Onions are a must-grow vegetable. To start, onions are very easy to grow and, properly prepared, bulbs will store reliably for up to six months. As with potatoes, there’s something deeply satisfying about the weighty harvest you can get from even a small area, and as the starting point to so many recipes, there’s every reason to grow your own.
Type of Onion
If you want to grow large onions, it's easiest to start with small bulbs, sold as transplants or sets. If you're growing sets into scallions or green onions, plant the bulbs 1-1/2 to 2 inches deep and 1 inch apart. If you're growing large onions, plant the bulbs 1/2 inch deep and 4 inches apart. When to plant them depends on how soon you can work the ground in spring. Onions can survive a light frost.
Keep onions moist until they reach the size you want and the green tops begin to tip over. When the soil dries out, dig up the bulbs and allow them to dry and cure in the sun (or a warm, dry, sheltered space if rain is forecast) for a week. This curing step helps the onions keep longer.
Some onions need more daylight hours to grow than others. Long-day onions stop growing their green tops and start forming bulbs when they receive 14 to 16 hours of light per day, making them a great choice for northern states (roughly the upper two-thirds of the United States, above the 36th parallel). In the Northern Hemisphere, the farther north you go, the longer the summer day length.
Short-day onions form bulbs when days contain 10 to 12 hours of sunlight, so they're a good choice for planting in the spring and fall in the lower third of the United States.
The ultimate size of onion depends not only on the type it is but the number and size of green leaves it forms. Each leaf indicates one ring of onion forming in the bulb below the ground. The larger the leaf, the larger the ring is.
Watch Them Grow
Onions grow their roots and leaves when temperatures are still on the cool side, 55 to 75 degrees F. When bulbs start to form, however, onions need warm temperatures and prefer low humidity. If there are a lot of cool, overcast days during a bulb's growing period, onion growth stalls.
Harvesting and Storage
Harvest time is approaching once most of the leaves have bent down towards the ground. Bulbs will continue to swell over the next few weeks before coloring up nicely in time for harvest.
When they’re ready, lift them up with a fork or trowel then move those destined for storing under cover to dry. Any form of cover, from an airy shed to a greenhouse is ideal. In warm, dry climates simply leave the onions where they are on the soil surface. Space bulbs out so there’s good airflow between them. Racks can help with this. This drying process, called ‘curing’, takes about two weeks and toughens up the outer skin of the onion so it will keep for longer.
Store onions suspended in nets, tied into bundles or woven into beautiful onion strings. Onions should keep until at least midwinter, and as long as spring.