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, also called green onions or scallions. 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. Fertilized 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.
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 https://www.burpee.com/vegetables/onions/, 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 favors 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
Spring onions, scallions and green onions describe the same vegetable: a young onion, 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 flavor 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.
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 scallions will come into the kitchen in pristine condition.
By Barbara Pleasant