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All About Bulbing Onions
Posted on : November 28, 2017

Onions are easy to grow! And garden fresh onions are just the best. They are crisp, full of flavor, and are a good source of anti-oxidants and anti-inflammatory minerals.

When you grow your own, you can pick them at any stage you like for the table. You don’t have to wait until they are fully bulbed. You can pick them as green onions in 25-30 days after planting. You can use small bulbs for “spring onions” in 60 – 90 days. Spring onions are slightly bulbed onions that can be grilled or roasted whole, or used like “pearl” onions with peas or wilted greens.

Day length matters
If you have ever planted onion “sets” that were just labeled “red”, “yellow” or “white” and were disappointed that they didn’t form lovely little softballs of sulfurous delight, it is likely that you were sold the wrong type of onions. It wasn’t anything you did wrong or a missing nutrient, no matter what Neighbor Sam says.

Onions form bulbs in response to day length. There are short day onions, long day onions, and a group called day neutral or intermediate day onions.  

The length of a day increases as you move away from the equator.  The South never sees days as long as the North, therefore Gulf Coast gardeners must grow what is known as a short day onion if we are to have any expectation of bulbing. 

Short day onions begin bulbing when the day length is between 10 & 12 hours.  Houston’s average day length on December 21, the Winter Solstice, is just over 10 hours.  Day length reaches 12 hours about March 15.  Day length never exceeds a few minutes more than 14 hours on the Summer Solstice, June 21.

We can’t grow intermediate day onions either. They need between 12 & 14 hours of day length. We hit 12 hours in mid-March, and don’t reach 14 hours until June.  By that time, temperatures in our area are not favorable to growing onions. 

Seeds, seedlings, or sets?
Onions can be grown from seeds, seedlings or sets. There are people who swear by planting from seed believing that they bulb faster, produce a larger bulb, and have less of a tendency to bolt. I have never tested this theory. However, I do grow scallions from seed pretty regularly and they do very well.

Seedlings are slender young plants that have been grown in the current season. They are dug when they are about pencil size, bundled, and shipped for transplanting. Their main advantage is ease of planting and nearly 100% survival rate.

We often call seedling onions “sets” around here, but an onion set is actually a tiny bulb that was grown last season. They are dug when they are less than 1” diameter, dried, and allowed to go dormant. Once they are planted in your garden, they resume bulb development. There are some varieties of sets that will do well here, but always check the variety to make sure it is a short day onion.

How to grow bulbing onions

Bed prep – All onions like a well-drained, loose, fertile soil.  Prepare the soil 8″ – 12″ deep. Add 1/3 compost by volume to garden soil or prepare the bed with Arbor Gate Organic Soil Complete.

Fertilizer – Onions are relatively light feeders. Start with ½ cup of Arbor Gate Organic Blend and 1-Tbls of Texas greensand per square foot. Side dress the seedlings with ¼ cup per square foot three weeks after planting, again when they are 8” tall, and one last time when they are beginning to bulb.

Planting – Snip the bands or twine holding the bundle together.  If the roots were not trimmed by the grower, or if you grew them from seed yourself, trim the roots to 1/2″ long.  Trim the tops so that the seedlings are all 4″ – 6″ tall.  This will help keep them from being top heavy when they are planted.

While some growers do not recommend pre-soaking, I have found that it gets them off to a quick start.  Place the seedlings in a small bowl with just enough water to cover the roots, no more.  You can do this the night before planting.  Do not leave onions soaking for more than a day.

Plant the base of the seedling 1″ – 1.1/2″ deep into prepared soil.  Make sure the youngest leaf is not buried.  Onions should be spaced 4″ – 6″ apart for full sized bulbs.  You can plant them as close as 2″ apart if every other one will be harvested as a spring onion, leaving the remainder to bulb fully.

If the seedlings are of good size and hardened off, and the soil has been prepared with compost, they can often just be pressed into the soil without using a dibble or making a furrow.  If they are too tender for this, you can make a furrow, space the seedlings, and pull the soil back over them in the row.  If planting in wide beds or square foot beds, use a dibble to make holes, set the seedling in the hole and press the soil around the roots.  A wooden pencil makes a great dibble.

Watering – Water well after planting.  Keep the soil evenly moist until the onions are making new growth. After new growth is observed, water once a week unless we have sufficient rainfall.  Onions prefer well-drained soils and even moisture, but should never be water-logged.

Aftercare – Keep bed free of weeds. Mulch before weeds appear.  Do not cultivate deeply as you may damage the roots or bulb.  Watch for disease or pests. Thrips are occasionally a problem with onions. Light row covers for prevention of thrips is the most effective control. If you observe thrips damage, use Monterey Garden Spray. Onion fly feeds on the roots at the larval stage. You can manage this pest with crop rotation and by using beneficial nematodes. Predatory ground beetles also prey on onion maggots – don’t squash them!

Harvest – Onions can be harvested at several stages – as green onions once they reach pencil size, as small spring bulbs, or once they have fully matured as large bulbs.  At maturity the leaves will begin to yellow and fall over.  It is not necessary to bend the leaves over manually.

Unless you need the garden space, onions can be harvested as they are needed from April – June. Harvest within a month of yellowing or when 3/4 of the leaves have fallen over, after which they should be dug, cured, and stored in the refrigerator for the longest storage.

Curing – Dig or pull the onions on a dry, sunny day.  Lay the freshly dug onions out in a sunny place to stop the roots from growing and kick start the curing process.  After a day or so, spread the onions out in a dry, shady location and allow them to dry until their skins rattle.  This can take 2 – 3 weeks.  A shady porch or garage floor covered with paper is a good location.  It is not necessary to trim the leaves, but if you wish to do so, leave at least 1″ so the onion will seal itself off at the top.  Use any onions with damaged or soft spots first.

Written by Angela Chandler

Angela Chandler is a lifelong gardener with a passion for learning and teaching. She tends a ½ acre garden in Highlands, Texas that includes ornamentals, fruits, a small experimental nursery, a flock of Buff Orpington chickens, and a Lab mix named Harley. Her gardening adventures would not be possible without her husband, Fred – always willing to help unload leaves, compost and help build beds. Angela is a member of the Harris County Master Gardener Association – Retired, and a member of the Garden Writer’s Association.

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