Onions

Onions are day-length sensitive in a way most vegetable gardeners don't expect from a root crop - the plant decides when to stop growing leaves and start forming a bulb based mainly on how many hours of daylight it's getting, not on the calendar date or the plant's size, and choosing a variety mismatched to your latitude is one of the most common causes of a disappointing onion harvest.

Long-Day, Short-Day, and Day-Neutral Varieties

Long-day varieties need roughly 14-16 hours of summer daylight to trigger bulbing and are bred for northern latitudes; short-day varieties trigger at only 10-12 hours and are bred for southern latitudes where summer days never get that long. Planted in the wrong region, a long-day onion in the deep South may never receive enough daylight to bulb properly, and a short-day onion planted too far north bulbs prematurely on a small, stunted plant. Day-neutral varieties split the difference and tolerate a wider range of latitudes, a reasonable default choice if you're unsure which category fits your zone.

Sets, Transplants, or Seed

Onion sets (small dormant bulbs) are the fastest and most forgiving way to start onions, transplants split the difference in speed and reliability, and starting from seed takes the longest but offers by far the widest variety selection. Sets are prone to bolting (flowering prematurely) more often than transplants or seed-grown plants, since a set is itself already a small onion that can misread its own maturity, so if bolt-resistance matters more to you than speed, seed or transplants are the more reliable choice.

Companions: Carrot Rust Fly Protection Runs Both Ways

The onion-and-carrot pairing is one of the more solidly evidence-backed relationships on this entire site - onion's strong scent measurably interferes with carrot rust fly's ability to locate carrot foliage by smell, a documented, mutually beneficial effect rather than one-directional folklore. Tomatoes and peppers benefit from the same pest-masking logic against different pests, though with a weaker evidence base than the carrot pairing specifically. Beets and asparagus share the shallow-root/taproot depth-complementarity logic common across several allium pairings, and lettuce and cabbage work as loose, compatible companions with broadly similar timing and needs.

Antagonists: The Legume Family, Consistently

Peas, bush beans, and pole beans are all documented antagonists to onions, and the mechanism is consistent across all three: alliums release compounds into the soil that measurably suppress the nitrogen-fixing bacteria legumes depend on, undercutting the very benefit that makes legumes valuable companions elsewhere in the garden. This is one of the more solidly evidence-backed antagonist relationships on this site, not a thin or speculative one, and it's worth keeping real distance between any allium and any legume bed section as a matter of routine planning.

Curing for Storage

Onions intended for long storage need curing after harvest - drying the whole bulb, tops still attached, in a warm, dry, well-ventilated spot for two to three weeks until the outer skin papers over and the neck fully dries down. Skipping this step and going straight into storage from a fresh harvest noticeably shortens how long the bulbs keep, since undried necks are a common entry point for storage rot.

Bolting and What to Do About It

A bolted onion - one that sends up a flower stalk instead of continuing to bulb - won't develop further as a storage onion and should be harvested and used promptly rather than left in the ground hoping it recovers, since it won't. Cold snaps after an early warm spell are the most common trigger for bolting in onions started from sets, which is part of why seed or transplants are the more bolt-resistant choice in a climate with unpredictable early-spring temperature swings.