Carrots and Scallions
Beneficial Pair — Good Match
This inherits the same real chemistry as the trial-tested carrot-onion pairing, but with a plant that's harvested small and young rather than as a mature bulb - whether that smaller scent presence produces a comparable rust fly reduction hasn't been tested as far as we know, so we rate it mixed rather than borrowing the full evidence-backed rating.
Mechanism: Pest Masking (Scent Confusion)
Scallions are alliums harvested young, before bulbing, and carry the same sulfur-compound chemistry documented in carrot-onion rust fly trials, but at a smaller, less concentrated scale than a full mature onion bulb. Because scallions are grown at a tight 1-2 inch spacing - tighter than nearly any other allium on this site - they can be interplanted almost directly within a carrot row rather than alongside it, which is a genuinely different physical arrangement than the alternating-row approach used with full-size onions.
Scallions are the closest thing to a true row-mate for carrots among the alliums on this site, simply because their spacing needs are so much tighter than a bulb onion's, letting the two share a row directly rather than needing separate lanes.
A Smaller Plant, Interplanted More Closely
Where carrots and onions are usually recommended in alternating short rows, scallions' compact size and tight spacing make direct in-row interplanting practical - tucking a scallion in every few inches along the same row as your carrot seed, rather than dedicating separate rows to each crop. Whether that closer arrangement produces a stronger or weaker scent effect than the wider onion spacing used in the trial-tested pairing isn't something we can confirm, but it's a genuinely more space-efficient way to combine an allium with carrots in a small bed.
Spacing
Scallions' unusually tight 1-2 inch spacing is close enough to carrots' own 2-3 inches that the two can be sown in the same row, alternating scallion and carrot seed, rather than needing separate adjacent rows the way a bulkier allium like onion typically does.
Watch For
Scallions mature fast, in as little as 60 days, and are typically harvested continuously as needed rather than all at once - pull them individually as you use them rather than waiting for a single harvest date, which keeps the row from becoming crowded as carrots continue sizing up around them.
Frequently Asked Questions
Does interplanting scallions this closely end up competing with the carrots for room, given how tight the spacing already is?
Not meaningfully - scallions are shallow, thin-rooted alliums that take up very little space below the soil line, so they don't draw on the same root zone a carrot's narrow taproot occupies. The tight spacing works specifically because neither plant is competing for the other's root depth, not because either one has given up room it actually needed.