How To Successfully Grow Onions From Seed

Onions are extremely versatile vegetables with so many different culinary uses. They can be eaten raw in salads or served up with cheese on sandwiches. They are used in soups, stews and casseroles, served as side dishes and used for flavouring. They can be boiled, fried, baked and pickled and that’s just for starters. According to food historians, onions have been cultivated and used in cooking for over 5000 years, which makes them the oldest cultivated vegetable recorded. Research suggests that onions probably originated in central Asia before spreading to other parts of the world.

Why grow onions from seed?

Fortunately, onions are quite easy to grow. If you want to grow them from sets then most of the tricky work has already been done. However, it’s much more satisfying growing onions from seed and this method affords you a much wider choice, in terms of varieties. It’s also cheaper to grow onions from seed than from sets. Onions that are grown from seed will take a little longer than those grown from sets, usually about 90-100 days in total but most gardeners will agree that seed grown onions are far superior in terms of quality. They are also less likely to bolt (flower and seed).

Sowing seeds indoors

Onion seeds can be sown under cover in modular trays, pots, flat trays or directly into open ground. If starting seeds off under cover then early February is the best month. A cold greenhouse should be fine but the ambient temperature will determine how long the germination process will take. Generally, if the outside temperature falls below zero for any length of time you can expect to wait up to about 2 weeks or more for germination. 

Whereas, if the outside temperature is well above freezing, germination can take place in as little as 4-5 days. A warm windowsill should provide a steady temperature. Sow seeds thinly into seed compost with 2-3 seeds per module or 3-4 seeds per 7.5cm pot. If sowing in flat trays then sow thinly. Apply water and cover the trays with glass or polythene.

Sowing seeds directly into the ground 

If you are too impatient and sow too early and in low temperatures then you might wonder whatever happened to your onion seeds. For best results wait until the outside temperature settles at around 16°C, which should be around the end of March. The seed bed should have been prepared well in advance by digging it over and incorporating plenty of organic matter such as compost or well-rotted farm yard manure. Rake the surface to form a fine tilth and then set out a string line to form a row. 

Use the edge of a hoe to create a shallow furrow about 1cm deep and sow the seeds thinly along the row. Lightly cover the seeds with soil and apply water using a watering can with rose attachment. Repeat the process for as many rows as you require, allowing 25cm-30cm spacing between rows. Depending on weather conditions and temperature, it can take up to 21 days for seeds to germinate outdoors.

The growing stage

Seedlings that have been grown under cover should be thinned when large enough to handle. The remainder should then be hardened off (acclimatised) for about 10-14 days prior to transplanting to their permanent positions at the end of March or early April, depending on weather conditions. Use a dibber to create small planting holes and handle the seedlings carefully. Seedlings should be planted at a distance of 10cm apart, which allows enough room for the bulbs to swell and grow unhindered. For seedlings that are growing outdoors, thin them out to 10cm apart as soon as they are large enough to handle.

Weeding, watering & feeding

As the temperature rises and the onion bulbs begin to grow and swell, ensure the areas between the plants are kept weed free. Keeping weeds at bay will discourage many types of pests. Ensure the plants are kept well-watered, especially during long dry spells. Not only do the bulbs require copious amounts of water in order to grow, if the plants are allowed to regularly dry out then they will be inclined to bolt early. Onions are voracious feeders and will need feeding with a liquid feed every 14 days.

General care 

During the growing season look out for plants that may produce flower stems. Remove these as soon as they appear to conserve the plant’s energy resources. Also, from about mid to late July, and if the bulbs have swollen sufficiently, stop applying liquid feed and reduce the amount of watering. If you continue to water and feed after this time then the onions will not store well, which could result in considerable waste. 

Harvesting & storage

Onions grown from seed in the spring should be ready for harvesting from August to September. The tops will begin to show a yellowing in colour when they are ready. Choose a warm dry day and lift the plants using a fork. Lay the plants on the soil to dry out in the sun before moving them complete with tops to a dry place for storage. They should be left to fully dry out or ‘cure’, which can take about 2-3 weeks. Placing them on newspaper, cardboard or on greenhouse staging will aid the curing process. 

Any bulbs that are bruised or damaged should be used first. Also, any that have thick stems will not keep for long. Discard any that feel soft to the touch. When fully dried remove the dead stems, place in boxes or net bags and store in a cool, dry and well ventilated place. You can, if you prefer, tie them onto onion strings and leave them suspended in a dry shed or garage. Keep them away from direct sunlight, which will encourage them to begin growing again. Check the onions regularly and remove any that are going soft or showing signs of rot or fungal growth.

Pests & diseases

Downy mildew can be a problem especially during long periods of wet weather. Remove any infected leaves as they appear. Leek rust is another fungal disease caused by damp conditions. There is no cure but removing infected leaves will help limit the spread. In both cases, planting too close can encourage the spread of fungal disease. Onion white rot is a soil borne fungal disease, for which there is no cure. To avoid this in future, do not plant onions into the same plot as the previous year as spores may still reside in the soil. 

Onion fly is the only serious pest to affect onions. The adult fly lays eggs at the base of the plant and the emerging grubs attack the bulbs. The leaves of affected plants will wilt and turn yellow. Covering the soil with horticultural fleece is the only option as there is no cure or remedy.

Have you got any Onion Growing Tips? Leave a comment below and let everyone know....


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