Onions are staples in most vegetable gardens, and are usually grown from sets. Growing onions from seed, however, is just as easy. It’s also far more affordable, as a packet of 100 seeds costs far less than an onion set, and offers you a greater range of choice. Read on for a complete guide to growing onions from seed.
When growing onions from seed, you’ll need to consider which variety best suits your growing conditions. In particular, how much daylight you get. Additionally, if you want to store the onions, you should select a variety with good storage capabilities.
Short-day onions form when day length is between 10-12 daylight hours. These are ideal for gardeners in the south or milder climates (hardiness zones 7 and above), and can be grown through the fall and winter for a spring harvest. Common varieties include: White Bermuda, Southern Belle, Red Creole, Vidalia, Burgundy, and Granex.
Sometimes called intermediate-day onions, these grow in all but the most extreme climates. Day-neutrals form bulbs when day length is between 12-15 hours. Common varieties include: Red Amposta, Early Yellow Globe, Candy, and Cabernet.
Suitable for hardiness Zones 6 and cooler, long-days form onions when day length is over 14 hours. This variety can be started off indoors before being transplanted out in the spring. Common varieties include: White Sweet Spanish, Ailsa Craig, Yellow Sweet Spanish, Red Zeppelin, Ring Master and Paterson.
How to Sow Onion Seeds
Onions require a long germination and growing period. As a result, they’re commonly started off indoors before being transplanted later.
Growing Onions from Seed Indoors
The process of growing onions from seed can begin indoors in early February.
Fill small pots or module trays with fresh seed starting mix, leaving a gap of about half an inch between the soil and the lip of the pot.
Place no more than 3 onion seeds in each module or pot. This gives them enough space to develop. Once you’ve sown the seeds, cover them with a thin layer of compost.
Firm the soil down and moisten with a gentle spray. Don’t use a watering can as a vigorous flow of water can disturb or drown the seeds.
Place the trays in a sheltered, light location such as a greenhouse or cold frame and keep the soil moist. Ideally the temperature should be between 50-60℉. Onion seeds usually germinate within 8 days, but this can take longer if conditions are less than ideal.
Thin the seedlings out once they’ve germinated, so that there is only 1 per module or pot. Then place them in a light location. As the onions grow, you’ll need to clip the tops to keep them at a manageable size: about 2.5 inches.
While onions are largely cold hardy, they’ll still need to be hardened off before transplanting. Start this process about 4 weeks before the last expected frost date.
Transplanting Onion Seeds
Select a location that receives at least 6 hours of sunlight every day and has loose, well draining, fertile soil.
Before transplanting, dig the soil over a couple of times, removing any stones that you find. Working in compost will help to add nutrients and organic material to the soil. Alternatively you can apply an all-purpose organic fertilizer.
To transplant, carefully remove the seedlings from the trays and tease the roots free. Make small holes in the ground and plant the seedlings. The gap between each seedling will vary depending on the variety of onion, so consult your seed packet before transplanting.
Once positioned, gently push the soil back in place and water gently.
Growing Onions from Seed Outside
You can also begin growing onions outdoors in the last week of March, once the winter frosts have passed. Dig the soil over and work in compost, as explained in the Transplanting Onion Seeds section, before sowing.
Onion seed germination requires a temperature of 50-60℉. Unless you live in a mild area, the soil can be cold in the spring. Placing cloches on the soil for a couple of weeks before sowing will warm the soil and encourage germination.
Water the soil and mark out a shallow trench. Sow your onion seeds in rows about 8 inches apart and cover with a thin layer of soil.
Thin out the seedlings once they reach about 4 inches in height. The amount of space your onions require varies between varieties, so consult the seed packet(s).
Be warned: thinning causes onions to release a distinctive aroma, which attracts the destructive onion fly. Covering the seedlings with a horticultural fleece will protect them.
Caring for Onion Plants
Growing onions from seed is no more labor intensive than any other method.
Onions have a shallow root system so will need to be watered regularly. Don’t let the soil dry out. This can cause onions to mature before they are fully grown. Don’t water wet onion beds, as doing so can encourage rot or disease.
Onions benefit from a regular, low-level supply of nutrients. Over-feeding will encourage unnecessary foliage growth, and may keep your onions from reaching their full size. Work a long-lasting blood, fish and bone mix into the soil once a month, being careful not to damage the plants’ roots.
In June, applying a light feed of sulfate of potash, will encourage the bulbs to ripen.
Weeding and Mulching
Weeds can take moisture and nutrients from your onions, possibly stunting growth. Weed the area regularly to help your onions thrive. Place an organic mulch around your onions when they reach 10 inches in height. Doing so will help to retain moisture, encourage growth, and deter weeds.
Preparing for Harvest
In late summer, draw mulch and earth away from the bulbs, exposing them to the sun. As the bulbs swell, cease watering and fertilizing. Onions are ready to harvest when their leaves turn yellow and the stem starts to bend.
During wet summers, stalks may stay green for too long, prolonging the ripening process. Pulling the bulbs up slightly will break some of the roots and encourage ripening.
Common Problems and how to Solve them
Growing onions is a largely easy, problem-free process. However, there are a few issues you need to be aware of.
Bolting is usually caused by a period of low temperature followed by a quick warm spell. Loose soil or an accidental disturbance of the root system can also trigger the process. Bolting is more common in onions grown from sets, so it’s likely when growing onions from seed.
If your onion bolts, quickly cut off the stalk. The onion will still be edible, but can’t be stored.
Mildew, mould, smut, or rust may appear on onion leaves during warm, humid spells. These, along with leaves turning white, are all signs of fungal disease.
Onions are more likely to develop fungal disease in soil with a high nitrogen level. Applying an onion-licensed anti-fungal spray can alleviate symptoms, but the plant is difficult to save once infected.
The most serious fungal disease is onion white rot. This causes a white mould to grow on the base of the plant. Should onion white rot strike, you’ll have to destroy the entire crop. Don’t place infected plants on a compost heap as the spores may spread to other areas of your garden.
Onion white rot spores live in infected soil for up to 8 years. Consequently normal crop rotation methods aren’t effective. Instead, avoid using the infected area for an extended period. You’ll also have to be careful not to transfer soil from the diseased patch to another area. If space is limited, constructing a raised bed filled with clean topsoil or compost over the infected area is an effective solution.
The most common pest is the onion fly. This pest will lay eggs around the base of the plant. Once hatched, destructive maggots will attack the root system, stunting growth and killing smaller plants.
The most obvious sign of onion fly infestation is yellowing or drooping leaves. Upon closer inspection, you’ll notice small maggots in the soil.
Cover onion seedlings with a horticultural fleece to protect them. This will also prevent attacks by allium leaf miners.
Pigeons in particular can quickly pull up row after row of young seedlings, destroying an entire crop. Covering onions with a horticultural fleece or net until they’re established will deter birds.
Onions get on with most plants, but there are a few plants that you should avoid placing near onion crops.
Plants to Avoid
Onions struggle when planted near:
Don’t grow onions in large groups. Growing onions from seeds or sets alongside other members of the allium family can encourage pests. Instead, space your alliums throughout your garden.
Plants that Thrive near Onions
Onions also deter other pests such as aphids. Consequently many plants thrive when planted near onions. These include:
Harvesting and Storing Onions
You can harvest onions when they reach a suitable size. If you want to store them, leave them in the ground until the leaves begin to turn brown or droop. You can lift the onions out after a week in this condition.
Lift onions by digging them out of the ground with a fork. Try not to dig too close to the onions in case you damage the bulb. You can’t store damaged onions.
Once lifted, let the onions completely dry, or cure, before storing.
Spread the onions out on racks or newspapers in a well-ventilated greenhouse. Curing should take 2-3 weeks.
Cured onions will have papery skin and completely shriveled leaves. Once cured, cut off the onion roots and remove any loose skin.
Store onions in a dry, cool, well-ventilated space away from direct sunlight. A garage, or cellar is ideal. Hanging the onions in a net bag will allow air to circulate around them, reducing the opportunity for mould to form.
Check stored onions regularly and remove any that have gone bad.
Harvesting Onion Seeds
Onion seeds are cheap and easily available, but you can also harvest your own. Onions are biennials, meaning that they won’t produce seeds until the second year.
In warmer climates, onions can remain in the ground. In colder areas you’ll have to dig up and store the onions for re-planting the following spring. Once re-planted, allow the onions to flower.
Spent flower heads should be deadheaded and placed in a paper bag to dry for about a month. Once dried, release the onion seeds by shaking the flower heads.
Be warned: F1 varieties don’t grow true to type, so they may be a disappointment.