Onion / Bulb onion (Allium cepa) growing guide

Quick facts

Category: roots bulbs
Difficulty: intermediate
Days to harvest: 90 to 130 days
Harvest type: single harvest then replant
Spacing: 10 cm between plants

Environment

Temperature: 10–26°C
pH: 6 to 7
EC (hydroponic): 1.4 to 1.8 mS/cm
Daily light: 17 to 25 mol/m²/day

Climate and zones

USDA zones: 3 to 10 (winter low around -40°C or warmer)
Frost tolerance: frost hardy (handles regular frost)
Season: cool (spring and fall crops)

Viable growing environments:

outdoor year-round (in zone)
outdoor in growing season (annual)
unheated greenhouse / hoop house
heated greenhouse
indoor (heated home)

USDA zone bounds reflect outdoor year-round survival. Anywhere outside the bounded zone range, this crop still grows as an annual in the warm months (outdoor_seasonal), under cover (greenhouse), or indoors under lights.

Growing systems

Onion works in:

media bed (ebb and flow)
wicking bed
soil bed

Growing media

The substrate the roots sit in. Choice depends on the system (clay pebbles don't fit NFT channels; rockwool isn't used in media beds) and the crop (onion works in the media listed below).

Medium	pH effect	Water retention	Bacterial surface
Soil-based mix (Potting soil)	varies by source	high	high
Coco coir (Coconut coir)	slightly acidic	high	moderate

Bacterial surface area matters for aquaponics: clay pebbles, lava rock, and pumice double as biofilter substrate. Low-surface media (rockwool, perlite, pea gravel) work in hydroponics but need a separate biofilter in aquaponics.

Nutrient demand by stage

NPK ratios are relative weights at each growth stage; the nutrient mix calculator scales them to absolute grams or ml. EC targets shift through the plant's life: seedlings need a much lighter solution than fruiting adults.

Stage	N	P	K	EC target (mS/cm)
seedling	2	1	1	1
vegetative	2	2	3	1.6

Companion-growing notes

Heavy uptake of phosphorus. Co-grown crops with the same demand will end up deficient even at "correct" EC. Plan around this in shared reservoirs.
Releases compounds through the roots that can mildly inhibit other crops in the same reservoir or bed. The effect is usually subtle but worth knowing if neighbors look stunted.

Aquaponics suitability

Compatible with typical aquaponics nutrient profiles. Fish waste provides enough nitrogen for healthy growth; supplemental potassium, calcium, and iron may still be needed depending on fish stocking density.

Care notes

A challenging hydroponic bulb crop due to the day-length requirement and long growing season. Media beds or large containers with 10 cm depth for root development. EC 1.5-2.5 mS/cm. pH 6.0-7.0. Temperature: 15–25°C. Moderate to high light (DLI 14-22 mol/m2/day). Choose the correct day-length type for your latitude; this is the most common beginner mistake. Start from sets (small bulbs, fastest to harvest), transplants (medium), or seed (slowest, 100-130 days). Green onions (harvested before bulbing) are far easier and faster for hydroponic systems than bulb onions. For bulb onion production: the day-length trigger initiates bulbing; before that point, the plant must have grown enough leaf area to support a good-sized bulb (each green leaf corresponds to one ring in the bulb). Harvest when the tops fall over and begin to dry. Cure bulbs in a warm, dry, ventilated space for 2-4 weeks before storage. For aquaponics media beds, onions grow well but the long crop time (4-6 months for full bulbs) means the bed is occupied for an entire season.

Notable varieties

A starting shortlist of cultivars worth knowing about. Not exhaustive: the seed catalogs list hundreds of named varieties. These are the ones home growers commonly choose between.

Cultivar	Type	Breeder / origin	Days	Notes
Yellow Sweet Spanish	open-pollinated		110	Long-day, large yellow bulb, mild flavor, stores 3-4 months. Standard northern-latitude home garden onion.
Walla Walla	heirloom		125	Long-day sweet onion (Italian origin via the Pacific Northwest). Very large, very mild, very poor storage (1-2 months). Specifically a fresh-eating onion.
Vidalia (Granex)	open-pollinated		110	Short-day sweet onion. The original Vidalia onions are this variety grown in Vidalia, Georgia (a protected designation). Other regions can grow Granex but legally can't call them Vidalias. Poor storage.
Red Wing F1	hybrid	Bejo	115	Long-day red storage onion. Stores 5-6 months, holds color through cooking, hybrid vigor gives better disease resistance than open-pollinated reds.
Candy F1	hybrid		100	Day-neutral, sweet yellow. The variety to grow if you're in the awkward zone-6-to-7 latitude where short-day and long-day types both struggle.