Growing Maincrop Onions from Seed

Most folks I know grow their maincrop onions from sets planted in spring, but I have always grown mine from seed. It’s pretty easy growing onions from seed, and there is a much greater choice of varieties, onion variates come in many shapes, sizes and colours. If you want to have large onions then the two main factors are, variety and date of sowing. Exhibition growers sow their seed in December, you need a long season to grow big bulbs. The main important factor in growing really big onions is the variety, the most successful gardeners keep saving their seed from their best plants from year to year, developing a good giant strain. You can produce big onions from bought seed, the most popular of these varieties are The Kelsae, Mammoth and Ailsa, and seed is readily available. For most of us who want just good sized onions for the kitchen, and good storage, there are many of good varieties.

Sowing Onion Seeds

For growing onions you need to sow your seed in January or February in trays of a good seed compost and cover with a quarter inch of sieved compost or vermiculite. I prefer to use vermiculite to cover all my seeds. Water lightly, then place the seed trays into a heated propagator or cover with a plastic tray lid or even place in a plastic bag, this way the onions will germinate faster. Keep the onion seeds at a temperature of around 55 degrees F [ 13 C ] until they germinate. This should take between 7 to 14 days depending on temperature.

Look after Seedlings

Keep the seedlings warm until all reach one to one and a half inches in height, this is often called the ‘loop’ or ‘crook’ stage. Grow on with less heat now as you want the seedlings to be strong and robust, not weak and soft. I like to place mine on a south facing windowsill [ I grow in half seed trays so they will fit on my windowsills ] and cover them at night with a clear plastic seed tray cover if the nights are very cold. Leave the seedlings in the tray till they reach 3 to 4 inches in height, they can then be pricked out into individual pots or into large modular trays of a good potting compost. Keep your seedlings moist but not wet, watering from the bottom helps to encourage to roots to do down into the compost and develop a good root system. Although they don’t want too much warmth, they will need protection from frost and very cold weather. I like to place my potted up seedlings in a plastic covered mini greenhouse, and place that in my cold conservatory or cold greenhouse. Ventilate well on warm days.

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Hardening Off

When you are growing onions from seeds you will need to acclimatize the young plants to the outdoor conditions. When your seedlings are at least 6 inches high, you can start to harden them off, I start by keeping the door of the mini house cover open at all times, then after a week or so, I remove the cover completely. After another week I place my onion plants in a coldframe to get ready for planting out. April is the month I usually plant them out, a lot will depend on the weather.

Planting Out

You may be able to plant out in late March in sheltered areas, or late April in more open plots. Beware of wind, It can cause more damage to the young plants than cold. Hopefully you will have already prepared your onion bed the autumn before. Adding lots of well rotted manure or garden compost. If you didn’t manage to prepare in the autumn, you can add good garden compost or add well rotted organic farmyard manure that can be bought in bags from garden centres. Adding a little calcified seaweed or super-phosphate will help. Work your bed to a fine tilth beforehand.

For good sized bulbs plant your seedlings approx 10 -12 inches apart, with the rows 14 – 16 inches apart. You can use closer spacings in raised beds or for slightly smaller bulbs. Plant the base of the onion seedling 1 inch below the soil level and firm lightly. For a bit of extra protection in windy or exposed sites, you can cover the young plants with plastic tunnels or cloches. Remove the protection after 2 – 3 weeks.

If you have prepared your bed well, there should be no need for extra feeding, If you do feed, stop by June as too much feed can cause soft growth leading to poor ripening and poor storage qualities.

Harvesting and Storing

Your onions should be ready for harvesting from mid august to mid September. You will notice that they star to ‘neck’ when ripening. This is when the tops bend over as the goodness flows out of the foliage, into the bulb as they start the ripening process. When the foliage has bent over you can help them ripen by loosening them from the soil, use a fork to do this, don’t pull them by the stem, as the stem can be damaged and cause storage problems. After a week, lift them from the ground with a folk and place somewhere sheltered to fully dry out. Staging in a greenhouse or garage bench is ideal. When dry, trim the tops and dry for another week before stringing up for storage. Store in a cool place in the light, dark will encourage growth. Tights can be used as a storage method, place a trimmed onion in the tight leg, then knot before adding another onion, keep repeating this process. Just cut below a knot to release one onion at a time to use. Many varieties will store till at least March.

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If you are wanting to show your produce, allow about 10 days before a show to prepare them. Check with the show schedule as different class require different preparation. Some will require the onions complete with foliage, others trimmed and bound with raffia. The rules with tell you what is required for each class, follow them to the letter!

Pests and Diseases

I find that growing onions is easy and they are fairly pest and trouble free, keep well weeded as they do not like any competition. Thrips can sometimes be a problem, they are little bugs that bury into the foliage and make it look unsightly, large infestations can cause problems as they effect the foliage’s ability to feed the bulb. Other problems are onion fly and white rot, these can be very serious so good cultivation and crop rotation is important. Many exhibition growers use wire supports to keep the neck and foliage of the plants upright, this helps them to develop larger bulbs. It can also be useful in a windy or exposed site. I grow my onions through square netting held taught with canes and can be raised as the plants grow.

If you are tempted to grow giant onions or try and beat the world record for the heaviest onion, the world record at the moment is a staggering 17 lbs 15 and a half ozs grown by pensioner Peter Glazebrook of Newark! Good Luck!


Helen Fowler
Born in Middlesbrough. Moved to live in rural North Yorkshire in late teens. Moved back to the town in my 30's to live near Stockton on Tees. Then after a divorce and a serious accident I moved back to rural North Yorkshire near Thirsk, where I live now. I am a passionate gardener, a keen amateur photograper, I love travel, music, anything artistic and I have a great love of nature and the natural world. I have gardened since my teens and I lived and worked on a farm for years. I have owned or have experience with most pets and domestic animals. I hope by sharing my own experiences and the personal knowledge I have gained over the years, to help and encourage others to gain the most from their gardening efforts.

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