Onions companion plants — what to plant with onion plant

Best companions for onions

These pairings each have a documented mechanism — volatile-based pest disruption, nitrogen exchange, microclimate effect, or shared cool-season timing. Strong-evidence pairings have peer-reviewed or replicated trial support; moderate pairings have a single trial or extension-service consensus; traditional pairings are popular but under-studied.

• Carrotsstrong

  The most evidence-backed onion pairing — mixed-row plantings reduce carrot-fly egg-laying by up to 70% (University of Bristol). The two crops cross-mask each other's scent and confuse both carrot fly and onion fly.

  Strong evidence — peer-reviewed or replicated trials

• Tomatoesmoderate

  Onion sulfur volatiles deter aphids and whiteflies that target tomato foliage. Plant onions at the bed edges or between tomato rows — they don't compete for the same root zone.

  Moderate evidence — single study or extension consensus

• Peppersmoderate

  Same allium-aphid-thrip mechanism as tomatoes. Onion tops stay short enough to share a bed with peppers without shading them.

  Moderate evidence — single study or extension consensus

• Lettucetraditional

  Onion scent helps mask lettuce from aphids; lettuce in turn carpets the soil between onion plants and suppresses weeds. Same cool-season planting window.

  Traditional pairing — limited formal evidence, observational

• Spinachtraditional

  Cool-season pairing — onions deter aphids that target spinach, and spinach covers the soil between onion plants.

  Traditional pairing — limited formal evidence, observational

• Cucumberstraditional

  Onion volatiles deter aphids and some cucumber-beetle activity. Plant onions on bed edges so the cucumber vines don't shade them out.

  Traditional pairing — limited formal evidence, observational

What to avoid near onions

Onions has measurable conflicts with the crops below — usually through shared disease pressure, nutrient competition, or chemical interference. Plant these in separate beds or with at least 3 feet of separation.

• Peasmoderate

  Onions release sulfur compounds (allicin) that disrupt the Rhizobium bacteria peas rely on for nitrogen fixation. Multiple extension sources recommend 2-3 rows of separation.

  Moderate evidence — single study or extension consensus

• Bush beansmoderate

  Same allium-Rhizobium interference as peas — bean root nodulation drops measurably when onions are grown close. Better separated.

  Moderate evidence — single study or extension consensus

Neutral pairings

These crops have no measurable positive or negative effect on onionsin the published literature — plant them or not, based on space and your zone's timing.

• Basil
• Garlic
• Radishes

How to lay out a onions bed

Pick 2-3 companions from the "best companions" list above and arrange them so the volatile-emitting plants (basil, alliums, aromatic herbs) sit within 12-18 inches of onions. Place any antagonists in a separate bed entirely — or keep at least 3 feet of clearance, with a non-host buffer crop between them.

Timing matters as much as pairing. Cross-check your zone in the USDA hardiness zone map and your sowing windows in the monthly planting calendar before committing the bed plan. For the bed-design fundamentals, see our vegetable garden layout guide; for soil prep and first-year setup, the 5-step vegetable garden plan covers it.

Why these pairings work for onions

Cool-to-warm-season allium. Sulfur volatiles (allicin) deter many soft-bodied pests but disrupt legume root bacteria. Shallow-rooted, low water demand.

Most of the best onionscompanions exploit one of three mechanisms: volatile-priming defence (where one crop's scent compounds switch on the other's pest-defence genes before any insect arrives), scent confusion (mixing chemistries so specialist pests can't locate their host plant), or nitrogen exchange (legumes feeding nitrogen to neighbouring heavy feeders via Rhizobium bacteria). Each pairing above is flagged with the mechanism in play.

Frequently asked questions

What is the best companion plant for onions?

The strongest-evidence companion for onions is carrots. The most evidence-backed onion pairing — mixed-row plantings reduce carrot-fly egg-laying by up to 70% (University of Bristol). The two crops cross-mask each other's scent and confuse both carrot fly and onion fly.

What should you never plant with onions?

Avoid planting peas near onions. Onions release sulfur compounds (allicin) that disrupt the Rhizobium bacteria peas rely on for nitrogen fixation. Multiple extension sources recommend 2-3 rows of separation.

How far apart should companion plants be?

For most pairings on this page, 12-18 inches between species is enough for the beneficial effect (volatile scent overlap, shared microclimate). Allelopathic interference (fennel, walnut) needs at least 4 feet of separation. Disease-sharing pairs like tomato and potato need 10+ feet or separate beds entirely.

Does companion planting reduce the need for fertilizer?

Partially — and only for specific combinations. Legume neighbours (peas, beans) fix atmospheric nitrogen via Rhizobium root bacteria and can deliver 30-50 lb of nitrogen per acre to following crops. That offsets some nitrogen fertilizer in the next rotation but doesn't replace phosphorus, potassium, or micronutrients. See onions's pairings above for the legume options.

When during the season do you plant companions?

Plant companions at the same time as the main crop wherever possible, so the volatile or scent-confusion effect is in place before pest pressure builds. For trap crops (radish for cucumber beetle, nasturtium for aphids), sow 1-2 weeks ahead of the main crop so the trap is already established when pests arrive.

Does companion planting work in containers or raised beds?

Yes — the volatile-based mechanisms (scent confusion, defence priming) work even better in dense raised-bed plantings because the volatile cloud stays concentrated. Nitrogen fixing also works in containers if you inoculate the legume seed with Rhizobium. The one thing containers can't replicate is the root-layer separation that some pairings rely on.

Sources

Pairing claims sourced from peer-reviewed horticultural literature (Plant Cell Reports, Journal of Agricultural and Food Sciences), US Cooperative Extension publications (Cornell, UMN, WVU, UF/IFAS, UVM), the Royal Horticultural Society's vegetable companion guidance, and the evidence reviews maintained at garden-myths.com. Pairings labelled traditional represent gardener consensus without controlled-trial confirmation. Curated by the Growli editorial team, last reviewed May 2026.

Keep going

• How to grow onions — full guide
• The full companion planting chart
• Complete companion planting guide
• Onions plant-care reference
• Monthly planting calendar
• USDA hardiness zone map