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How to Grow Onions from Seed


Onions can be grown and pulled fresh from the garden or out of your onion store almost all year round from just a couple of carefully-timed sowings.

Both peas and broad beans are two of the top-selling vegetable seeds but onions are not. However onions are one of the most used kitchen vegetables in the world.

If Growing onions is not on your grow list then read this carefully – the Japanese varieties have filled the June-July gap of the old days, and the close-spacing recommendation increases the yield.

Fungicide-treated and pellet onion seeds are available. Germination and seedling growth are slow in spring and also erratic in hot weather.
Expected germination time: 21 days

grow onions from seed 

What soil is best to grow onions from seed in.

It is best to choose an open, sunny site with good drainage. Dig thoroughly in autumn, incorporating a liberal quantity of manure or compost. Liming will be necessary if the soil is too acid.

Before sowing or planting it is necessary to prepare a traditional ‘onion bed’. Apply a general fertilizer and rake the surface when the soil is reasonably dry. Tread over the area and then rake again to produce a fine, even tilth. In other words the seeds needed to grow onions are pretty small, so the finer the soil the easier time the seeds will have in germinating and growing properly.

Prize winning onions.

Many exhibitors grow their show onions in a permanent bed in order to build up fertility, but in the kitchen plot it is a much better idea to change the site annually.

Sowing and Planting onion seeds

Sow very thinly in drills 1/2in (1cm) deep and 9in (23cm) apart. Water very gently if the soil is dry, and cover with soil.

Thin the spring-sown crop in 2 stages – first to 1-2in (2-5cm) when the seedlings have straightened up and then to 4in (10cm) apart. Lift the seedlings carefully – the soil should be moist and all thinnings removed to deter onion fly.

Seedlings raised under glass should be transplanted 4in (10cm) apart, leaving 9in (23cm) between the rows. The roots must fall vertically in the planting hole and the bulb base should be about ½ in (1cm) below the surface. Plant firmly.

Salad onions should be planted in rows which are only 4in (10cm) apart – thin the seedlings, if necessary, to 1in (2.5cm) spacings.

Seeds of Japanese varieties should be sown at 1in (2.5cm) intervals in rows spaced 9in (23cm) apart. Thin seedlings to 4in (10cm) intervals in spring.

Expected germination time: 21 days

Approximate number per ounce 8000

Expected yield from a 10 ft row 8lb (4kg)

Life expectancy of seed 1- 2 years

Approximate time between sowing and lifting:
46 weeks
(august sown)
22 weeks
(spring sown)

Sow indoors
January - march

Sow outdoors
March - April

Easy to grow.

Onions can be grown all year round.

For an August or September crop sow as soon as the land is workable in the spring (late February-early April depending on the location of your garden).

Sow in mid August for an earlier crop – Japanese varieties mature in late June – standard varieties such as Reliance and Ailsa Craig are less hardy, less reliable and later cropping (late July onwards), but they can be stored.

In cold areas and for exhibition bulbs sow under glass in January, harden off in March and transplant outdoors in April.

Salad onions should be sown in March-July for a June-October crop. Sow in August for onions in March-May.

Looking after the crop

Hoe carefully or weed by hand – dense weed growth will seriously affect yield. Water if the weather is dry (not otherwise) and feed occasionally. Feed an autumn-sown crop with Instant Bio in March.

Break off any flower stems which appear. Mulching is useful for cutting down the need for water and for suppressing weeds. Stop watering once the onions have swollen and pull back the covering earth or mulch to expose the bulb surface to the sun.


The salad varieties should be pulled when the bulbs are ½ - 1in (1-2.5cm) across. The harvesting season is March - October.

When the bulb is mature the foliage turns yellow and topples over. Leave them for about 2 weeks and then carefully lift with a fork on a dry day.

The onions which are not for immediate use must be dried. Spread out the bulbs on sacking or in trays – outdoors if the weather is warm and sunny or indoors if the weather is rainy.

Drying will take 7-21 days, depending on the size of the bulbs and the air temperature. Inspect the bulbs carefully – all damaged, soft, spotted and thick-necked onions should be set aside for kitchen use or freezing. The rest can be stored – the exceptions are the Japanese varieties which are not suitable for storage.

Store in trays, net bags, tights or tie to a length of cord as onion ropes. Choose a cool and well-lit place; they will keep until late spring.



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