Onion Companion Plants
Allium cepa · onion family · rotation group: allium
The best companion plants for onion are Carrot, Broccoli and cabbage, Lettuce, Beet, and Strawberry. Avoid planting onion near Bush bean, Pea, Asparagus. Of the 10 pairings recorded here, 2 are supported by published research and 8 are traditional practice with no controlled evidence behind them.
What to plant with onion
| Plant | Claimed benefit | Evidence | What the evidence actually shows |
|---|---|---|---|
| Broccoli and cabbage | Onions interplanted with brassicas reduce caterpillar damage. | Research-backed | In an Iowa study cited by extension, thyme, onion and nasturtium each reduced cabbage looper and imported cabbageworm damage in broccoli; note the effect may require a substantial planting of the companion, not a token row. |
| Carrot | Intercropping onions with carrots reduces onion thrips on the onions and carrot rust fly on the carrots. | Research-backed | The 1984 mixed-cropping trial found reduced Thrips tabaci infestation on intercropped onions as well as up to 65 percent less carrot fly damage, with the effect strengthening as the onion-to-carrot density ratio rose. |
| Beet | Beets and onions share a bed without competing. | Traditional | A conventional pairing with no evidence beyond compatible root depth and season. |
| Chamomile | Chamomile improves onion flavor and vigour. | Traditional | A folk claim with no mechanism and no data; flavour effects from neighbouring plants have never been demonstrated in any crop. |
| Lettuce | Lettuce uses the gaps between slow-growing onions. | Traditional | Onions are poor weed competitors, so a fast catch crop is practical, but no interaction has been tested. |
| Strawberry | Onions edging a strawberry bed keep pests down. | Traditional | No strawberry-specific data; the pairing borrows credibility from the carrot result. |
| Tomato | Onions grown under tomatoes deter aphids and spider mites. | Traditional | Widely recommended, mechanistically plausible given documented allium volatile effects on other flies, but untested on tomato pests. |
What not to plant with onion
| Plant | Claimed problem | Evidence | What the evidence actually shows |
|---|---|---|---|
| Asparagus | Onions compete with and weaken an asparagus bed. | Traditional | No data; the practical objection is that asparagus is a long-lived perennial whose crowns should not be disturbed by annual digging for bulbs. |
| Bush bean | Onions stunt beans by interfering with their nitrogen-fixing rhizobia. | Traditional | This is the single most confidently repeated antagonism in garden writing, usually attributed to allicin disrupting Rhizobium, but no controlled trial supporting it was found and the mechanism appears only in secondary garden sources. |
| Pea | Alliums inhibit pea growth. | Traditional | Same unsupported allicin-and-rhizobia story as with beans; several extension companion-planting tables reproduce it, but tables are not trials. |
The honest verdict
Of everything gardeners recommend planting alongside onion, 2 pairings have been tested properly: Carrot, Broccoli and cabbage. Those are the ones worth planning your bed around.
The remaining 5 pairings on this page are traditional practice. They circulate on every companion planting chart online, but we could not find controlled evidence testing them. They are not wrong so much as unverified — most cost nothing to try, and interplanting flowers and herbs has its own well-documented benefit of pulling pollinators and predatory insects into the garden.
Every row above links to the publication it came from. See how we grade evidence.
Growing onion: the numbers
| Sun | Full sun (6+ hours) |
|---|---|
| Water | Moderate |
| Spacing | 4 in apart · 15 in between rows · 9 per square foot |
| Mature size | about 18 in tall |
| Soil pH | 6–7 |
| Days to maturity | 100–120 days |
| Frost tolerance | Hardy — takes hard frost |
| Containers | Yes — 3 gallon minimum |
| Typical yield | one bulb per set, transplant or seed |
| Common pests | onion maggot, onion thrips, cutworms, wireworms |
| Common diseases | Fusarium basal rot, Botrytis neck rot, pink root, bacterial soft rot, downy mildew |
When to plant onion
Zone 6b shownStart seeds indoors: Jan 16 – Jan 30 — 10–12 weeks before your last frost
Transplant outdoors: Mar 20 – Apr 2 — 3 weeks before last frost
Direct sow outdoors: Mar 20 – Apr 2 — 3 weeks before last frost
Expected harvest: Jun 28 – Jul 18 — 100–120 days from planting
Those dates are for zone 6b. Get your own dates by ZIP code →
Varieties worth knowing
seeds- Walla Walla — long-day sweet, large mild bulbs, poor keeper
- Utah Sweet Spanish — long-day, standard large yellow Spanish type
- Yellow Granex — short-day, the Vidalia standard, for southern latitudes
- Evergreen White Bunching — non-bulbing green onion, perennial clumping, ~50 days
Variety names link to seeds on Amazon. As an Amazon Associate we earn from qualifying purchases. These are the varieties named in the extension sources for this crop, not a paid placement — where a source recommended a variety for a specific reason, that reason is the note beside it.
Put it in a bed
The garden planner checks every square against this ledger as you place crops — if you drop onion next to something recorded as a bad neighbour, the square turns red and tells you why.
Records for the plants named above:
Same rotation group (allium) — don't follow one with another:
Onion companion planting questions
What should not be planted next to onion?
How far apart should onion be planted?
Does companion planting with onion actually work?
When should I plant onion?
Sources for this record
- Growing onions in home gardens, University of Minnesota Extension
- Onion, Home Vegetable Gardening, University of Illinois Extension
- How to Grow Onions in Your Garden, Utah State University Extension
- Onion Planting and Spacing, Utah State University Extension
- Vidalia Onion Production Guide (B1198), University of Georgia Extension
Record compiled 2026-07. Spot an error or a study we missed? Tell us and we'll correct the entry.