How to Grow Onions & Shallots from Seed

Growing onions and shallots from seed is often seen as the ‘next step’ in vegetable growing - and with good reason. While onion sets are quick, easy, and convenient, sowing from seed gives you access to heritage and specialist varieties, bigger bulbs, improved storage, and less risk of bolting. It’s also the method used by serious growers and exhibitors aiming for show-bench standards.

If you’ve ever wondered why gardeners rave about varieties like Kelsae, Ailsa Craig, or Bedfordshire Champion, the answer is simple: you can only grow them from seed.

In this comprehensive guide, we’ll walk you through the process of growing onion seeds and shallot seeds step by step - from sowing your seeds in January to harvesting beautifully cured bulbs later in the year.

Whether you’re growing onions or shallots from seed, the end goal is the same: firm, fertile, well-drained soil.

• Choose an open, sunny site
• Avoid planting where leeks, garlic, or onions grew last year (important for disease prevention)
• Soil should be free-draining but moisture-retentive
• Aim for a neutral pH (around 6.5–7)

Prepare the ground in advance, incorporating organic matter if needed, and rake level. Onion roots prefer firmness - resist the urge to over-dig.

When to Sow (UK)

For maincrop onions and shallots, sow from January to February.

This early sowing is critical. Onion bulbing is triggered by day length, not size. By sowing early, plants have time to build strong root systems and thick bases before the long days of June signal bulb formation.

How to Grow (Step by Step)

1. Fill seed trays or module trays with peat-free seed compost
2. Sow thinly (or 3–4 seeds per module)
3. Cover lightly - about 0.5cm
4. Water gently
5. Place in a heated propagator at 15–20°C

Once germinated, move seedlings to a cool, bright location. Warmth is for germination only - keeping them too warm afterwards causes weak, leggy growth.

Onion seedlings don’t emerge like most plants. They appear as a loop or hook of grass, still carrying the seed coat underground. Over a few days, the loop straightens - a small but unmistakable sign you’re growing alliums. Brush your hand gently over them, and you’ll notice something else - even at this stage, they smell unmistakably of onion.

When seedlings reach around 10cm tall, trim them back to about 7–8cm (3 inches) using scissors. This trick stops seedlings from flopping over, encourages thicker stems, and pushes energy into root and base development. You can repeat this once or twice before planting out.

When to Plant Out

Plant out from March to April, once seedlings are pencil-thick and the worst frosts have passed. Harden them off gradually over 7–10 days.

How to Plant

• Space onions 10cm apart
• Shallots 15cm apart
• Plant so the base sits just below soil level
• Firm gently - good soil contact is key
• Water well.

Watering

Consistent watering is vital, especially during late spring and early summer dry spells. Drought stress leads to smaller bulbs and increases the risk of bolting.

Feeding

A light feed with a balanced vegetable fertiliser in late spring supports bulb development. Avoid excess nitrogen - lush leaves don’t equal big bulbs.

Mulching

A light mulch helps retain moisture and suppress weeds, particularly on lighter soils.

Weeding

Onions hate competition. Weed regularly by hand - shallow roots mean hoes can do more harm than good.

Removing Flower Stems

If a plant bolts, remove the flower stem immediately and use that onion fresh. It won’t store.

When to Harvest

Harvest when the foliage naturally yellows and collapses. Don’t bend the tops yourself - this damages the bulb and invites disease.

How to Harvest

Lift on a dry day and cure bulbs in a warm, dry, and airy place for 2–3 weeks until the skins are papery and the necks are fully dry.

• Damping Off: A fungal disease affecting seedlings. Use fresh seed compost, clean containers, and fresh tap water.
• Onion Fly: Cover young plants with mesh.
• Poor Germination: Onion seed loses viability quickly (1–2 years). Always buy fresh seed - don’t trust the packet from the back of the shed.

A Note on “Growing Onion Sets from Seed”

Gardeners often search for how to grow onion sets from seed when they actually mean growing onions from seed for harvest the same year.

You can grow your own sets by sowing seed thickly in summer and lifting small bulbs for planting next year - but that’s a separate technique – see our Onion Sets Guide.

Growing onions and shallots from seed is the gold standard - the method used by exhibitors, heritage growers, and anyone chasing maximum size, flavour, and storage life. It does take more care than sets, but the rewards are significant.

With our full range of D.T Brown onion seeds, a reliable propagator, and quality peat-free seed compost, you can give your crop the best possible start.

All our sets are guaranteed to arrive firm and mould-free upon arrival – that’s the D.T. Brown quality promise.