Companion Planting Tool
Check which plants help or hinder a crop, and test a specific pairing before you plant it.
Companion planting advice is a mixture of solid observation and garden folklore, and this tool presents it honestly as that: the traditional pairings generations of gardeners have used, flagged where the reasoning is known (root depth, shade, pest confusion, shared diseases) and left as tradition where it is not. What it will not do is dress folklore up as laboratory fact.
Pick a crop to see its traditional friends and foes, and optionally pick a second plant to test the pair directly. The strongest rule in the whole subject is not in any pairing list: never follow a crop with its own family in the same soil, which the Crop Rotation Planner handles.
How to read these lists
The pairings come from traditional companion planting sources, cross-checked across several standard references; where sources disagreed, the pairing was left out rather than guessed. Some entries have clear mechanisms: onions beside carrots confuse carrot fly by scent, tall corn shades lettuce in summer, and the classic bean and corn pairing trades nitrogen and a living pole. Others persist because generations of gardeners swore by them, which is evidence of a kind, just not the laboratory kind.
What actually has trial support:
- Marigolds (Tagetes) suppress certain root nematodes in soil they grow in, the best documented effect in the whole subject.
- Mixed plantings and flowers near crops genuinely help pest control by sheltering predators and confusing host-finding insects.
- The clearest harms are mechanical, not mystical: walnut roots (juglone), vigorous mint, and heavy feeders planted tight against light ones.
- Shared family diseases are the hard rule: tomatoes after potatoes, or cabbage after cabbage, is how soil disease is farmed.
Worked examples
1. Tomatoes and basil. The famous pair: basil is listed as a tomato friend in nearly every tradition (and tomato hornworm confusion is the usual claim), and practically they share water needs and the basil fills the bare soil under the vines.
2. Beans and onions. Kept apart in the lists, with a plausible mechanism: allium root exudates are reported to check legume growth and their nitrogen fixing bacteria. Separate beds cost nothing, so the tradition is cheap to respect.
3. Carrots and lettuce. Friends by structure rather than chemistry: shallow lettuce shades the soil for slow germinating carrots, and is harvested before the carrots need the room. Many of the best pairings are really timetables.
Pairings with a practical mechanism
| Pairing | Verdict | Mechanism |
|---|---|---|
| Corn + beans | Friends | Beans fix nitrogen; corn is the pole they climb |
| Carrots + onions | Friends | Mixed scents confuse carrot fly and onion fly |
| Cabbage + celery | Friends | Celery’s scent confuses cabbage white butterflies |
| Tomato + potato | Keep apart | Same family, same blight; the strongest rule on this page |
| Beans + garlic | Keep apart | Alliums are reported to stunt legumes and their root bacteria |
| Cucumbers + potatoes | Keep apart | Potatoes invite the blight and beetles cucumbers suffer from |
Notice how many strong entries are about shared pests and diseases rather than friendship chemistry. That is where the reliable value of companion planting lives.
Common mistakes
- Planting by the chart and ignoring spacing. A ‘friend’ planted six inches away still competes for light and water. Pairings assume normal spacing, not crowding.
- Expecting marigolds to guard the whole garden. Their nematode effect works in the soil they actually grow in, over a season. A token row at the gate is decoration.
- Breaking family rotation for a pairing. No friendship justifies tomatoes in last year’s potato bed. Rotation outranks companionship every time.
- Mint anywhere near a bed. Mint is a thug that escapes. It belongs in a pot, sunk or standing alone, whatever any chart says about it repelling pests.
Putting the result to work
Sketch the beds with families first (rotation), then use these lists to arrange neighbours inside that plan, then set spacings with the Plant Spacing Calculator. That order (family, friends, spacing) is how experienced gardeners actually plan, whatever the charts imply.
How do I check a companion pairing?
- Pick the main crop to see its full friend and foe lists.
- Optionally pick a second plant to test that exact pair.
- Read the verdict: friends, keep apart, or no strong tradition.
- Check the mechanism column before acting on any pairing that costs you bed space.
- Always let family rotation overrule a pairing.
Is companion planting scientifically proven?
Parts of it are. Nematode suppression by Tagetes marigolds, pest confusion in mixed plantings, and predator shelter from flowers all have trial support. Many specific pairings rest on tradition and observation rather than controlled trials. This tool presents the traditional lists labelled as such, with mechanisms stated where they are known.
What are the three sisters?
Corn, beans and squash grown together, from Indigenous American agriculture: corn provides the pole, beans fix nitrogen and climb it, and squash shades the soil with big leaves, holding moisture and suppressing weeds. It is the most famous companion system in the world and a genuinely practical one in warm climates with room for corn blocks.
Which pairing matters most to get right?
The negative one: keeping families apart across years. Tomato after potato shares blight; brassicas after brassicas share clubroot. Those harms are well documented, unlike most friendship claims, and they are the reason the rotation planner outranks this tool in planning order.
Do marigolds really protect vegetables?
French marigolds (Tagetes patula) grown as a dense crop suppress certain plant parasitic nematodes in that soil over a season, which is documented. A few marigolds as edging are mostly ornamental; claims that they repel insects across a whole bed are much weaker than the nematode effect.
Why is fennel on so many ‘keep apart’ lists?
Fennel is allelopathic: its root and leaf compounds inhibit the germination and growth of many garden plants, and it crosses readily with dill. The standard advice, followed here, is to give fennel its own corner or a pot, away from vegetables.
Pairings follow widely published traditional companion planting references, cross-checked for agreement; mechanisms are stated only where documented. Family disease warnings follow standard plant pathology guidance.
