Vegetable
Bulb onion
Allium cepa · Amaryllidaceae
Also called: Onion, common onion, dry bulb onion
Bulb onion (Allium cepa) is a moderate-water vegetable well suited to Tucson and the low desert. It thrives in full sun, with a slow growth rate.
Bulb onion at a glance
- Water use
- Moderate (established)
- Sun
- Full sun (6+ hours); maximum sun drives both top growth and bulb sizing.
- Mature size
- Tops 12-24 in tall; bulbs 2-4+ in depending on variety.
- Growth rate
- Slow
- Bloom
- White to greenish-white globe (biennial; flowers only if it bolts - undesirable in bulb crops), Grown for bulbs, not bloom; harvest late spring/early summer in Tucson. Bolting (flower stalks) is a defect that hurts storage.
- Cold hardiness
- Cool-season; frost-hardy and overwinters in Tucson. Critically, choose SHORT-DAY (or intermediate-day) varieties for Tucson's latitude - long-day types won't bulb properly here.
- Soil
- Loose, well-drained, fertile loam rich in organic matter; remove rocks/clods and amend caliche so bulbs expand. Tolerates slightly alkaline soils.
- Native range
- Central Asia; not native to the Sonoran Desert.
- Best used as
- Cool-season bulb vegetable, green/bunching onions when pulled young, raised beds and in-ground rows, long-storage crop after curing
- Wildlife
- Flowers (if bolted) attract bees; the pungent foliage deters many browsers.
- Toxicity
- Edible and non-toxic to people. TOXIC to dogs and cats (all Allium species cause hemolytic anemia) - keep harvested and growing onions away from pets.
How to grow Bulb onion in Tucson & the low desert
Watering
Keep soil consistently moist through the growing season - shallow roots need frequent water; in Tucson water deeply about every 2-3 days. Taper off and stop watering when tops yellow and fall over to cure the bulbs.
Fertilizer & nutrients
Heavy nitrogen feeder during leaf growth (more leaves = bigger bulbs); side-dress with nitrogen every few weeks while tops grow, then stop once bulbing begins. Adequate phosphorus and potassium support root and bulb development.
Pruning & care
No pruning. Do not cut green tops during growth (each leaf becomes a ring); harvest when tops naturally flop and yellow, then cure.
Notes
In Tucson plant SHORT-DAY varieties. Sow seed in fall (Sept-Oct) or set out transplants/sets in late fall through winter (Oct-Jan) for harvest in late spring/early summer. Day length triggers bulbing, so variety choice is the single most important decision. Cure harvested bulbs in a dry, shaded, airy spot before storing.
Sources: University of Arizona Cooperative Extension (Pima County) low-desert vegetable planting calendar; Pima County Master Gardeners; Arizona Master Gardener Manual