Growing Onions in Containers
I find onions, like a lot of root vegetables, are some of the cheapest vegetables in the grocery store. You can buy them in large bags, by the pound. But if you read through a seed catalogue you'll discover a huge selection of onion varieties to choose from: not just the "white, yellow and red" like you find a the grocery store. There are lots of different colours, sizes, and maturity times. Onions are related to garlic, leeks and shallots. They are biennials but grown in gardens as annuals. You can grow them for their bulbs or eat the tops before they mature. Or grow green onions which don't form large bulbs, grown specifically for their tender green leaves.
Container grown Spanish Onions.
Onions are not large plants and develop a fairly shallow root system. This makes them well suited to container gardening. They do not require full sunlight but young plants will appreciate overhead sunlight to get started. Something I find true for all root crops. Being a shallow rooted plant, they will appreciate moisture and nutrients in the upper layers of the potting media. They don't need too much water but don't let them dry out. Don't give the leaves a chance to wilt and wither. And don't give them an excuse to bolt. A sudden drop in temperature or lack of adequate moisture can trigger bolting. Plants want to reproduce and when stressed they assume the end is near and will try to do it early. Flowering diverts energy and resources from bulb and leaf formation and (apparently) can alter the taste of the onion. But I do like the look of onion flowers. They photograph nicely.
Onions take a long time to reach maturity: up to 120 days, depending on the variety. Onions can be started from seed, indoors and out. And they can also be stated from sets. These are small dormant bulbs that are simply planted in the soil where they can continue their growing cycle. Sets are grown by planing onion seeds late in the season and pulling the bulbs in the fall before they mature. If you choose to start from sets you may not find as many varieties available to you. For me, seed is the way to go.
Onions like to do their growing in cooler weather. And they put their energy into bulb formation during the hot long days of summer. Long-day varieties, the most common, produce bulbs after they sense 14+ hours of daylight. There are shorter season varieties that will enter bulb production with less daylight. And there are even short season onions that can be planted in the fall for spring and early summer harvest. Since they take such a long time to reach maturity, you will likely be best starting with seeds early indoors to get a jump on the season.
I've started onions indoors in the early spring by densely sowing seeds in a small tray. As they sprout I thin them out a little but not much. They eventually grow into a fairly dense and tangled mess. When I'm ready to plant them outside I empty the tray and separate the seedlings, selecting the largest for transplant. In my container, I poke 1 inch holes every 3 inches and plant the seedlings in them. It takes a while but eventually they establish themselves in their new home and grow to maturity.
Most of the bulb will grow above the soil surface which makes it easy to check on their progress. Keep the soil moist to prevent bolting and promote juicy bulb formation. In time, the leaves of the plants will turn brown and fall over, signalling they are nearing maturity. Even after all of the leaves have withered, keep the bulb in the ground for a while to finish forming. To store onions, they need to dry out a little. If you pull them and let them sit out in the sun for a few days or a week, the outer skin will dry and toughen. They will store longer this way. But I tend to use mine right away. Rinse them off and start slicing.
I've had good luck with late season White Spanish onions from seed grown in a large planter that is up along the railing of my balcony where it gets unobstructed sunlight from just after noon until dusk.
A few green onions growing in a bit of extra container space.
Green Onions, Scallions and Bunching Onions all refer to onions that are grown more for their tops than their bulbs. These onions form small bulbs but that part of the plant isn't as desireable as their green tops. The tops are usually cut and used in the kitchen as a garnish or flavor enhancer. The leaves can be harvested at pretty much any point in the plants life cycle. Or you can apply a cut-and-come-again approach and just a cut a leave or two as required, leaving the plant to grow and produce more leaves. A popular garden hack is to replant the lower part of a green onion purchased from a grocery store. With roots still attached, they should continue to grow and produce leaves.
Growing onions for tops requires less patients and less space then growing for bulbs. Green onions do not need much space to grow. I usually start in the spring with a dense planting: perhaps one every square inch. I pull the plants as I need them while the rest grow larger. And the crop gets thinned out as the season progresses. I don't pay much attention to maturity times for green onions but the seed packets I've seen state it to be about 60 days We don't use a lot of green onions. Maybe an onion a day for and an omelette in the morning or an egg salad sandwich.
Spring onions are grown like scallions but they are actually larger onions usually grown for bulbs that have been harvested early, in the spring, before the bulbs start to grow and the leaves turn brown.