I will confess that onions are one of my favorite crops (and foods). But they are also one of the crops that most shows how we need to understand the plants we are growing before we can grow them well.
Onions are related (both in botanic and practical ways) to other bulbous plants, like Daffodils, Lilies, and other plants you may have bought (or traded for). If you look closely (the key to good gardening) you will see that storage organ (the bulb) is above the roots, and separated from them by a sort of “frontier” where one world meets another. Those two worlds are the above ground and the below ground, and their meeting point is the “crown” of the plant. (For more detail see this page on Garden Smarts.)
What this means in terms of garden culture is that onions, contrary to our ordinary understanding, are a leaf crop, not a root crop. Thus early on they want more nitrogen than a root crop; it also means that they can be transplanted without much risk of ruin. (Beets are about the only true root crop that survives transplanting well…)
Here is a little step-by-step pictorial primer on how we transplant onions. In this case they were started in a cold frame in the middle of March and then set out in the open garden in mid-May. From earlier experiments I have determined that it is even better to sow them in mid-Fall (October here in USDA Zone 6-7) and transplant in early spring.
The process, in either case, is not very complicated. Simply make 3-4 furrows along the length of the bed (depending on the width of the bed and how much space you want to give the onions (a Kelsae Sweet or an Ailsa Craig needs a lot more room than an Italian torpedo onion like Tropea — the former should have at least 4-6 inches, while that latter can make do with only one or two).
Lay the onions in the furrow at the chosen spacing, and then just use your finger to push the roots into the soil. Once the roots are set, use the sides of your hands to “right” the plants and you are done (except watering them in if rain is not imminent). That’s it…you’re done!
In the case of the planting pictured, we covered the bed with shade cloth to give the onions a little protection from the late spring sun — 2-3 days is usually plenty.