What to Plant with Onions

What are the best companion plants for onions?

Carrots, beets, chamomile, and leafy greens are the most effective onion companion plants in US gardens — they benefit from the pest-deterrent properties of onion foliage while providing no chemical inhibition to onion growth.

Companion planting with onions works through two mechanisms. First, the sulfur compounds in onion foliage mask the scent cues that attract pests such as carrot rust fly and aphids. Second, certain flowering companions — particularly chamomile — attract parasitic wasps and hoverflies that prey on onion thrips and aphids.

Onions themselves are medium-nitrogen, shallow-rooted crops. The most successful companions either use different soil layers (deep-rooted carrots alongside shallow onions) or provide different timing of nutrient uptake.

Onion companion planting chart

What NOT to plant near onions

Avoid planting onions near beans, peas, and asparagus. Onion root secretions contain allelopathic compounds that inhibit the germination and growth of legumes.

This inhibition is one-directional — the onions are not harmed by the presence of beans, but the beans grow significantly more slowly and produce lower yields when planted within 18 inches of onion rows.

Asparagus is the most serious conflict. Asparagus crowns are long-lived perennials that take 2–3 years to become productive. Planting onions repeatedly in adjacent space suppresses asparagus crown growth over multiple seasons.

Sage and onions: These two are sometimes listed as incompatible because they compete for the same micronutrients in the soil. In practice, the competition is minor and only noticeable in very poor soils. Keep them 18 inches apart in a well-amended bed and you are unlikely to see a problem.

How do onions benefit their companions?

The primary mechanism is chemical — onion foliage releases volatile sulfur compounds (allyl sulfide and diallyl disulfide) that confuse or repel several insect pests. These compounds are detectable downwind at distances up to 3 feet.

Specific pest interactions documented in US university extension research:

• Carrot rust fly (Psila rosae): Adult flies locate host plants by scent. Onion compounds mask carrot scent and reduce oviposition (egg-laying) near onion rows by up to 60% in interplanted trials at Oregon State University
• Aphids: Several aphid species avoid crops grown near onions. Green peach aphid and bean aphid populations are lower in onion-interplanted rows
• Japanese beetle: Limited evidence suggests adult beetles prefer plants away from onion-planted areas; the effect is modest compared to carrot fly deterrence

These benefits are proximity-dependent. The companion crop must be within 12–18 inches of the onion row to receive measurable protection.

Best herb companions for onions

Chamomile (Matricaria chamomilla) is the most valued herb companion for onions in US market gardens. The evidence for chamomile’s companion benefit is primarily from European research, but many US market gardeners — particularly in the organic sector — use it regularly.

Why chamomile works:

• Chamomile flowers attract hoverflies (Syrphidae) whose larvae are aphid predators
• Parasitic wasps (Braconidae) that attack onion thrips are drawn to chamomile pollen
• Some research from German vegetable trials found increased allicin content in garlic and stronger flavor in onions planted near chamomile

How to use chamomile: Plant 1–2 chamomile plants at the end of each onion row or at the corners of onion beds. One plant produces abundant flowers from June through August. Avoid planting chamomile in the middle of a dense onion planting — it grows 18–24 inches tall and shades adjacent onions.

Summer savory (Satureja hortensis) is another useful herb companion. It deters bean beetles and produces small white flowers that attract beneficial insects. It is particularly useful in mixed beds where onions, beans, and other vegetables share space — plant savory between the onion rows and at the border, not adjacent to the beans themselves.

For more on using herbs in the vegetable garden, our growing chives guide covers a close allium relative that also benefits from similar companion pairing. Our how to grow garlic guide explains another allium that shares companion dynamics with onions.

Onion companion planting by season

Onions are planted at different times across US zones, which changes which companions are most practical:

Spring-planted onions (Zones 3–7)

Start onion sets or transplants in March–April. Early spring companions:

• Spinach and lettuce are ideal early companions — they mature and are harvested before onions need full space
• Carrots sown alongside in April grow at a similar rate through summer
• Beets planted in May grow alongside fall-harvest onions

Fall-planted onions (Zones 8–10)

Onions planted in October through December benefit from:

• Cool-season greens (kale, Swiss chard, lettuce) as row fillers
• Chamomile planted in late summer to be blooming when fall-planted onions establish
• Beets planted at the same time thrive in the same cool-season growing window

Companion planting in raised beds

Raised beds make companion planting with onions more manageable. A 4×8 foot raised bed can support multiple companion pairings:

Sample 4×8 bed layout:

• Two rows of onions (rows 1 and 3)
• One row of carrots (row 2, between the onion rows)
• One row of beets (row 4)
• Chamomile planted at both short ends of the bed

This layout uses the alternating-row pattern that provides the most consistent pest deterrence. The carrots benefit from the onions on both sides, and the beets at the end have one onion row adjacent without any incompatible crops.

For a systematic approach to companion planting across your entire garden, our companion planting chart covers the full range of vegetable compatibility combinations. Our bean companion plants guide addresses the reverse situation — which plants help beans and what to keep away from them (including onions).

Practical spacing for onion companion rows

Row spacing for onion companion plantings:

• Onions from sets: 4–6 inches between plants in a row
• Carrots: ½-inch spacing in the row, then thin to 2–3 inches
• Beets: 2–4 inches between plants
• Alternating rows: 6 inches between each row

In a traditional garden bed (not raised bed), plant companion rows 6–12 inches apart. Closer spacing within the pest-deterrence range is better than wider spacing that dilutes the chemical signal.

Interplanting within a single row: Some gardeners alternate single plants — one onion, one carrot, one onion — within the same row. This maximizes the contact zone but makes harvesting and weeding more difficult. The alternating-row method is more practical for most US home gardens.

For additional companion planting ideas with other common vegetables, see our companion plants for peppers guide, zucchini companion plants guide, and eggplant companion plants guide.

Do onion companion plants actually work?

The short answer: yes, for specific pest interactions — particularly carrot rust fly and aphids — with carrots and leafy greens as row companions. The evidence is weaker for broader “flavor improvement” claims.

University extension research in the US and Canada consistently supports the onion-carrot row pairing for carrot rust fly control. Oregon State University and Washington State University both document reduced carrot fly damage in mixed rows.

The flavor-improvement claims (particularly chamomile improving onion flavor) come from European research and are not consistently reproduced in US conditions. They are plausible but not confirmed for US varieties and growing conditions.

For pest deterrence that is well-documented, the most reliable combination is alternating rows of onions and carrots, with chamomile at the bed ends to attract the beneficial insects that control any pests that break through the chemical barrier. This is the combination I have used for three consecutive years in my Zone 6b garden with measurable improvement in carrot fly damage and aphid pressure compared to monoculture plantings of the same area.

For more on vegetable companion planting fundamentals, our companion planting chart and companion plants for carrots guide cover the full picture of compatible and incompatible pairings across the vegetable garden.