Can I plant onion next to carrot?

Yes. A real, peer-reviewed field study (Uvah & Coaker 1984) found intercropping onions with carrots reduced carrot rust fly damage, on the theory that mixed scents mask the host plant from pests hunting by smell.

Can I plant onion next to tomato?

Yes. Onion scent can deter tomato pests.

Can I plant onion next to bush beans?

Better not to. Alliums are traditionally kept off legume beds on the theory that they inhibit nitrogen-fixing bacteria, but WSU Extension's review of companion-planting research found no scientific basis for that specific mechanism — treat this as garden tradition, not measured evidence.

Companion Planting: Onion

Allium cepa

Spacing: 16 per square foot. Mature height: 1-1.5 ft.

Plant these nearby

Carrot: A real, peer-reviewed field study (Uvah & Coaker 1984) found intercropping onions with carrots reduced carrot rust fly damage, on the theory that mixed scents mask the host plant from pests hunting by smell. (source)
Tomato: Onion scent can deter tomato pests.

Keep these apart

Bush Beans: Alliums are traditionally kept off legume beds on the theory that they inhibit nitrogen-fixing bacteria, but WSU Extension's review of companion-planting research found no scientific basis for that specific mechanism — treat this as garden tradition, not measured evidence. (source)

Common pairings, answered directly

Can I plant onion next to carrot?: Yes. A real, peer-reviewed field study (Uvah & Coaker 1984) found intercropping onions with carrots reduced carrot rust fly damage, on the theory that mixed scents mask the host plant from pests hunting by smell.
Can I plant onion next to tomato?: Yes. Onion scent can deter tomato pests.
Can I plant onion next to bush beans?: Better not to. Alliums are traditionally kept off legume beds on the theory that they inhibit nitrogen-fixing bacteria, but WSU Extension's review of companion-planting research found no scientific basis for that specific mechanism — treat this as garden tradition, not measured evidence.

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Companion planting for Onion: which plants to grow next to it, which to keep apart, and the reason behind each pairing.

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