Can I plant peanut next to onion?

Better not to. Alliums are traditionally kept away from legumes on the grounds that they inhibit nitrogen-fixing bacteria, but that specific mechanism is folklore rather than measured for peanuts. What IS measured is that peanut itself is allelopathic and suppresses the growth of other plant species nearby, per NC State's Extension Gardener Plant Toolbox, so give it distance from any neighbor, not just onion.

Companion Planting & Garden Value: Peanut

Arachis hypogaea

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

Plant these nearby

No well-established companions.

Keep these apart

Onion: Alliums are traditionally kept away from legumes on the grounds that they inhibit nitrogen-fixing bacteria, but that specific mechanism is folklore rather than measured for peanuts. What IS measured is that peanut itself is allelopathic and suppresses the growth of other plant species nearby, per NC State's Extension Gardener Plant Toolbox, so give it distance from any neighbor, not just onion. (source)

Common pairings, answered directly

Can I plant peanut next to onion?: Better not to. Alliums are traditionally kept away from legumes on the grounds that they inhibit nitrogen-fixing bacteria, but that specific mechanism is folklore rather than measured for peanuts. What IS measured is that peanut itself is allelopathic and suppresses the growth of other plant species nearby, per NC State's Extension Gardener Plant Toolbox, so give it distance from any neighbor, not just onion.

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

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