When is the optimal time to plant onions for bulb development?
Written by
Tina Carter
Reviewed by
Prof. Charles Hartman, Ph.D.Timing is more crucial to success with onions than any other single factor. Plant onions when the *soil temperature* is at least *40°F*, I use a $10 probe thermometer to measure *soil temperature.* If you miss this window, your onions will be pencil-thin bulbs. Southern gardeners can also plant short-day varieties, like 'Texas Grano', in the fall for harvests in early spring.
Soil Readiness
- 40°F (4°C) minimum for germination
- Test at 4-inch (10 cm) depth morning and afternoon
- Clay soils warm slower - add black plastic mulch
Day Length
- Short-day varieties need 10-12 hours of sunlight
- Long-day types require 14+ hours
- Intermediate works in zones 5-7
Frost Risks
- Sets survive 28°F (-2°C) with row covers
- Seedlings die below 25°F (-4°C)
- Use NOAA frost date maps for precision
People who garden in the city deal with challenges, specific to their environment. My onions on a rooftop in Brooklyn grew well in 15-gallon (57-liter) pots, but I had to water them every day. You want to use a potting mix that is approximately 30% perlite for drainage. If you don't have much space I would advise you to skip using bulb types and use green onions instead, because they will produce quicker in confined space.
Soil preparation is the most significant difference between hobby gardeners and professionals. Every fall, I rototill 3 inches (7.6 cm) of compost into my beds. Then I plant in the spring and add 10-20-10 fertilizer at the rate of 1 lb/100 sq ft (0.45 kg/9.3 m²). You should check your soil pH every year because onions will fail to grow properly and develop stunted necks that will snap if the pH is below 6.0.
Read the full article: When to Plant Onions: Expert Guide for Perfect Bulbs
