When to Plant Onions in London, ON
Published: April 24, 2026 · Updated: April 27, 2026
Onion timing is determined by day-length as much as frost dates. Use short-day varieties south of 35°N latitude (TX, FL, CA) and long-day varieties north of 35°N (OH, MN, NY).
London's Canadian Zone 6a (USDA 5b) southwestern Ontario climate delivers adequate rainfall, warm summers, and 170 frost-free days. Shorter than Toronto's season but long enough for reliable production of most warm-season crops with appropriate variety selection.
Onions Planting Calendar for London
Start seeds indoors: February 22–March 1
Transplant outdoors: April 5–15
Direct sow outdoors: April 5–15
Harvest window: July 4 – August 3
Minimum soil temperature: 2°C (35°F)
Days to harvest: 90–120 days
Sun requirement: Full sun
⚠ CRITICAL: Choose variety based on your latitude. Short-day varieties for south of 35°N (Texas, Florida, California). Long-day varieties for north of 35°N (Ohio, Minnesota, New York). Intermediate varieties work in the middle band.
London Climate Notes
London has 150-170 frost-free days — adequate for most warm-season crops when transplants go in by late May. The region's rich soils support heavy feeders like tomatoes and brassicas with minimal amendment.
Growing Tips for Onions
- Match variety to latitude: short-day south of 35°N, long-day north of 35°N, intermediate in between.
- Growing from sets (small bulbs) is fastest; growing from transplants gives the most variety options.
- Reduce watering as tops begin to fall over — dry conditions encourage tight, firm bulbs that store longer.
- Cure harvested onions 2–4 weeks in a warm, dry, airy location before storing — uncured onions rot quickly.
Companion Planting in London
Pair onion with Carrot, Tomato, Lettuce, Beet for mutual benefit. Avoid planting near Bean, Pea, Asparagus, which compete with or inhibit onion growth.
Pests and Problems to Watch in London
The most common pest and disease pressure on onions in London comes from Onion Thrips, Onion Maggot, Downy Mildew, Neck Rot. Floating row covers through the first 4–6 weeks after planting block adult pests from laying eggs, and a weekly scouting routine catches infestations before they damage the crop.
Other Ontario Cities
Frequently Asked Questions
When do I plant onion in London?
London's last spring frost is around May 3. Start seeds indoors February 22–March 1. Transplant outdoors April 5–15.
What Canadian hardiness zone is London?
London is in Canadian Zone 6a (USDA equivalent 5b). The southwestern Ontario temperate climate delivers 170 frost-free days from May 3 to October 20, which shapes every planting date in the local calendar.
How long is London's growing season?
London has 170 frost-free days — from May 3 in spring to October 20 in fall. That is more than enough time to finish a full onion crop (90–120 days to maturity) before the first fall frost.
Can I grow onion in containers in London?
Yes. Container growing on balconies and decks is practical in London — choose a 5-gallon or larger dark-coloured container to warm the root zone, use a high-quality potting mix, and water daily during hot summer weather. In milder climates, containers extend both spring and fall windows by several weeks.
What is the first fall frost in London?
London's average first fall frost is October 20. Most onion in London is a single spring-timed planting designed to harvest before the first fall frost.