Onion sets are individual onion plants that have had their growth interrupted. While they are still immature, they are pulled up and dried by the grower for planting just like spring flower bulbs. Once they are replanted, they continue to mature normally, forming large onion bulbs by around the summer solstice. Many growers appreciate the convenience of planting onions from sets rather than seeds - and they certainly are faster. The trade up is that onion sets are rarely named varieties, so they produce more generic onions.
Soil should be well-drained, loose, and rich in nitrogen; compact soil affects bulb development. A neutral PH is recommended. Onions are heavy feeders and need constant nourishment to produce big bulbs. If you remember, add aged manure or fertilizer the fall before planting. At planting time, mix in some nitrogen fertilizer, such as Gaia 4-4-4 too, and side dress every few weeks with compost until the bulbing process begins. Plant onion sets as soon as the ground can be worked in the spring, usually late March or April. Make sure temperature doesn’t go below 20 degrees F. Plant the smaller sets 1 inch deep, with 4 to 5 inches between each plant and in rows 12 to 18 inches apart. Think of onions as a leaf crop, not a root crop. When planting onion sets, don’t bury them more than one inch under the soil; if more than the bottom third of the bulb is underground, bulb growth can be restricted. Practice crop rotation with onions to avoid disease.