Select a location with full sun where your onions won’t be shaded by other plants.
Soil needs to be well-drained, loose, and rich in nitrogen; compact soil affects bulb development.
Till in aged manure (Zoo Poo) or fertilizer the fall before planting. Onions are heavy feeders and need constant nourishment to produce big bulbs.
At planting time, you can mix in some nitogen fertilizer, too, and side dress every few weeks until the bulbing process begins.
Seeding? Onion seeds are short-lived. If planting seeds indoors, start with fresh seeds each year. Start seeds indoors about 6 weeks before transplanting.
Plant onions 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.
For sets or transplants, 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.
Fertilize every few weeks with nitrogen to get big bulbs. Cease fertilizing when the onions push the soil away and the bulbing process has started. Do not put the soil back around the onions; the bulb needs to emerge above the soil.
Generally, onions do not need consistent watering if mulch is used. About one inch of water per week (including rain water) is sufficient. If you want sweeter onions, water more.
Onions will look healthy even if they are bone dry, be sure to water during drought conditions.
Make sure soil is well-drained. Mulch will help retain moisture and stifle weeds.
Cut or pull any onions that send up flower stalks; this means that the onions have “bolted” and are done.
Soil Prep for Onions
Onions will grow in practically any kind of soil but, one that's rich in decayed organic matter and humus and drains well is best. Heavy soils that stick together after rain will bake hard when the sun comes out, making it difficult for the bulbs to expand. Another problem with heavy soils is that water stays on the surface in puddles. That can drown plants! Southern gardeners can overcame this problem by making raised planting beds that are about 10 inches wide and four to six inches high. When it's time to plant onion sets in January, your raised seedbeds won't be too wet to plant. The planting beds drain well and leave the soil moist but not packed, soggy or impossible to work.
Adding Organic Matter
Adding plenty of organic matter to heavy soil will help to loosen it up and improve drainage. With light, sandy soil, the organic matter will help hold the water after rain or irrigation. If your soil has plenty of organic matter -- such as decayed leaves or compost -- worked into it, it will stay moist after a rain or irrigation. Organic matter acts like a sponge, holding moisture near the surface. That's good for onions, which have shallow roots and can't tap water or nutrients deep in the soil.
If you're wondering about soil acidity or alkalinity (pH), onions do best if the soil pH is between 5.5 and 6.5, a little on the acid side. They'll grow outside this range, but not quite as well. You can easily test your soil's pH by using a soil-testing kit, available at garden centers. Soil testing is also done by county cooperative extension agents.
Onions love fertilizer and they can take about twice as much as most other vegetables. Your onions won't mind it a bit if you add plenty of well-rotted or dried manure, compost or commercial fertilizer to the soil before planting. In addition, try adding a handful of bonemeal or rock phosphate, which is high in phosphorus, when you prepare your soil for planting. Phosphorus stimulates early bulb formation and root growth. It helps give onions a rapid start.