Onions and Asparagus
Beneficial Pair — Good Match
This is inherited advice rather than a measured result - the scent-masking idea is plausible on its face, but the beetle's known overwintering habit points to garden sanitation as the more consequential factor, something no companion plant addresses.
Mechanism: Pest Masking (Scent Confusion)
Onion's sulfur-compound scent is traditionally credited with masking the cues asparagus beetle uses to locate asparagus ferns. Asparagus beetle actually overwinters in old, dead fern debris left standing near the plant, which means fall cleanup of that debris is a more direct intervention than anything a neighboring crop's scent can offer.
This pairing's traditional scent-masking claim competes with a much better-understood fact about how asparagus beetle actually survives the winter.
Sanitation Beats Scent Here
Asparagus beetle overwinters in the dead, standing fern growth left behind after asparagus dies back for the season, which is a documented part of its life cycle. Removing that debris each fall interrupts the beetle's overwintering site directly, while onion's scent-masking effect on the beetle's host-finding remains an untested extension of a claim made for other crops.
Because asparagus is a long-lived perennial bed rather than an annual planting, onions here function more as a permanent bed-edge companion than a rotating crop - a practical difference from most allium-pest-masking pairings on this site, where both partners are typically annuals planted fresh each season.
Spacing
Asparagus is a long-lived perennial bed that shouldn't be disturbed - plant onions at its outer edge, 12-18 inches out, rather than tilling in among established asparagus crowns.
Watch For
Cut back and remove dead asparagus fern growth every fall rather than leaving it standing over winter - this single practice does more against asparagus beetle than any companion planting choice.
Frequently Asked Questions
My asparagus bed is over ten years old - does the 12-18 inch onion spacing still apply?
Treat it as a minimum, not a fixed number. A mature asparagus bed's crowns and roots spread outward considerably beyond where they sat in the first few seasons, so the safe margin for tilling in onions at the edge of a decade-old bed is wider than it would be around a bed only two or three years old. Dig shallow furrows for onion sets rather than tilling deeply, and stay clearly outside the fern's drip line rather than measuring from where the bed's edge used to be.
Should I replant onions around the asparagus bed every year?
You can, since onions are annual, but the asparagus itself stays put for many years - treat the onion planting as a yearly addition to a permanent perennial bed rather than something that dictates how the asparagus itself is managed.