Growing Onions: Sets or Seedlings?

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Starting onion seeds indoors

Onions are one of my pet crops, and bulb onions are always the first vegetable I start indoors from seed. Properly handled onion seedlings consistently mature into plump bulbs, with little risk of bolting (the biggest risk when growing bulb onions). In addition, I also find homes for a small bag of onion sets, because sets are the easiest way to grow spring onions. When it comes to growing onions, you can have it both ways.

Avoiding Bolting Onions

By nature, onions are biennial plants. Their natural rhythm is to grow from a seed, to a plant, and then into a dormant bulb in their first year. The following spring (after being exposed to cold but sub lethal winter temperatures), the bulb will begin growing again and soon produce a flower spike. Fertilised flowers produce seeds, and the life cycle is complete.

We gardeners abbreviate this cycle by growing onion seedlings and harvesting them the same season. Growing onions from sets (small onion bulbs sold for planting) creates a more explosive situation, because onion sets are entering their second year of growth, when it’s normal for them to flower. Botanically speaking, an onion set that’s been stored in cool conditions is a biennial on the brink of bloom.

Onion sets
Onion sets

There is some good news, at least for gardeners in northern latitudes. European researchers have found that sets of a few varieties can survive "heat treating" – a procedure that slowly kills the dormant embryo within the onion set, though the set itself survives. Heat-treated onion sets are held at 77-78°F (25-26°C) and 85-95% humidity for up to three months, and only the toughest survive. Those survivors seldom bolt, so they seriously simplify growing onions.

Heat-treated onion sets are widely available in the UK, but in the US this procedure has not caught on, probably because it works with only a few long-day varieties. The short-day varieties that grow into wonderfully sweet onions in the southern US can’t be grown as sets, with or without heat treatment. One of the secrets to getting top quality from day-neutral varieties (which perform great in the central US) is to keep them growing fast and vigorously, with no interruptions. This can only be done by growing onion seedlings. Onion seedlings are still in their juvenile stage of growth, so they are not easily coaxed into bolting.

Several of the larger American seed company including, Johnny's and Territorial Seeds are now offering bundles of bulb onion seedlings of superior varieties, though they are costly and sell out quickly. And as with all vegetables, growing onion seedlings yourself makes it possible to try varieties you will never find as seedlings or sets, at any price.

Among environmental factors, prolonged exposure to temperatures below 45°F (7°C) can make an onion seedling sense that it has been exposed to winter, which triggers a hormonal response that favours flowering. If you are growing onions from plants (seedlings) and still having bolting problems, delaying planting until warm weather arrives may be your ultimate solution.

Spring Onions from Sets

The spring onion is a young onions, still in its juvenile stage, featuring a few green leaves and a tender white shank. Most spring onions are mild enough to eat raw, and they provide onion flavour for the table while you wait for bulb onions to grow. Spring onions are harvested long before bolting becomes an issue, so you can use inexpensive sets to grow them. Special "bunching" varieties also can be grown from seed to produce spring onions, but they are much slower to establish compared to growing onion sets, which can be stuck into the soil neck-deep just about anywhere.

Onion sets planted in egg cartons
Onion sets planted in egg cartons

I often use green onions grown from sets to mark boundaries between spring sowings of salad greens and different types of potatoes. Spring onions are a great little crop for containers, too. Sets planted only one inch (3 cm) apart in a well-drained pot are usually ready to eat in only four to five weeks. And, although egg cartons are too small for growing seedlings, biodegradable egg cartons make fun mini-nurseries for spring onions by the dozen. After 10 to 14 days, the entire carton can be transplanted to the garden and protected from late freezes with cloches or garden fleece (row cover).

There is one messy detail to anticipate when growing spring onions from sets. When you pull the onion, there will likely be a halo of rotting set that must be cut off and composted. But if you take along a sharp knife and trim your spring onions as you pull them, the set’s unsavory remains can be quickly dispatched, and your spring onions will come into the kitchen in pristine condition.

By Barbara Pleasant

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Show Comments


"What about the major issue of cost of seed versus sets? That's a big consideration especially if you're growing lots of onions. Also compared to shop prices of the same amount."
Julia on Friday 18 March 2011
"I'm not sure cost is a major issue - I've just planted upwards of 40 bulbs each of organic Red Baron and a white variety I can't remember the name of. And I've probably got around 30 more left to do. Each bag cost me £1.20, so pretty reasonable when you consider that you don't have to expend the time and energy getting seedlings started. Admittedly you would probably get a lot more than 70 onions from a packet of seeds, but for me 70 of each variety is probably enough! Obviously P and P costs would be involved too, but can be offset if you're ordering a lot of other stuff at the same time."
James Bartlett on Friday 18 March 2011
"I live in NH (zone 5). It is my first year trying onions, and have started both scallions and yellow onions from seed, indoors. They are in small plastic seed starting trays and growing under lights at about 75 degrees Farenheit. Will they transplant ok into the garden - or will moving them damage them?"
Cathy on Friday 18 March 2011
"Hi Cathy, I hear it's sunny in NH today! I'm in Oregon and it's pouring now, but I want to get my seeds out there too. I have tried the same thing you are doing with the seeds, and found the plants to be very delicate but most survived as long as they were hardened off before planting. I'd wait till they got some strength to the stem and then when planting, I'd put a remay cloth cloche or some kind of housing to keep them from the elements till they established, hope that helps!"
stephanie on Friday 18 March 2011
"Loved the onion growing tips and egg carton suggestion especially. Novice onion grower, but going to try a set of red this year in the raised bed. Zone 5 in northern utah, usa - we'll see what comes up. Good comments, gardeners are such a wonderful lot"
Sharon Franz on Friday 18 March 2011
"I've had a great deal of trouble growing onions here in zone 5b, Indiana. I've never had them bolt, but they rarely get up to size to make a good bulb before the solstice. This year I started my onion seedlings at the beginning of February, hoping that this is early enough. We also sit right on the "long-day/short-day" dividing line, which can't help (I err on the short-day side). But my biggest problem is that my seedlings *never* look so nice as yours do in that photo. They are always flopped over, hardly standing up at all. Any suggestions there?"
Robyn M. on Friday 18 March 2011
""floppy" onion starts...could be a few things. most important is: temperature, water and water. if set have too much water, the roots swell, can't transport the water to and from the 'green' plant above soil level. not enough water, well, that's the deal. the cell structure of the plant is supported by the moisture within the delicate framework of the inner cell walls. need enough water to hold it up, kind of like filling individual water chambers in some water play thing, if the water leaks out, it flops over. One very other important thing is light. if the plants are trying to find the sun, the Phototropic properties of plants, they will go to great lengths, literally to get enough lumens to process photosynthesis for growth/energy. best is to keep the ambient temp a bit cool, sufficient humidity where they are growing, good air circulation and don't drown, or dehydrate the root. it's a tough job at first but worth the "nursery" effort no doubt!! "
pdxcitykid on Saturday 19 March 2011
"Beyond the money, I find that bulb onions are great fun to grow from seed. Sets almost always bolt in my climate, but seedlings never. The one trick to growing stocky onion seedlings not mentioned so far is to cut them back every two weeks, to about four fingers high. The seedlings in the photo were given a crew cut with sharp scissors the day before. I stop trimming them when I start hardening them off. Spindly onion seedlings take off fast when planted after the soil has warmed a bit, in mid to late spring."
Barbara Pleasant on Saturday 19 March 2011
"Ah, I suspect I'm drowning my onions--thank you very much for the advice!"
Robyn M. on Saturday 19 March 2011
"We switched from sets to seedlings because for asian cooking we want the long straight greens of "grunions" (our nickname). For us, sets grow a bottleneck above their swelling bulb that makes the greens stunted and flopped over, and the little pearl onion is darn hard to slice with a big ass cleaver! (our preferred chopping knife). Seedlings do take longer, but a spring-planted bed will keep you in grunions until winter—perhaps even until spring if your site is well situated. "
Karen Dale on Vashon Island, WA on Saturday 19 March 2011
"Barbara my question is about spring onions is it advisable to use grass for mulching , is there any danger of growing grass instead, competing with the onions"
Jovenal on Sunday 20 March 2011
"I have a pack of onion seeds called "Granex Hybrid" (large, sweet yellow). It says to sow after danger of frost in spring (162 days harvest). I am zone 7b (virginia). Does this mean I could start this from seed now and potentially have a full bulb by fall? I have another pack of onion seeds that does specifically say to start before frost in fall."
Nina on Monday 21 March 2011
"J, I mulch my onions with grass clippings because it's the easiest mulch to spread between onions. However, I wait until late spring, when the soil is warm and slugs are finished with their spring fling, before putting down the mulch. ***Nina, Granex are short-day onions, so start the seeds as soon as possible. In your climate, you should do well with intermediate day varieties like Candy, and some long day varieties if you can find them as seedlings. Good luck!"
Barbara Pleasant on Monday 21 March 2011
"Barbara, I start my onion seeds indoors, avg.66 degrees. Year after year the seeds sprout, get to about 1/32" diameter and then stop growing, shortly after that they begin to slowly die off. I've tried a half dozen different varities. I have them under a 5 bulb grow lamp. I check them every day. I only water when the top soil begins to get dry. I've tried a weak vegetable fertilizer in the water too. What am I doing wrong?"
Jeff P. on Tuesday 22 March 2011
"Do you mean you can plant the entire paper egg carton kind of like peat pots? Thank you, Carla"
Carla on Wednesday 23 March 2011
"Yes, the whole egg carton can be planted. It breaks down just like peat pots."
Jeremy Dore on Wednesday 23 March 2011
"I grow onions from sets and from plants, I haven't tried seeds yet. I love the idea of using the egg carton! Thanks- GJ"
Gardening Jones on Wednesday 23 March 2011
"LOVE the egg carton idea. We have chickens and everyone gives us their cartons. Plenty to go around. I have always grown my onions from sets in hopes of getting big fat onions for storage. It have never happened fo rme here in zone 5b in NH. Last year was the closest I have ever gotten. Half sized onions that tunred out to be great keepers. I still have some! This year I am growing onions from sets (about 80) some from seed, started last week. Also picked up some seed for bunching onions. I hope I get the best of all world here this year with onions. They have been my only veggie challange."
Jenn on Wednesday 23 March 2011
"Jeff, sounds like you have germination down and are then running into a root-rot disease problem. Use a fresh bag of seed starting mix and very small cells or containers, and give the little onion seedlings as much water as they can handle. I start feeding them when they begin spending days outside getting used to real sun. With onion seedlings, plenty of water and food is key. "
Barbara Pleasant on Wednesday 23 March 2011
"Several weeks ago your advice concerned SPINACH. For the past 60 years I've planted spinach seeds in the vegetable patch, in rows, and in 2010, I had a particularly bad germination rate - from one packet of seeds I managed just 30 plants. This year I have taken your advice - and it works! The packet says "Average contents 265 seeds" and I've now got over 300 plants, in pots, waiting to be planted out. Thanks."
Doug Beard on Friday 25 March 2011
"is it ok to trim spring onion seedlings , will it have any effect on the crop ? "
shirley hardwick on Friday 29 April 2011
"Very young onions benefit greatly from periodic trimmings, which help keep them upright. Once they are outside, exposure to wind will make them sturdier, so trimming becomes less necessary. Onions that are to be harvested green, as scallions, can be trimmed all you like. You can even harvest them by cutting them off at the ground and they will quickly regrow."
Barbara Pleasant on Sunday 1 May 2011
"I've always had the best luck from sets, they are fast and more resistant to disease. I get bigger onions in my northern climate, only the odd one usually bolts. "
Ed Hunter on Thursday 4 August 2011
"how does the drain water affect the height of spring onion?"
rose v. on Tuesday 20 September 2011
"Rose, I'm not sure what you mean by drain water. However, height of spring onions is most strongly affected by variety and intensity of sunlight."
Barbara Pleasant on Tuesday 20 September 2011
"I swear by onions started indoors from seed, under lights, in late Jan -early Feb. I second the suggestion to trim leaf tops to c. 3"high when they are getting tall - c. 4"-5" and flopping over. Keep soil evenly moist, feed only with 1/2 strength liquid fertilizer (I use seaweed emulsion) and NEVER feed when the soil is dry. I water flats from the bottom by setting them in a pan of lukewarm water til the surface shows moisture - don't soak them. One tip not yet mentioned for insuring good bulb size is to be sure to set seedlings at least 4" apart when transplanting into the garden with rows 8"-10" apart. To get the huge onions from varieties like Ailsa Craig Exhibition heirloom, the seedlings need to be planted 6"-8" apart to allow enough room for bulb development. AND regular weeding is very important. "
Christie, gardener from central Massachusetts on Friday 17 February 2012
"I live in Hopetoun on the S/W coast of WA Australia.I am unable to find any reference to onion sets in Australia but would enjoy experimenting with producing some sets for my garden.Just how these little sets are produced remains a mystery to me and I would be grateful for any info on the subject"
Hugh Marshall on Wednesday 1 August 2012
"would it be a problem planting sets in limed soil"
Graham Nall on Tuesday 19 March 2013
"As long as the lime has been well worked in and the limed soil has been rained upon, onions should not mind this soil amendment."
Barbara Pleasant on Tuesday 19 March 2013
"my trouble is the onions I planted look more like leeks. they just aren't bulbing. My garlic is doing the same thing. Am I missing some fertilizer or something? The tops just never fell over. I have some onions in the ground now that I planted last spring and they survived the winter and are sprouting again but no bulbs. "
Pat on Sunday 14 April 2013
"Pat, you will not see the garlic bulb forming because it is below the ground. As soon as the plants start deteriorating, you will find bulbs at the bottom when you dig them up. Do your onions have rounded, hollow leaves or flat ones? Flat leaves indicate leeks. Onions that have been in the ground for a year will probably flower and then set very small bulbs, behaving like the true biennials that they are. The best bulbs come from first-year plants. "
Barbara Pleasant on Monday 15 April 2013
"Do spring onions supply the same affect as normal onions, when concerning pair planting?"
Alex on Friday 17 April 2015
"I'm not sure what you mean by pair planting. Onions look to their own needs, and provide no special benefit to other plants. They are useful for interplanting because they grow straight up and take up little space."
Barbara Pleasant on Saturday 18 April 2015
"I heard that uf planting onions near tomatoes they will taje on a slight onion flavor? "
pat on Saturday 18 April 2015
"No, but the tomatoes will shade out the onions. The relationships between plants are mostly spatial ones."
Barbara Pleasant on Wednesday 22 April 2015
"I've just bought a bundle of red onion seedlings that I want to plant today. My question is, do I plant them indoors (in biodegradable containers), or can I go ahead and stick them in the ground? I'm in Eastern Washington (the dry side of the state - Zone 7a) and we're coming out of an unusually cold winter. "
Valerie Landon on Friday 17 March 2017
"Valerie, it can't hurt to wait until the weather settles, but you don't have to spoil your onion plants with individual pots. Spread them out a bit and plant them in flowerpots, so they are a finger's width apart, and grow them indoors or in a warm spot for a few days to a few weeks."
Barbara Pleasant on Saturday 18 March 2017
"Last fall lime was applied to my onion area indiscriminately. Spring I planted onion sets. Most came up but then died back after a few weeks. Applied some ammonium sulfate. Now some are growing. I am in zone 6B. Do I pull them and replant in spring or leave them?"
Gloria Freeman on Sunday 25 September 2022

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