Onion benefits significantly from companion planting. The plant's main challenges — Onion Thrips and Onion Maggot pressure, and the pollination needs that drive fruit set — are directly addressed by the right neighboring plants. Below are the companions with the strongest evidence, their specific benefits, recommended spacing, and the plants to keep onion away from.
Companion Planting for Onions
Published: April 21, 2026
Best Companions for Onions
| Companion | Benefit | Spacing |
|---|---|---|
| Carrot | Loosens soil near tomato roots; attracts parasitic wasps | 6 in |
| Tomato | Repels asparagus beetle; shares beneficial insect habitat | 24 in |
| Lettuce | Fast-maturing ground cover between tall plants; shades out weeds | 8 in |
| Beet | Shares shallow root space without competing with alliums | 6 in |
| Chamomile | Accumulates calcium; attracts hoverflies and wasps | 12–18 in |
Plants to Avoid Near Onions
| Avoid | Reason |
|---|---|
| Bean | Onion and garlic sulfur compounds stunt bean nitrogen fixation |
| Pea | Inhibited by onion and garlic root compounds |
| Asparagus | Inhibited by onion family; compete for similar deep nutrients |
Onions in a Raised Bed — Layout
In a 4×8 raised bed, plant onions at 4-inch spacing in rows 10 inches apart (48 to 64 onions total). Interplant carrots in rows between the onions — onion sulfur compounds deter carrot fly, and the different root depths prevent competition. Add lettuce at the bed edges as a fast crop that harvests before onions size up. Never plant beans or peas in an onion bed.
When to Plant Companions with Onions
Plant carrot with onion in the cool-season window — both prefer temperatures in the 55 to 70°F range. Cool-season companions can go in at the same time as direct-sown onion (soil at 35°F minimum) or 1 to 2 weeks ahead of transplants. Herb companions like rosemary and chives are perennials that stay in place year-round. Interplant fast cool-season companions (radish, lettuce) between rows for a quick 30 day harvest before the main crop fills in.
Companion Planting & Onion Pest Control
Onion’s most common pests — Onion Thrips, Onion Maggot, Downy Mildew — are directly addressed by the right companions. Carrot loosens soil near tomato roots; attracts parasitic wasps. Tomato adds a second defense — repels asparagus beetle; shares beneficial insect habitat. These effects are strongest when the companion is planted within 18 inches of the main crop, and when multiple companion species are layered together rather than used singly. Beneficial insects (hoverflies, parasitic wasps, ladybugs) drawn in by companion flowers provide additional control against aphids and caterpillars — flowering companions like dill, borage, and marigold all attract these predators.
Frequently Asked Questions
What is the best companion plant for onion?
Carrot is the best all-around companion for onion — loosens soil near tomato roots; attracts parasitic wasps. Plant carrot 6 in from onion for the strongest effect. Tomato is a close second — repels asparagus beetle; shares beneficial insect habitat.
What should I not plant near onion?
Avoid Bean and Pea. Bean onion and garlic sulfur compounds stunt bean nitrogen fixation. Keep these at least 6 feet from your onion or plant them in a separate bed to avoid the competition and shared pest pressure.
Can I grow onion and bean in the same garden?
Yes, in separate beds. Bean and onion onion and garlic sulfur compounds stunt bean nitrogen fixation, but placing them 6+ feet apart with a buffer of other crops usually prevents problems. Rotation is also important — don't plant the same family in the same bed two years in a row.
How far apart should companion plants be from onion?
Pest-deterrent companions like carrot work within 12 to 18 inches of onion. Nitrogen-fixing legumes work at 18 to 24 inches. Trap crops (nasturtiums, for example) need 18+ inch spacing so pests congregate on the trap rather than the main crop. Flowering companions for pollinator attraction can be anywhere in the bed — their bee-attracting effect reaches 10 to 20 feet.
Does companion planting help with onion thrips on onion?
Yes, for the specific companions that target Onion Thrips. Carrot and Tomato are documented onion thrips deterrents and should be planted within 18 inches of onion. Companion planting is one part of integrated pest management — combine it with floating row covers during peak onion thrips season and hand-picking for best results.