Companion Planting in Practice: Which Pairs the Layout Planner Flags, and How Far to Trust Them

Companion planting is the idea that some crops do better beside certain neighbors and worse beside others. Some of it rests on practical reasons, such as two crops sharing the same disease or one crop shading another. A lot of it is garden tradition, passed from book to book, with little testing behind it. Both kinds turn up on every companion chart, side by side and usually unlabeled.

The Garden Layout Planner includes a small companion table. When you enter a bed size and tick two to six crops, it lists "Good Companions" and "Keep Apart" pairs alongside how many of each crop fit. This guide shows what is in that table, how the tool reads it, and how much weight to give each answer.

What the table knows

The planner offers 20 crops, but only 10 have companion entries of their own: tomato, basil, lettuce, carrot, onion, bean, cabbage, broccoli, potato and cucumber. The table lists, for each of those, crops it is traditionally grown with and crops it is traditionally kept away from. For example:

• Tomato: good with basil, carrot, marigold, onion and lettuce; keep apart from potato, cabbage, broccoli and kale.
• Bean: good with cucumber, potato and strawberry; keep apart from onion and garlic.
• Cabbage: good with onion, beet and potato; keep apart from tomato, strawberry and bean.
• Potato: good with bean, cabbage and marigold; keep apart from tomato and cucumber.

The other ten crops (pepper, radish, pea, zucchini, kale, spinach, beet, marigold, strawberry and garlic) have no entry of their own. They only show up in a pair when one of the ten listed crops names them. When you tick one of them, the planner lists it under "No companion data" so you know the table has nothing on it either way.

How the planner decides

For every pair of crops you tick, the planner checks both crops' entries:

1. If either crop lists the other as one to keep apart, the pair goes under Keep Apart.
2. Otherwise, if either crop lists the other as good, the pair goes under Good Companions.
3. Otherwise the pair is not mentioned at all.

So a warning always wins. Onion and pea is a good example: carrot lists pea as a good companion, but onion lists pea as one to keep apart, so ticking onion, pea and carrot gives two good pairs (onion and carrot, pea and carrot) and one to keep apart (onion and pea).

Four worked examples in a 4 x 8 bed

The classic: tomato, basil, carrot, onion

Each crop gets 8 square feet. The planner counts 2 tomatoes, 11 basil, 128 carrots and 72 onions, lists four good pairs (tomato and basil, tomato and carrot, tomato and onion, carrot and onion) and nothing to keep apart.

A mixed bed: tomato, basil, carrot, onion, potato, bean

With six crops each gets 5.3 square feet: 1 tomato, 7 basil, 85 carrots, 48 onions, 5 potatoes and 21 beans. The planner lists five good pairs and three to keep apart: tomato and potato, carrot and potato, and onion and bean.

Brassicas: cabbage, broccoli, onion, beet, strawberry, bean

Six good pairs (including cabbage and beet, broccoli and onion, strawberry and bean) and three to keep apart: cabbage and strawberry, cabbage and bean, and onion and bean. Beet and strawberry are listed under No companion data: every pair they appear in comes from another crop's entry.

No data: zucchini, kale, spinach, garlic

The planner finds no pairs either way. Its messages say "No traditional companion pairs found in the table for these crops" and "No keep-apart pairs found in the companion table for these crops", and then list all four under "No companion data", with a note that an empty list is not an all-clear for them. That is the right way to read it: none of these four has an entry in the table, so they have not been checked against each other at all. Garlic is a good example of why it matters, since bean lists garlic as a crop to keep apart; the table simply has no entry of garlic's own.

Which pairings have a practical basis

Sorting the planner's pairs by the reason behind them makes them easier to use:

Shared pests and diseases

Tomato and potato are in the same plant family and are both hosts for late blight, a disease that can spread quickly between them. Keeping them apart, and not following one with the other in the same bed the next year, is common advice for this reason. Cabbage, broccoli and kale are all brassicas and share pests such as cabbage worms; grouping them makes a single row cover protect all of them, which is a practical reason to plant them together rather than apart.

Light and space

Tall crops shade short ones. Tomato with lettuce works partly because lettuce tolerates the afternoon shade a tomato casts in summer, and is often harvested before the tomato fills its space. Any pairing of a tall and a short crop works only if the short one is on the sunny side; the planner does not know which side that is. Check the bed with the Sunlight Requirements Guide and our guide to measuring hours of direct sun.

Different rooting depths and timing

Pairs such as lettuce and carrot or lettuce and radish combine a shallow-rooted crop with one that roots deeper or matures at a different speed, so they compete less for the same space at the same time.

Tradition with thinner evidence

Claims that basil improves tomato flavor, that onions and beans stunt each other, or that marigolds protect everything nearby are widely repeated. Some may have a basis in particular conditions, but they are not reliable rules. The planner includes them because they are standard on companion charts. Treat them as optional.

How to use the results

• Take Keep Apart seriously when the reason is disease. Tomato and potato, in particular, belong in separate beds and in a rotation that does not follow one with the other.
• Treat Good Companions as a menu, not a recipe. They are pairs worth trying, not combinations that guarantee a bigger harvest.
• Check the counts. The planner shares the bed equally and counts by area; its column is labelled "Plants That Fit (area-based estimate)", and that estimate can overstate what fits in whole rows. Check each patch in the Plant Spacing Calculator; our 4 x 8 raised bed guide shows basil dropping from 11 to 8 when laid out in rows.
• Use it for rotation too. The same family groupings that matter for disease matter year to year. Our guide to planning next year's beds uses the planner for a simple rotation.

What the planner does not cover

• Crops outside its list of 20, and companion data for ten of those 20.
• Distance. A pair flagged to keep apart in the same bed may be fine at opposite ends of a long bed; the tool does not know where you put them.
• Pest control. Companion planting is not a substitute for checking plants. If you see damage, the Garden Pest Identifier helps narrow down the likely culprit, and your local extension service can confirm it.
• Your soil, climate and varieties, which usually matter more than which neighbor a crop has.

A note on safety

Marigolds appear on many companion lists, but not every plant called a marigold is the same species, and some ornamental companions are toxic if eaten. Keep flowers grown for companion planting apart from edible herbs and salad when harvesting, teach children which plants are food, and do not eat anything you have not positively identified. Onions and garlic, common companions, are toxic to dogs and cats, so keep culls and trimmings away from pets.