Country Gardener

Widely regarded as the authority on gardening in the south west

Knowing your onions!

An early start is often the trick for growing what is a must-have vegetable in the garden.

Onion sets are easy to grow and should be the first thing on the list for the new season.

Your garden turns a small set into a full-sized bulb without any attention from you during the summer so it is not a difficult choice when it comes to adding them on to your vegetable growing list. Sets are the immature plants that are raised from seed the previous summer. Because they were sown at a very high density, they do not reach sufficient size to bolt. They just carry on growing instead.

Plant sets whenever you can given reasonable warmth and dryness from February onwards, pushing them gently into the soil so the tips are level with the surface. It’s a good idea to cover them with pea sticks or fleece to prevent birds pulling them out.

The chance of bolting is decreased if you avoid planting in cold, wet soil. They thrive in a sunny, well-drained situation. Keep weed-free, especially early on. It is important to move the onion bed around every year to prevent the build-up of diseases like onion white rot.

They don’t take up much room, are an easy and tough crop, and by growing your own you can get cultivars (cultivated varieties) not available at markets.

First, you’ll need to choose which onions to grow.  Consider the colour - they come in reds, yellows, and whites. Consider the size, which often relates to use, such as the small white pearl onions for stews or large ones for slicing.  Consider if you want to eat and use them fresh or store them, as some store much longer than others.  Of course consider taste - do you like sweet or strong flavours?

onion setsPerhaps most important culturally is their day length requirement. Onions, depending on cultivar, require various amounts of light in order to form bulbs.

Whether you’re planting sets, purchased plants, or those you started yourself from seeds, plant out about two to four weeks before the last usual frost date in your area.  For northern gardens, this may mean planting out late-April to mid-May.  To avoid problems and get the best crop, don’t plant them in the same area onions or their relatives have grown the last couple years, nor where legumes (peas and beans) have recently been grown.  Onions grow best in a well-drained, loose soil with lots of organic matter. A sandy loam with additional compost is ideal.

Usually onions have no problems, except for weeds and perhaps for onion maggots.  To discourage this pest, don’t plant too early in spring, and use row covers for the first few weeks to discourage the flies laying eggs.  Onions don’t compete well with weeds, so keep them weeded, particularly when young. Mulching after planting with straw or grass clippings reduces weeding.

Another key to an excellent crop is to keep the soil moist during the first couple of weeks after planting out. Then, at the end of the season, the opposite applies. As plants mature - no more new leaves and the tops fall over - reduce watering, and pull soil away from the tops of the bulbs. This encourages them to go dormant and start the drying process.

Plants should be dormant before harvest, or the bulbs won’t store well. The sign to harvest is when most of the tops have fallen over. Harvest when the weather is dry, remove loose soil, then place in a warm and airy place, out of direct sun, for three weeks before storing.

Want to grow bigger onions...

If so, there are some tricks of the trade.

  • Start early and make sure you have the right ‘large bulb’ variety.
  • Add plenty of nitrogen into the soil. 
  • Avoid onion sets, start from seed.  
  • Water frequently and mulch often to retain moisture through the growing season. 
  • Sandy loam soil is the best for large onions.
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