There are basically three ways to plant onions: direct seeding, transplants, and sets. Tonight I’m going to talk about sets, because that is what I put in the garden before the sun went down.
Holes awaiting the insertion of the sets
Sets are onions that were planted as seed the previous year. Sets are usually purchased, but you can raise your own sets but you’ll need to direct seed them late June so they are large enough to harvest by fall. Bigger isn’t better when it comes to sets. Big sets may split into two bulbs an have a tendency produce a flower stalk very early. BTW don’t try to store it or let it mature further. It will not put any energy into growing larger and in will have a significantly reduced storage life. Pull and use or dehydrate at once, or consider saving the seed.
Once you have your sets, sort into large and small. Plant the larger sets for green onions and plant the smaller ones as storing onions. The smaller sets will produce the bigger onions. Onions will tolerate a light frost. I try to plant mine in mid April here in Michigan. Plant onion sets, pointy end up, 1½ to 2″ below soil. Pack the soil around the bulbs. Choose a location with at least 6 hours of direct sunlight. More is even better.
Sets are not the only thing to do with onions however. You also need to look into transplants and direct seeding. If you want to raise a winter’s supply of fresh onions, you’ll need to plant onions from seedlings in addition to onions from sets. I’ve noticed that onions from sets don’t store as long as those from seeds. A way around this is to dehydrate your onions and store them that way. It may be a bit smelly, but it is our preferred method of storing onions.
Red onion sets next to a still open hole.
Sets are onions that were planted from seed last year. Sets for white, yellow and sometimes red onions are available from mail order catalogs and even some grocery stores. You can raise your own sets but you’ll need to direct seed them in the garden by July so they can grow big enough to harvest and store in fall. Buy onion sets that are firm and marble size but have not sprouted. Big sets may split into two bulbs or produce a flower stalk very early. (When an onion sends up a flower stalk, pull the onion and use it; it will not continue to grow larger nor will it store well.) Sort sets into those large than a dime and those smaller. Plant the larger sets for green onions and plant the smaller ones with adequate spacing: they’ll produce bigger onions.
Because sets tolerate light frost, you can plant them outside when temperatures reach 48 degrees, around the middle of April in the Twin Cities. Plant onion sets, pointy end up, 1½ to 2″ below soil. If you’re planting big sets for green onions, space them close, almost touching. If you’re planting them for full size onions, space them 3 to 4″ apart. Firm the soil around the bulbs. Choose a location with at least 6 hours of direct sunlight. Sun all day is even better.
If you want to raise a winter’s worth of onions, you’ll need to plant onions from seedlings in addition to onions from sets, as onions from sets don’t store as long as the others.
Transplants:
Raising your own seedlings for transplants gives you a jump on spring in late winter, more choices of varieties to raise, plus it’s less expensive than purchasing transplants. Transplants are available through mail order and some garden centers. Regardless of the source, transplants will give you bigger onions by fall. Onions need to be planted indoors 8 to 12 weeks before transplanting outside, so plant the seeds in February to transplant onion seedlings outside in May. Buy only as much seed as you will use this year. Onion seeds don’t remain viable for much over a year.
The most important thing to remember when buying onion seed is day length, especially if you’re ordering from a national mail order firm. In Minnesota, we need to buy “long day” onion seeds because our summer days can be 16 hours long.
In the south, day length varies less and averages about 12 hours of light per day. Onions for southern gardeners are “short day” onions. You can raise short day onions in Minnesota but they will bulb as soon as day length reaches 12 hours and you’ll get small onions.
Long day varieties start to bulb when day lengths are about 14-16 hours. If you start onion seeds indoors, keep lights on only 12 hours each day to give the plants a suitable night. Onion seedlings will form bulbs too early if exposed to long days at any time during their development. You will not get anything bigger than sets.
Plant onion seeds ¼-½” deep in a sterile seed-starting media. They can be planted 3 to a cell in recycled four or six packs or sow thickly in rows if you plant in a deep (4″ to 6″) flat or other container. Keep tops trimmed 3-4″ tall. Water regularly to maintain adequate soil moisture.
Onion seedlings need to be hardened off before transplanting outside, after danger of frost has passed. Expose young plants to outdoor conditions gradually, over a two week period.
Plant seedlings 4″ apart and just deeply enough to catch in the soil. When mature, they’ll appear to sit on top of the soil.
Once outdoors, keep onions well watered, about an inch a week. Watering is critical when the bulbs start to swell. Water stress produces stronger flavored bulbs. Add fertilizer, based on a soil test, at planting, then again when leaves are 6″ tall and lastly when bulbs are starting to swell.
Direct seeding:
If you want to plant onion seeds directly in the garden, you’ll probably be most successful with green onions (also called scallions). Our short season makes success less likely when direct-seeding bulb-type onions.
Plant onion seeds in a sunny location where soil drains well. Cover the seeds with ½” of soil and water them regularly. Harvest when the white portion is pencil thick.
Harvest:
