How to Grow Onions from Seed (Heirloom UK Guide)
, by Ellen Rignell, 6 min reading time
Everything you need to know about Growing Onions from Seed in the UK (with a focus on Heirloom Varieties)
Introduction: Seeds vs. Sets - Why Choose Seeds?
As a seed saver and seed seller, I feel I need to declare my strong bias for growing onions from seed.
When you grow onions from seed you have a much more exciting range of varieties available to you - gorgeous heirlooms with incredible flavours.
Yet, the vast majority of gardeners in the UK grow onions from sets. Why? Growing from sets is thought to be an easier, fail-safe method but as you'll read in this blog post, I find growing from seed is simple and gives you the excuse to sow something early in the year.
Growing from sets has some definite drawbacks. Aside from the lack of variety available as sets, my key bug bears are that sets are more susceptible to bolting and they don't take as well to storage.
Growing from seed does mean you have to start sowing early, but the plants will be so much more robust and resilient.
Choosing Heirloom Onion Seeds for UK Gardens
Heirloom seeds are THE BEST seeds for home gardeners to use. Here's why:
- They have a wealth of genetic diversity - each plant is a true individual, not a clone like F1 hybrid seeds. This gives the plants a robustness and resilience needed for growing in nature friendly & organically managed vegetable gardens.
- Heirlooms have a superior flavour. These varieties have been grown for generations and bred for not just good growth characteristics but also great taste. Modern varieties can't compete with their depth of flavour.
- Growing these varieties preserves our horticultural heritage - both the genetics within the varieties and the stories of people that have saved seed from these plants for generations.
My Top 3 Heirloom Onion Varieties
The sweetest onion in our catalogue with a rich flavour only found in homegrown onions. Long Red Florence is a very versatile Italian heirloom variety which can be cropped as quick-growing, bright purple salad onions or left to mature into elongated, red bulbs.
Our Desert Island onion. Cuisse de Poulet du Poitou is a banana shallot bursting with savoury-sweetness. Its mild enough to be served raw but really sings when cooked low and slow. The bulbs are about the size of an onion - you get proper shallot flavour without the extensive peeling. And they're beautiful - elegant, elongated bulbs with a rusty-rose skin.
Rossa di Milano is a stunning, deep red Italian heirloom with broad shoulders, tapering down to a narrow root. One we're really excited to bring to UK gardeners. A reliable variety to grow from seed, we sow in February and harvest in late summer. A brilliant storage variety - worth a try if you're after lovely sweet red onions which keep all winter.
The UK Onion Seed Sowing Calendar: When to Sow
The KEY to growing onions successfully from seed is to start early.
When to Sow
Sow undercover (in a propagator or greenhouse) from Valentines Day to March. Sow outside from March onwards. If the weather is very cold, the plants will benefit from some horticultural fleece but generally young onion plants are pretty hardy.
How to Sow
My preference is to either sow in module trays, putting 3-5 seeds in each module. This is known as 'multi-sowing'. They can be sown in quite small modules - but be warned - onions plants are often sat in their module trays from mid-Feb to April so the plants often run out of nutrients. You may need to give them a liquid feed to top them up.
I also sow onion seeds directly into crates full of compost (similar to sowing in a seed bed). I then bareroot transplant the plants into their final planting position (more on this later). I find this method gives healthier plants than when grown in modules and eliminates the need for liquid feeding.
Planting out and Aftercare for Bumper Onion Harvests
Where to Plant
Although onions aren't the heaviest feeders in the garden (courgettes, sweetcorn and brassicas are the hungriest!), they do appreciate being grown in a spot which has been composted this season or last.
They prefer a well-drained soil so avoid putting them in the soggy patch of your garden. The varieties I have in our catalogue are adapted to heavier soils and being grown in a damp climate.
How & When to Plant
Plant from end of March-April. Plants always benefit from fleece but often this isn't strictly necessary. Onion seedlings can withstand a light frost (-3C) without much damage. Make sure you harden your plants off sufficiently before planting. I do this by moving them out of the greenhouse and covering them with fleece at night for a few days before planting.
Multi-sown module grown plants are spaced at 15-30cm apart.
For the onions sown in seedbed crates I plant single seedlings at 10-15cm apart. Generally plants sown this way can wait until end of April or even mid May to be planted out.
Onion Seedling Aftercare
The main thing with onions is to keep them well-weeded. Onions are easily out-competed by weeds because their roots are very shallow. Hoe your freshly planted seedlings twice after planting. This should eliminate the need for hand weeding.
Water your plants in dry spells. Avoid watering once the leaves start to yellow (this is part of the drying down process, essential for storage).
Harvesting and Curing Heirloom Onions
When to Harvest
Most gardeners harvest onions once the foliage has started to flop over and turn yellow in the late summer. However, I encourage you to harvest them fresh (anytime from July onwards). They are very delicious and sweet at this stage and it spreads out the harvest more evenly. Early harvested bulbs won't store well, you need to use them up within a couple of weeks.
Curing
If you are harvesting onions for storage, you'll want to harvest them in late summer. The flopping foliage is a sign that the plant has finished growing. To get the best storage out of an onion, you need to put them out the soil (use a fork if you need to) and hang them upside down on racks in a greenhouse.
This gets the onions nice and dry - you can then trim the excess roots and foliage and store in mesh bags, hung up in the shed.
Conclusion: Give it a Go!
Are you convinced? If you want to try growing onions from seed, check out our range of heirloom varieties here.
Happy Gardening!
