Onion Companion Plants

Claim. In UK field trials, carrot fly damage fell only when onions greatly outnumbered carrots (about four onion rows per carrot row) and only until the onions began to bulb; at garden-typical ratios the effect was small or absent.

Mechanism. Onion volatiles are thought to mask carrot odour used by the fly to find hosts; the effect needs a high density of actively growing onion foliage.

Studied in. Field trials with carrot–onion mixtures at several ratios. (field)

Conditions. Sow together; protection ends when onions bulb. About 4:1 onion to carrot rows, interleaved. Onions must be in leafy growth during carrot fly egg-laying periods.

Management. Maintain the high onion ratio; Use a mesh barrier as the reliable control.

Applies when. Rarely practical in a home bed; treat it as a minor supplement to mesh covers.

Limitations. Weak at equal ratios; no protection after onion bulbing, which often coincides with the second carrot fly generation.

Evidence. Tier B: One or more relevant controlled field studies or an authoritative synthesis tied to the crop, effect and arrangement. (field trial).

Sources. Uvah 1984; University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record onion--carrot--pest_suppression v1. Request a correction.

Claim. Beets and onions are commonly listed as good neighbours. They have compatible needs and sizes; no benefit beyond ordinary compatibility has been demonstrated.

Mechanism. Proposed by tradition; not demonstrated in a controlled comparison.

Conditions. Planted together Both plants must be present at the same time.

Applies when. Optional experiment, not a guarantee.

Limitations. Compatible, not beneficial.

Evidence. Tier D: Traditional knowledge or structured community observations without a controlled comparison. (tradition).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record beet--onion--spatial_complement v1. Request a correction.

Claim. Alliums and beans are traditionally listed as enemies, sometimes with the claim that onions inhibit bean growth. No controlled evidence was found in either direction.

Mechanism. Claimed by tradition; no harmful interaction demonstrated.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner does not separate these plants on this claim; it evaluates space, light, water and timing normally.

Limitations. No mechanism identified.

Evidence. Tier D: Traditional knowledge or structured community observations without a controlled comparison. (tradition).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record onion--bush-beans--flavor_or_growth v1. Request a correction.

Claim. Climbing beans and onions are traditionally listed as incompatible. No controlled evidence found.

Mechanism. Claimed by tradition; no harmful interaction demonstrated.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner does not separate these plants on this claim; it evaluates space, light, water and timing normally.

Limitations. No mechanism identified.

Evidence. Tier D: Traditional knowledge or structured community observations without a controlled comparison. (tradition).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record pole-beans--onion--flavor_or_growth v1. Request a correction.

Claim. We did not find a source that establishes a benefit or harm between carrot and onion.

Mechanism. None documented.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner evaluates spacing, light, water and timing normally.

Limitations. The documented effect runs from onion to carrot (carrot fly, conditional). Whether carrots reduce onion fly has no usable evidence.

Evidence. Unknown: No source meeting the inclusion rules. (none).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record carrot--onion--other v1. Request a correction.

Claim. We did not find a source that establishes a benefit or harm between collards and onion.

Mechanism. None documented.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner evaluates spacing, light, water and timing normally.

Limitations. No relationship documented.

Evidence. Unknown: No source meeting the inclusion rules. (none).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record collards--onion--other v1. Request a correction.

Claim. We did not find a source that establishes a benefit or harm between kale and onion.

Mechanism. None documented.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner evaluates spacing, light, water and timing normally.

Limitations. No relationship documented.

Evidence. Unknown: No source meeting the inclusion rules. (none).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record kale--onion--other v1. Request a correction.

Claim. We did not find a source that establishes a benefit or harm between onion and lettuce.

Mechanism. None documented.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner evaluates spacing, light, water and timing normally.

Limitations. Commonly listed as compatible; no evidence found.

Evidence. Unknown: No source meeting the inclusion rules. (none).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record onion--lettuce--other v1. Request a correction.

Claim. We did not find a source that establishes a benefit or harm between mustard greens and onion.

Mechanism. None documented.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner evaluates spacing, light, water and timing normally.

Limitations. No relationship documented.

Evidence. Unknown: No source meeting the inclusion rules. (none).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record mustard-greens--onion--other v1. Request a correction.

Claim. We did not find a source that establishes a benefit or harm between parsnip and onion.

Mechanism. None documented.

Conditions. Planted together Both plants must be present at the same time.

Applies when. The planner evaluates spacing, light, water and timing normally.

Limitations. No relationship documented.

Evidence. Unknown: No source meeting the inclusion rules. (none).

Sources. University of Minnesota Extension 2023

Reviewed August 15, 2026 by Garden Planner editorial team (pending independent horticultural review). Record parsnip--onion--other v1. Request a correction.