy closes.
  • Relay: sow the next crop a few weeks before the current one ends, for example autumn carrots or coriander into a late-summer tomato bed.
  • Succession: follow a cool-season crop (lettuce, peas, spinach, broad bean) with a warm-season one (beans, tomatoes) in the same spot.
  • Summer and winter beds: one bed hosts two sets of crops a year. Frost-tender crops (tomato, pepper, eggplant, cucumber, squash, corn, beans, basil) go in the summer layout. Cool-season crops (cabbage, broccoli, cauliflower, peas, broad beans, spinach, leek, wheat, barley) go in the winter layout. Tolerant crops (carrot, lettuce, onion, garlic, radish, potato) and perennials fit both. The planner only suggests plants that suit the layout you are working on, so it will not put broccoli next to tomatoes. This calendar is a rough guide for temperate and Mediterranean climates.
  • Plants sharing a bed also need similar water.

Rotation between years

  • Do not return the same botanical family to the same spot for 3 to 4 years (2 to 3 is better than none). Pests and diseases that wait in the soil then starve. Each plant page shows its family and the plants that share it.
  • A common order runs from hungry to undemanding: heavy feeders (tomato, corn, cabbage) → light feeders (carrot, onion, garlic) → soil builders (beans, peas, broad bean).
  • Feeding groups differ between sources; treat them as a rough guide.

Give every plant a job

  • Instead of asking only “are these two friends?”, ask “which jobs does this bed still lack?”. A balanced bed has a soil builder (a legume), a flower that feeds beneficial insects, something covering the soil, and a support for any climber.
  • Legumes fix nitrogen and enrich the soil life. They do not feed the neighbour directly: the benefit comes to the next crop.
  • Beneficial insects need variety: a wide mix of flowers feeds many predators, while one or two species does little. Plant them inside the bed.
  • Cereals (wheat, barley, sorghum) can do two jobs. As a cover crop they smother weeds and add organic matter when cut, but they do not fix nitrogen, so pair them with a legume. Grown with peas, a cereal uses the soil more completely than either crop alone (field trials average about 26% more per area), and the peas cling to its stiff stalks and lie down less.
  • Sorghum is a strong weed suppressor, but its roots and fresh residue release a compound that stunted or killed tomato, lettuce and broccoli transplants. Keep them apart, and plant them only after the residue has broken down. It is also a favourite host of the corn earworm, which then moves on to nearby tomatoes, peppers and eggplants.
  • A trap crop is a plant that pests prefer. Put it a little apart from the crop it protects.

What the research says

  • Supported: physical effects (shade, support, ground cover, roots at different depths), flowers that feed beneficial insects and pollinators, trap crops, a diverse bed that confuses pests, and shared pests or diseases between relatives.
  • Not supported in open gardens: plants that “repel” pests by their smell (it can work in a greenhouse), “plants love each other” charts, and yield promises for the Three Sisters in a home garden. We left these out.
  • Marigold is a good example: dense French marigold suppresses root-knot nematodes, but it also discourages beneficial insects, and its scent does not protect nearby crops in the open.
  • Keep-apart rules stay only where a mechanism is documented, such as potato and tomato sharing late blight, or corn earworm moths moving from drying corn silks to nearby tomatoes. Widely repeated claims with no study behind them, such as “onions stunt beans”, were dropped.

📚 Sources

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