How to Grow Onions From Sowing to Harvest

Plant Onions in the Spring!

February 5, 2019

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Onions are a must-grow vegetable! Why? Where to begin? To start, onions are very easy to grow and properly prepared bulbs will store up to six months! You can get a weighty harvest from even a small area. Plus, they are the starting to point to many recipes! We’ll show you exactly how to sow, grow, and harvest your onions.

As well as yellow and red, there are white varieties of onion which are often bigger and mild flavored. (If you have our Garden Planner, view a list of varieties for onions. When you add onions to your plan, you can then view the Plant List to see recommended sowing, planting and harvesting dates for your garden’s location.)

Grow onions in a sunny, open position, in free-draining soil that has been enriched with compost or well-rotted manure. In wet, heavy soils, grow onions in raised beds or on mounds instead.

Starting Onions Indoors

Planting onion seeds into plug trays or pots to transplant later avoids the need for thinning out and is economical with seeds. Sowing in a greenhouse or cold frame means you can sow at least a month sooner than outdoors.

Fill plug trays with seed-starting or general-purpose potting mix and sow four to eight seeds per cell. Cover over with half an inch of potting mix, and water with a fine spray.

Transplant the seedlings while they’re still quite small to avoid disturbing the delicate roots. Plant each clump of seedlings about 4in apart, and water them in.

Planting Onions Outside 

Sow onions outside as soon as the soil is workable in the spring and has warmed a little.

Rake the soil then mark out seed drills half an inch deep and twelve inches apart.

Sow seeds very thinly and cover them back over.

Water along the rows to settle them in.

Once seedlings pop up, thin them in stages until they’re 2 inches apart for lots of smaller onions, or 4 inches apart for fewer, but larger, bulbs.

Cover early sowings or transplants with row covers to speed things along at the start of the season. This may also help reduce the tendency to bolt, or flower, which makes the bulb too tough to eat. 

Some varieties of onion may also be sown in late summer for an extra early crop the following spring or early summer.

Onion Sets

Another option is to buy onion transplants for immediate planting, or you can plant “onion sets” (part-grown onions).

Onion sets save time sowing, and are a cinch to grow. However, they don’t store as well as onions grown from seed or transplants, and they are more likely to bolt (heat-treated varieties are available that are more resistant to bolting).

Plant sets in mid spring once the soil is workable and has warmed up a little. Space them 2 to 4 inches apart, and plant them deep enough to leave just the tips poking up from the ground. Some sets may also be planted in early autumn, to give a harvest up to two months earlier next summer.

Onions transplanted from plug trays can be thinned out once they’ve grown on a little if you want bigger bulbs. Use the thinnings as green onions.

Keep onions well watered in dry weather. Hoe between rows regularly, but hand weed within the rows to avoid damaging the bulbs or roots.

Harvesting and Storing Onions

Once most of the leaves have bent down, it’s almost harvest time. Allow them to swell and color up for a few more weeks. 

To harvest, lever them up with a fork or trowel.

Onions that are to be stored need to be cured first in an airy shed, greenhouse or other dry, well-ventilated area. In warm, dry climates you can simply pull the onions and leave them on the soil surface to cure. Space bulbs out for good airflow. Curing takes about two weeks and toughens up the outer skins so they will store for longer – until at least midwinter, and as long as spring.

Once cured, store onions in nets, tied into bundles or woven into onion strings. 

To learn more about growing onions, see the Almanac’s Onion Plant Guide.

For help planning your garden for the maximum yield, explore our online Garden Planner with a free 7-day trail!

2019 Garden Guide

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