Choose onion type (sets, seeds, or transplants)
Select a sunny site with well-drained soil
Prepare soil by loosening to 8–12 in (20–30 cm) and mixing in compost
Check soil pH and aim for about 6.0–7.0 if possible
For onion sets:
Plant in early spring when soil can be worked
Space plants 4 in (10 cm) apart in rows 12–18 in (30–45 cm) apart
Place sets shallow so the tip is just at or slightly below the soil surface
Water to settle soil
For onion seeds:
Start indoors 8–10 weeks before last frost or sow outdoors when soil is workable
Sow thinly and keep evenly moist until germination
Thin to 2–4 in (5–10 cm) apart after seedlings establish
Transplant seedlings if needed to reach 2–4 in (5–10 cm) spacing
For transplants:
Plant after last frost
Space 2–4 in (5–10 cm) apart in rows 12–18 in (30–45 cm) apart
Set seedlings at the same depth they grew in their container
Water consistently:
Keep soil evenly moist during establishment and early growth
Reduce watering as bulbs begin to bulk
Mulch lightly after seedlings establish to help retain moisture and suppress weeds
Fertilize:
Apply a balanced fertilizer at planting if soil is poor
Side-dress with nitrogen when plants are actively growing, following label rates
Weed regularly to prevent competition
Manage pests and diseases:
Remove infected plants
Use appropriate organic or labeled treatments if needed
Watch for maturity:
Stop fertilizing when bulbs start to enlarge
Allow tops to fall over naturally or begin curing when ready
Harvest:
Harvest when tops yellow and fall over
Lift bulbs carefully to avoid bruising
Cure and store:
Cure in a dry, well-ventilated area for 2–4 weeks
Cut tops down to about 1 in (2–3 cm) after curing
Store in a cool, dry, ventilated place appropriate for the variety