How to Grow Onions Successfully

Onions balcony garden

Are onions really worth growing in your garden? They're inexpensive to buy in most markets, and there are so many more intriguing vegetables to grow, right? Not at all! You can grow flavorful, super-sweet onions in your garden, and it's easy to do once you know a few tricks.

Onion Types, Demystified

Short day vs. long day vs. day-neutral - what does it all mean?

Onion plants need cool weather to grow their green tops and warm weather to ripen the bulbs. They don't begin to form bulbs until both temperature and day length are suitable for the variety, which means that distance from the equator is important in growing onions.

In areas closer to the equator, day length does not vary greatly from one season to the next, while in areas further from the equator, the difference in day length is dramatic.

Short Day Onion Plant Varieties

'Short day' onions are suitable for areas with even day length, such as the southern part of the United States; they will form bulbs too soon if grown further north in response to the long summer days. These onions do not usually store well.

Some good short day varieties include 'Texas SuperSweet, 'Yellow Granex' which is also known as 'Vidalia,' 'White Granex,' and 'White Bermuda.'

Long Day Onion Plant Varieties

'Long-day' onions are suitable for the north; they don't form bulbs in the south because the days aren't long enough. They need fourteen to sixteen hours of daylight to bulb satisfactorily. They generally store better than 'short day' varieties and have a more pungent flavor.

Some long day varieties to consider include 'Walla Walla,' which is a well-known sweet onion, white or yellow 'Spanish' onions, or 'First Edition.'

Day-Neutral Onion Plant Varieties

Day-neutral varieties will grow well anywhere. For these, you'll mostly want to pay attention to the days until harvest, being sure that your growing season is long enough to let them grow to maturity.

Some good day-neutral varieties to try in your garden are 'Cimarron,' 'Red Candy Apple,' and 'Superstar,' all of which take 80 to 90 days to mature.

Planting Onions: Seeds or Sets?

Onions have a bit of a reputation for being difficult to grow. Timing and soil are everything when it comes to growing these pungent members of the allium family. Onions are usually started in one of two ways: via sowing seed or by planting sets (which are basically small onions).

Starting From Seed

If you have the space indoors, as well as the patience, starting from seed might be a good option. It's definitely more cost-effective, since seeds are quite a bit less expensive than sets and you get more for your money. However, you need to grow onions inside for six to 12 weeks before your last frost date, so that's a significant amount of time and space if you have limited space under your grow lights.

If you have a long growing season, you can sow onion seeds directly into the garden.

To start from seed, follow the timing instructions on your seed packet, and sow the seeds a quarter inch deep in seed starting mix. Keep the flats or pots well-watered, and work on hardening them off once your garden soil can be worked in spring.

You can plant onion seedlings outside after your last spring frost date. They won't really grow much until temperatures reach the 50s and the days start getting longer.

Onions will take 120 to 175 days to mature if planted from seed, depending on variety.

Starting From Sets

Sets are onions that have already gone through one season of growth and then have been harvested when small, at the end of that first season. When you plant them in your garden, you're essentially giving them a second season of growth, and often, being biennial plants, they'll send up a flower stalk at the end of the season - incidentally, you'll want to trim off any flower stalks you see, since you want the plant's energy to go to the bulb.

Like most bulbs, onion sets can withstand a bit of cold; in fact, you can plant sets when air temperatures start hovering around the high 20s and low 30s, as long as the soil is able to be worked.

Plant the sets about two inches deep and space them three to six inches apart. Onions grown from sets take about 90 to 100 days to harvest, again depending on variety.

Growing Onions in Your Garden

close-up of growing onion plantation

Once you've figured out the best type of onion to grow in your area, and decided whether to start with seeds or sets, you're ready to grow.

The onion bulbs won't start growing until average daytime temperatures reach 50 degrees or warmer, but they'll start sending up more green growth even before the weather warms up.

Light and Soil Requirements

Onions need full sun (at least six hours per day) and moist but not soggy, well-drained soil. Sandy loam is ideal. No matter which type of soil you have, it's a good idea to amend it with a nice amount of compost before planting.

Fertilizing

Onions are fairly heavy feeders. In addition to planting them in fertile soil, you'll want to fertilize them every four to six weeks with a high-nitrogen fertilizer to encourage bulb growth (the bulbs of onions are actually a structure formed from fleshy leaves that make up the layers of the onion.)

Don't go overboard, though; if the onions are starting to push themselves up out of the soil, they're growing too fast and you'll want to hill some soil over them and cut down on the fertilizer a bit.

Pruning

If any flowering stems form, remove them from the plant as soon as you notice them. This keeps the plant's energy focused on bulb growth rather than flower production.

Pests and Diseases

While they don't generally have many pest or disease issues, there are a few things you'll want to watch out for when growing onions.

  • Onion thrips are occasionally a problem. They can be hosed off with a stream of water or washed off with a soap spray.
  • Onion root maggots, which burrow into the bulbs and eventually kill them, can be a real problem. If you know you've dealt with these pests before, be sure not to plant onions in the same area (crop rotation is always a good idea) and consider spreading diatomaceous earth on the surface of the soil at planting time. This is an effective deterrent, since it irritates the undersides of the maggots, often killing them before they reach the onion bulbs.
  • Onions are rarely eaten by pests. In fact, onions or chives are often used as companion plants to repel deer, rodents, and other garden pests.
  • If onions are planted in very wet, heavy soil, they can sometimes rot. If this happens, there's nothing you can do to save them. Dig them out and compost them, and consider a different spot next year.

Harvesting Onions

You can eat onions before the bulbs mature; in fact, many people prefer "green onions" to mature ones.

It's easy to tell when onions are mature: the tops yellow and fall over. There's no need to bend the tops to try to hurry maturity; they'll fall over by themselves when the plant is mature.

After the tops have fallen, carefully dig up the onions and set them aside to dry and cure. If the weather is sunny, they can cure right on the ground. If it's rainy, move them to a warm, dry place that has good airflow.. Some people dry onions on a screen for better air circulation.

Don't cut off the tops. Let them dry too, until the necks of the onions are brown and rather thin.

You can store onions by braiding the tops together in a French braid and hanging them somewhere dry and cool. Weave a strong piece of twine into the braid so the weight of the onions won't break it.

Lots of Flavor, Right From Your Garden

It's easy to bypass growing onions. They seem, sometimes, like more work than they're worth, and sometimes it's hard to justify making room for them in the garden. But there are amazing, flavorful varieties of onions that are so much more interesting than the standard onions available in most grocery stores. Grow your own, and you'll never want to go back to store-bought!

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How to Grow Onions Successfully