Onions and Cabbage
Beneficial Pair — Good Match
This is repeated far more than it's been tested - the scent-masking mechanism is unconfirmed for cabbage moth specifically, though the practical onion-variety preference is a straightforward matter of available bed space rather than a disputed claim.
Mechanism: Pest Masking (Scent Confusion)
Onion's sulfur-compound scent carries the same traditional cabbage-moth-masking claim used across several allium pairings on this site. Cabbage itself is a space-demanding crop with a wide mature footprint, which is why gardeners commonly favor bunching or scallion-type onions over full bulb onions for this specific pairing - a practical detail more than a chemistry difference.
Onions carry the familiar scent-masking claim seen across several allium pairings on this site, but the more useful practical detail here is which onion type actually fits well alongside cabbage's own demanding footprint.
Why Bunching Onions Are the Common Choice
Cabbage needs real room to mature - a full-sized head can require well over a foot of clearance on each side by harvest time. Bulb onions add their own space requirements on top of that, while bunching or scallion-type onions stay compact and slender, fitting into the gaps around cabbage without meaningfully competing for the space a maturing head needs.
This is less about any chemical difference between onion types and more a straightforward matter of fitting two space-hungry crops into the same bed without either one crowding the other out.
Spacing
Bunching onions take considerably less room than bulb varieties, fitting comfortably at 4-5 inches from a cabbage plant without competing for the wide space a maturing cabbage head needs.
Watch For
If using full bulb onions instead of bunching types, give cabbage significantly more clearance - a maturing bulb onion's own space needs compound with cabbage's already-generous footprint in a way that can crowd both crops by midseason.
Frequently Asked Questions
Unlike sage or rosemary elsewhere on this site, do onions need drier soil than cabbage wants?
No - this is actually one of the more compatible pairings on that front. Bunching onions want fairly consistent moisture much like cabbage does, so unlike the Mediterranean herb companions that need a drier corner of the bed, onions can share cabbage's regular watering schedule without needing a separate dry zone set aside.
Is there a real chemical benefit specific to onions and cabbage moth?
Nothing confirmed that we located - the claim follows the same general allium scent-masking pattern used elsewhere on this site without independent testing for this specific pest and crop.