Onion
Allium cepa
Day length, not the calendar, decides when an onion stops making leaves and starts making a bulb.
Pick the right day length
This single decision determines whether you get onions or overgrown scallions. Long-day varieties begin bulbing at 14-16 hours of daylight and suit the northern states; short-day types bulb at 10-12 hours and suit the deep south. Zone 6b/7 sits in the overlap, so intermediate (also sold as day-neutral) varieties are the reliable choice. A long-day onion planted here bulbs late and small; a short-day one bulbs before it has built enough leaf to make a large bulb.
Every leaf becomes a ring
Each leaf produced before bulbing becomes one concentric ring. Bulb size is therefore capped by plant size at the moment day length triggers bulbing. Everything you do in early spring is about growing as much leaf as possible before that switch flips.
Sets, transplants or seed
Sets are easiest and earliest but bolt more readily and are often sold unlabelled. Transplants give the best size and variety choice. Seed started indoors 10-12 weeks before last frost is cheapest with the widest selection but needs the longest lead time. Whichever you use, plant as soon as the soil can be worked - onions tolerate frost well.
Feed hard early, then stop
Push nitrogen through the leaf-building phase, side-dressing every two to three weeks. The moment necks swell and bulbing starts, stop nitrogen completely. Late nitrogen produces thick necks that never cure and rot in storage.
Shallow roots need steady water
Onion roots are sparse and stay in the top few inches, so they cannot chase moisture. Give about an inch a week consistently; irregular water causes splits and doubles. Stop watering once tops begin to fall.
Weeds beat onions
An onion has almost no canopy and loses badly to competition. Mulch or hoe scrupulously from the start - a weedy onion bed produces small bulbs no matter what else you do right.
Bolting
A flower stalk means the bulb has stopped growing and will not store. Sets larger than about three-quarters of an inch, or plants hit by a long cold spell after starting growth, are the usual causes. Pull bolted plants and use them fresh.
Pests and disease
Onion thrips cause silvery streaking in hot dry spells; a hose spray helps. Onion maggot tunnels into bulbs in wet springs - rotate and avoid planting into fresh manure. Downy mildew and botrytis neck rot both follow wet weather and crowded plantings, so favour airflow and avoid overhead watering late in the season.
Harvest and curing
Harvest when about half the tops have flopped naturally; do not bend them down yourself, which just bruises the necks. Lift on a dry day and cure in a single layer somewhere warm, dry and airy for two to three weeks until necks are papery and skins rustle. Uncured onions rot. Trim, then store cool and dry - good keepers last six months, while sweet varieties last weeks and should be eaten first.
- Days to harvest
- 90-120
- Type for 6b/7
- Intermediate day
- Spacing
- 4-6 in
- Row spacing
- 12-18 in
- Sun
- Full, 8 hrs+
- pH
- 6.0-6.8
- Water
- 1 in/week, steady
- Storage
- 1-6 months by type