Days to germination: 6 to 12 days
Days to harvest: Spring (green onions) 60 – 70 days, bulb onions 100 – 120 days
Light requirements: Full sun
Water requirements: Regular watering
Soil: Loose and high in nitrogen
Whether you grow thin scallions, or the usual fat bulbed white ones, onions are one of the most popular garden vegetables because they are used in so many dishes. Red onions like Redwing offer a bit of color as well as milder onion flavor.
Scallions (also called spring onions or green onions) are often eaten raw, or added to cooked dishes as seasonings. The white bulb is usually eaten when raw, but the green leaves are added in for cooking.
Larger onions are more often cooked but can be eaten raw as well (especially some of the milder varieties). The leaves of bulb onions are seldom used.
Other than adding great flavor to foods, you can get chromium, vitamin C and fiber from your onions. The pungent nature of onions makes it naturally anti-bacterial and can help with digestive problems.
Starting from Seed
You have 3 options for starting your onion plants. You can seed them directly into the garden, start the seeds indoors for transplanting, or plant small purchased bulbs (called sets) out into the garden. Sets are the easiest method that will produce the quickest harvest but are also the most expensive way to grow onions. Thin green onions are usually just grown from seed, not sets.
If you’re using transplants, then you should start your seeds 8 or 9 weeks before your frost date, to grow your seedlings to the right size for transplanting. You can use regular seed flats, onions have sturdy roots and transplant well. Seeds should be just 1/4 inch under the soil. They should start to sprout in just over a week.
Regardless of your method, you’ll want your onion plants outside in the garden about 3 weeks before your last frost date. They can handle a little frost. So if you are just direct seeding, you’ll want to get your seeds planted outside at that time. Plant them the same depth (1/4″) as indoors, and keep them 2 to 3 inches apart. Green onions can be planted about an inch apart or just scatter the seeds, and thin them later.
Transplants should go out at the same time. Dig up the soil to loosen it and remove any large stones. Onions bulbs don’t grow down very deeply, so you don’t have to remove stones for more than a few inches.
For onion sets, they’ll also go into the soil at the same time as any transplants. Onion sets don’t really need to be buried, but rather just pushed slightly into the ground on the surface. Set them upright with the point upwards.
To get a staggered harvest period for green spring onions, plant more seeds or sets later in the season and you can have fresh onions all summer long.
If you plant your onions around your other plants, their smell can help keep some insect pests out of your garden.
Onions are different than many other underground vegetables in that nitrogen in the soil is beneficial rather than a problem. Adding a fertilizer that is high in nitrogen will really stimulate your plants to grow bigger onions. But take care not to use the same fertilizer near radishes or broccoli, which don’t produce well with too much nitrogen.
Other than that, just keep them watered and weeded through the summer.
Both bulb and spring onions work fine in containers, though the bulb type will obviously need a larger pot than the smaller green type. Use any of the three starting methods already mentioned above.
Kincho is a very good variety of spring onions for pots because they don’t bulb, and take up the least amount of space.
Pests and Diseases
The strong smell of onions is a deterrent against many insect pests, but that doesn’t mean onions are completely problem free.
Onions leaves can get Botyrtis blight, sometimes called blast blight because it hits fast and hard. Pale patches will appear on the leaves, and they quickly spread to the entire plant. Fungicide can sometimes help if you catch the problem quickly enough. Otherwise, you will have to dig up the plants and try seeding elsewhere in the garden.
A related problem is onion white rot. Your first sign of it will be yellowing and then wilting of the leaves. Dig up one of the onions and check the roots. If you find a mass of white fungus instead of roots at the end of the bulb, then you have onion white rot. Again, there is nothing you can do except dig up the plants and avoid this part of the garden for onions or garlic for several years.
Harvest and Storage
The “days to harvest” figures provided above refer to how long it takes when you grow your onions from seed. If you grow from sets, it will be about half that time.
Green onions can be picked and used as soon as they get large enough to warrant pulling. You can tell when bulb onions are nearly ready to harvest when the leaves start to bend over. When all the leaves have fallen, your onions are ready. You really shouldn’t manually bend the leaves over as some gardeners seem to do. Let the plant mature naturally.
You can pull them sooner if you wish, and you will just smaller onions.
If you planning on storing your onions, you should “cure” them first. After you’ve pulled the bulbs up, lay them in the sun for 4 or 5 days. Cover them up or bring them inside at night though. The outer skins will get dry, and they will store much better. You can cure them inside as well, but natural sunlight works better.
Large bulb onions can then be stored in a cool, dark and dry place for several months. They need a ventilated container, or even a net bag. With a little luck, you may still be using your own garden onions by the time new ones are coming up the next spring.