Can I plant pea next to carrot?

Yes. Peas fix nitrogen that benefits neighboring root crops.

Can I plant pea next to onion?

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: Pea

Pisum sativum

Spacing: 8 per square foot. Mature height: 3-6 ft.

Plant these nearby

Carrot: Peas fix nitrogen that benefits neighboring root crops.

Keep these apart

Onion: 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 pea next to carrot?: Yes. Peas fix nitrogen that benefits neighboring root crops.
Can I plant pea next to onion?: 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 Pea: which plants to grow next to it, which to keep apart, and the reason behind each pairing.

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