We all need onions for good health. And they are easy to grow in the heirloom garden. Plant them in a warm sunny site, early in the season. Onions are day lenth sensitive. That means that when the certain day length occurs, they begin to produce the bulb, no matter how large the top has grown. If the top is small, the bulb will be small. Try to get the right heirloom onion for your area. They are usually classified as short day or long day onions.
I start my heirloom onions from seed in February in the house, under lights. It is easier to plant sets or plants and they work almost as well. But I am particular and it is difficult to get plants for onions that keep all winter. They have weak, shallow roots so they are not very efficient feeders. Give them lots of good organic matter if possible. If not, don't worry about it. Grow them anyway.
If you are a beginner, you can buy sets at most garden centers. There is just not alot of choice. Often they aren't labeled as to what variety they are. To plant them, just place them on the ground 3-5″ apart and push into the soil about an inch down and close up the hole with your fingers. When you plant them close, they will produce smaller bulbs. Encourage onions to grow as fast as possible as you will get a larger bulb. This simply means be sure to keep them weeded and watered when necessary. Since roots are shallow, it is not necessary to give alot of water at once. A mulch is helpful to keep the soil moist and weed-free.
Onions may bolt (go to seed without producing a bulb) if they were vernalized. This means that it was exposed to under 50 for two weeks when its stem was over 1/4″ . The plant reacts as if it went through a winter when the weather warms again, and begins to form seed. You can still eat the onion, but the bulb will not be very large and it will not store well.
When the tops turn yellow and die, the bulbing is complete. Withhold water and let them dry out. Leave them in the ground for another week and then harvest. Take the bulbs out of the soil, and let them dry and cure for a few days in the sun. If it is very warm, and the sun is hot, put them in the shade or they can be cooked by the sun. When tops are dried completely, trim the top to about 1″. Put them in a warm dry place to finish curing for 2-3 weeks. Store them in a mesh bag in a cool dry place; 32-50 is best. Too much moisture will encourage sprouting. Some heirloom onions will keep till February or March of the following year.