Allium cepa

Sowing and cultivation guidance

Allium cepa seeds germinate best in full sun and well-drained, moderately fertile soil kept evenly moist. Sow indoors well before last frost, then transplant when soil is workable; or direct sow once hard frosts have passed. In mild-winter climates, late-summer sowing can establish plants before winter. Provide generous spacing to allow bulb expansion and follow packet guidance for final distances; the plants remain compact in height but need room underground. Water consistently, avoiding waterlogging, and mulch lightly to conserve moisture. Keep weeds down and thin early to the strongest seedlings. Harvest timing depends on bulb size and day length; cure bulbs in a dry, airy place after lifting.

• Sowing window: start indoors before last frost; direct sow after hard frosts; in mild-winter climates, late-summer sowing is possible.
• Culture: full sun; well-drained, moderately fertile soil with consistent moisture; avoid waterlogging.
• Edible: bulb and green tops are used as a culinary crop when mature.
• Pollinator-friendly: if allowed to flower, umbels can attract visiting pollinators.
• Container-suitable: performs in deep, well-drained containers that allow for bulb formation.
• Potentially toxic: can be harmful to pets and some animals if ingested; site and dispose of waste responsibly.

How to grow Allium cepa from seed?

Sow fresh onion seed shallowly (about 0.5–1 cm deep) in trays or modules filled with a fine, well-draining mix, keeping it evenly moist and in full sun or under bright grow lights. Ideal germination temperatures are cool to moderate; steady warmth speeds sprouting, but avoid overheating and keep good airflow to reduce damping-off. Thin crowded seedlings early to prevent spindly growth. Provide consistent light from emergence, and brush tops lightly or run a fan to produce sturdy foliage. Onions are compact plants but need space for bulbs later, so start in modules if you plan to transplant with minimal root disturbance.

When to sow onion seeds after the last frost, and how does day length affect choice?

Start seeds indoors well before the last frost so seedlings are ready to transplant when soil is workable; direct sow outdoors only after the risk of hard frost has passed. In mild-winter climates, sow in late summer so plants establish before winter and bulb the following season. Choose varieties matched to your local peak day length: short-day types bulb at roughly 10–12 hours of daylight, intermediate-day at about 12–14 hours, and long-day at around 14–16 hours. Selecting the right day-length type for your light conditions is key to reliable bulbing and storage quality.

What is the best soil for onion bulbs?

Onions thrive in full sun and well-drained, moderately fertile soil that holds consistent moisture without becoming waterlogged. Work in well-rotted compost to improve structure, avoid fresh manure, and aim for a loose, friable bed free of stones so bulbs can expand evenly. Raised beds or ridged rows help drainage in wetter climates. Mulch lightly to conserve moisture and suppress weeds, but keep mulch from burying the necks. Maintain steady moisture through active growth; swings between drought and saturation can cause split bulbs or premature bolting.

How should I transplant onion seedlings, and what spacing do bulbing onions need?

Harden off seedlings for about a week, then transplant when they have 3–4 true leaves and the soil is workable. Plant so the white shank sits just at or slightly below the surface, taking care not to bury the growing point. Allow room for bulb expansion: space 10–15 cm between plants in rows 25–35 cm apart for full-size bulbs; use closer spacing (5–8 cm) for smaller bulbs or bunching onions. Water in well to settle soil around roots, and keep beds weed-free—onions dislike competition. Follow any packet guidance specific to the variety’s expected bulb size.

Can I grow onions in containers, and what watering schedule works best?

Yes—use containers at least 20–25 cm deep with ample drainage, filled with a free-draining, compost-enriched mix. Space plants roughly 10–12 cm apart, or grow bunching types more closely. Container onions need steady moisture; water when the top couple of centimeters of mix feel dry, keeping the medium evenly damp but never soggy. In warm, windy spells, containers may need more frequent watering. Water at the base in the morning to reduce foliage disease risk, and reduce watering once tops begin to fall over near harvest to encourage curing.

How do I cure and store onions?

Harvest when most tops have naturally fallen and necks are soft, lifting carefully to avoid bruising. Cure bulbs in a warm, dry, airy place out of direct, harsh sun until necks are fully dry and outer skins are papery—typically a couple of weeks. Brush off soil, trim roots, and cut tops to about 2–3 cm (or braid intact tops). Store sound bulbs in mesh bags, crates, or slatted trays in a cool, dry, well-ventilated, dark spot. Check periodically and use any damaged or thick-necked bulbs first; mild, sweet types generally store for a shorter period than firm, pungent ones.

Guidance here is general and intended for varied growing conditions. Results can differ with microclimate, soil structure, drainage, and seasonal timing (e.g., sow after last frost or when soil is workable). Instructions on individual product pages provide accurate, variety-specific details such as sowing depth, spacing, and timing. Those pages also include sourcing and compliance information relevant to each product to help ensure practices align with applicable standards.

Growing Guidance and Sourcing

These pointers support dependable results with Allium cepa seeds across a range of conditions. Choose a sunny, open site with well-drained, moderately fertile soil. Sow under cover and transplant when soil is workable, or direct sow after last frost; in mild climates, autumn sowing may be possible. Keep seedlings evenly moist and weed-free; avoid heavy nitrogen late, which favours leaves over bulbs. Space 10–15 cm apart in rows 25–30 cm for uniform development. Onions benefit from steady moisture during bulbing and reduced watering as tops begin to yellow. Harvest when most tops have fallen and necks have tightened, then cure in a dry, airy place before storage. Match varieties to local daylength and your growing window for best sizing. Additional variety notes, seed handling guidance, and related resources are available at SeedsChoice, supporting informed choices on bulb types, sowing approaches, and cultivation practices without prescribing a single method.