How to grow onions from seed in the UK - a step by step guide

My grandad grew 3 crops on his allotment: potatoes, beetroot and onions. He was a gardener of the 'Dig for Britain' generation and he took great pleasure in growing enough vegetables to get him through the winter. He also took great pleasure in growing big vegetables - like any true Yorkshireman should. Our gardening styles are very different but what I have inherited from him is a love of growing onions. I grow them from seed every year rather than sets. 

Here's our tried and tested guide to growing onions from seed.

1. Choose the right varieties

Onions are a kitchen staple which are very rewarding to grow. Start by choosing a few interesting varieties which are known to grow well from seed. Here's a few of our favourites:

Rossa Di Milano - Beautifully sweet onions with merlot red papery skins. Bulbs are broad at the top, tapering towards the roots. A proper italian heirloom with great vigour and beautiful flavour.

Cuisse de Poulet - A banana shallot type with gorgeous rusty rose skins and elogated bulb. The flavour of this french heirloom shallot elevates any dish. 

Stoccarde - A reliable, dual-use onion which has stood the test of time. Sow densely in March, harvest spring onions in May and mature bulbs in August. Dry the bulbs out before you store them and they'll keep until the following spring.

Cipollini - Cute little flattened onions which mature quickly, making them perfect to grow from seed. They're renowned for their sweetness and are perfect roasted whole.

2. Start your seeds early

• Sow onion seeds from Valentines day until the end of March.
• Use an unheated greenhouse, polytunnel or similar. They like it to be cool and bright.
• Pay attention to watering. It's easy to overwater your plants early on in the year. Keep the soil moist, not wet.

3. Multi-sow your onion seeds

Onions grow happily side by side. Fill up module trays with seed compost and shallowly sow 3 seeds per module.

4. Transplanting seedlings

Your seedlings will be ready to be transplanted outside when you can see white roots at the bottom of your module trays. You should be able to pull the module plants out of the tray without the compost totally collapsing. Depending on when you sowed your seedlings, this will happen between late March and late April.

• Hardening off - Remember to gently aclimatise your seedlings to cool, outdoor temperatures. Move your plants outside the greenhouse or polytunnel for a few nights before planting. Cover them with fleece at night while they're hardening off.
• Bed Prep - onions are heavy feeders so try to improve your garden soil with compost before planting. For early plantings, this is best done the previous autumn. Mulch on top of the soil and allow the worms to do the hard work or incorporate by digging.
• Planting - plant your modules 15-20cm apart.

5. Looking after your onions

Onions are a fairly low maintenance crop but we find they really benefit from:

• Weeding regularly and thoroughly. You can hoe your transplants if you catch the weeds early or hand weed if the weeds are a bit bigger. Onions don't like competition so its important to keep weeds to a minimum.
• Watering -  For plump, healthy bulbs, keep your onions, water your plants in thoroughly after planting. You'll need to use less water for onions grown from seed compared to onions grown from sets. Even so, keep an eye on your onions - they might need watering in a dry summer.

6. Harvesting and storing onions

Your onions will let you know when they need to be harvested. Their leaves will turn yellow and keel over - usually in mid summer. This is the best time to harvest onions for storage.

Many gardeners don't realise that you can also harvest your onions early for fresh use - the bulbs will be extra pungent and juicy. You can use the whole plant, stems and all and you'll barely need to peel the bulb. This is my favourite way to cook with home grown onions.

If you'd like to store your onions, you need to follow this process to help them store for as long as possible.

• Harvesting - Pull up your onions once the tops have turned yellow and keeled over. You can leave them on the soil to dry out further if its warm and dry. More often than not, it's wet. When this is the case, move them straight into your greenhouse or polytunnel.
• Curing - To ensure your onions store well, you need to cure them in a polytunnel, greenhouse or similar. Lay them out in a single layer to dry for a couple of weeks.
• Storing - Once cured, you can trim the stems off your onions - or plait them if you're feeling creative. Store them in mesh bags or crates in a well ventilated, rodent free place. Remember to eat the biggest onions first - the smallest ones tend to keep the longest.

 

We hope this guide will give you the confidence to try growing onions from seed this year! Read our Gardening Blog for more tips and have a look at our onion and shallot seed collection here. Happy Gardening!