Onion Growing Guide

Sets vs. seed vs. transplants

• Sets (small dormant bulbs) are the easiest and fastest method, planted directly in the garden, but offer fewer variety choices and can be more prone to bolting
• Seed gives the widest variety selection and often the largest bulbs, but needs to be started indoors 8–10 weeks before the last frost for a full-size harvest in most of Canada
• Transplants (young seedlings, either homegrown or purchased in bundles) split the difference — earlier and larger yields than sets, less lead time than growing from seed
• Whichever method you choose, confirm the variety is bred for long-day conditions, which covers most of Canada except the very southernmost regions

Planting

• Plant as early as soil can be worked in spring — onions tolerate light frost well as young plants
• Set sets or transplants 4–6 inches apart in rows 12–18 inches apart, in loose, well-worked soil
• Plant sets so the tip is just barely covered or level with the soil surface
• Choose a site in full sun with fertile, well-drained soil; heavy clay restricts bulb expansion

Watering & care

• Keep soil consistently moist through the growing season, especially during bulb formation in mid-summer
• Keep beds well weeded — onions have shallow roots and compete poorly with weeds for moisture and nutrients
• Stop watering once tops begin to yellow and fall over, which signals the bulbs are finishing up
• Avoid high-nitrogen fertilizer later in the season, which delays bulb maturity

Harvesting & curing for storage

• Harvest once roughly half the tops have yellowed and fallen over on their own — don't bend tops down manually to force this, which can reduce storage life
• Lift bulbs carefully with a fork on a dry day and let them lie on the soil surface for a day if weather permits
• Cure in a warm, dry, well-ventilated spot out of direct sun for 2–3 weeks, until the outer skins are papery and the necks are fully dry
• Once cured, trim tops to about an inch and store in a cool, dry, well-ventilated location — mesh bags or braided strings both work well
• Check stored onions periodically and remove any that start to soften, to prevent spread to the rest of the batch

Common problems

• Small bulbs: often the result of using a short-day variety, inconsistent watering, or weed competition
• Bolting (sending up a flower stalk): common after a cold snap in young plants, or with sets that were stored improperly — bolted onions still store poorly and should be used first
• Onion maggot: use floating row cover from planting to prevent flies from laying eggs at the base of plants
• Soft rot in storage: caused by bulbs that weren't fully cured or dried before storage, or bruising during harvest

Frequently asked questions