When is the optimal planting time for onions?
Written by
Olivia Mitchell
Reviewed by
Prof. Martin Thorne, Ph.D.The right time to plant is entirely dependent on your climate zone. Everywhere, it is early spring once any danger of frost has passed. Soil temperatures of 50°F are best for sets. I rely on forsythia blooms as my natural clue. Your timing has a direct bearing on the size and success of your harvest.
Consider overwintering varieties in moderate climates. Plant these in autumn, six weeks before your first frost. They start developing roots before going dormant and then explode (in a good way) in the spring. My 'Electric' onions manage to get through winters down to 20°F beneath a mulch of straw. Frost dates in your area will indicate when to plant in autumn.
In measuring soil temperature before planting, use a probe thermometer, inserting it at a depth of four inches. Neglect no detail. The temperature should be at least sixty-five to seventy-five degrees for seeds, but below that, down to fifty will do for sets. I take the temperature daily for three consecutive days at 8:00 a.m. The warmer the soil is, the better your planting will be.
Spring Planting
- Start seeds indoors 8 weeks before last frost
- Harden off seedlings for 7 days before transplanting
- Apply balanced fertilizer at planting time
Autumn Planting
- Direct sow 6 weeks before first frost
- Mulch heavily with straw after ground freezes
- Remove mulch gradually during spring thaw
Soil Readiness
- Test drainage: dig 12-inch hole, fill with water
- Ideal drainage: 1-2 inches per hour
- Amend clay soils with compost for better structure
Select day-length appropriate varieties for your location. Short-day onions will be found that variety 'Granex' will suit southern zones, where the days are no longer than 10 to 12 hours; the long-day types, such as Ailsa Craig, will require the long 14 to 16 hour days of a northern summer. Your latitude will determine the varieties you can plant to produce properly bulbous onions.
Track your local frost dates through your agricultural extension agency. Many universities have tools for you to search your zip code. I mark my calendar with my estimated 2-week planting windows. Your fine-tuning for your microclimates makes all the difference. South slopes will warm sooner than low-lying areas.
Try subscription planting. Start some sets early under cloches. Two weeks later, put in another crop. A comparison will help you determine your own best planting date. I have records of early plantings made in mid-spring, which produced my largest bulbs. Your garden journal is your best guide in determining when to plant.
Read the full article: How to Grow Onions Successfully at Home
