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