How to Grow Onions
Onions grow best in full sun in light, well-drained soil. If you have clay soil, amend the soil by mixing in compost or well-rotted manure prior to planting. If you have a source of phosphorous available such as bone meal, mixing this in at the time of planting is also beneficial.
Plant your seedlings 4 - 5" apart, 1 - 1-1/2" deep in rows 12 - 24" apart. Water them well after planting and watch to see if the soil drains well.
Water your onion plants once a week. Avoid overhead water if possible, as this can encourage foliar disease. In the first stage of development, your plants will grow their leaves. When the days reach 11 to 16 hours in length, depending on variety, the bulbs will begin to develop. At this stage the leaves stop growing as the plant directs nutrients to bulb formation. If flowering heads begin to grow, cut and remove them to keep the onion's energy moving to the bulb.