Onion: autumn sets or indoor sowing, the two routes

The onion (Allium cepa) offers a double calendar few gardeners know: it can be sown under cover from winter, but hardy varieties can also be planted as sets from autumn. Two strategies, two seasons — here is how to choose yours.

Two routes of propagation

• Sets (small onions to replant): less disease-resistant, but easier to plant and grow. The fast route.
• Sowing: the range of varieties available as seed is much wider than as sets. The economical, varied route.

Autumn set planting — it's now

Bulbs of hardy yellow onions — the 'Stuttgarter' type — are planted in autumn, between September and November, or in early spring between January and April. Autumn-suited varieties include 'Snowball', 'Stuttgarter', 'Centurion' and 'Patate'; simply avoid periods of hard frost for planting.

Autumn-planted sets give "early" onions ready before summer. For hardiness, the listed varieties withstand about -15 °C.

Undercover sowing — the other route, from winter

This is where sowing makes full sense:

• Two undercover sowing windows: late winter (February to April) for summer onions, and late summer (August-September) for overwintering onions.
• Allow about 10 days to emergence, at a germination temperature near 15 °C.
• Technique: sow in trays, cover the seeds lightly, firm gently and water.

The February-March undercover sowing benefits from a germination kit: holding a steady 15 °C in midwinter is exactly the job of the heating mat, and the LED lamp prevents leggy seedlings. The complete kit bundles all three.

Transplanting and growing

• Transplant when plants reach about ten centimetres: one plant every 15 cm, in rows spaced 20 cm apart.
• Direct sowing is possible: sow thinly in rows 20 cm apart, then thin to one plant every 15 cm once they reach 10 cm.
• Soil: light, loose, drained and humus-rich, in a sunny spot. Water needs stay low to moderate.

Varieties to know

• For storage: 'Jaune de Mulhouse', 'Rouge de Brunswick', 'De Stuttgart'.
• Early white onions: 'De Rebouillon', 'Très hâtif de la reine', 'Très hâtif de Vaugirard'.
• For autumn: 'Snowball', 'Stuttgarter', 'Centurion', 'Patate'.
• For flavour: 'Rouge Long de Florence', semi-early, with an elongated violet-red shape.

Harvest and storage

Harvest runs from July to September. The full cycle takes 100 to 220 days on average depending on the route — patience, then, and lifting when the foliage falls over.

Diseases and rotation

• Two main enemies: onion mildew and white rot.
• Do not replant onions or other alliums (leek, garlic, shallot…) in the same spot for at least 4–5 years.

Going further: see also our shallot guide — its autumn planting runs October to January. And to anticipate winter sowings, sign up for the newsletter in the footer.

Growing calendar

	JAN	FEB	MAR	APR	MAY	JUN	JUL	AUG	SEP	OCT	NOV	DEC
Sowing
Planting
Harvest