Season Extension
A season-extension calendar for Canada
Short-season gardening is less about a single planting day than about layering protection so the useful season stretches at both ends. This calendar is a framework rather than a fixed schedule: the right dates depend on your local last-frost and first-frost timing, which varies widely across the country. Treat the months below as relative stages and shift them to match your own conditions.
First, anchor the calendar to local frost dates
Before applying any schedule, find your area's average last spring frost and first autumn frost. These two dates frame the open-garden season; everything season extension does is push activity earlier than the first and later than the second. Local agricultural extension resources and historical climate records are the reliable place to confirm them.
Why relative dates: a coastal garden and a prairie garden can differ by many weeks. A calendar that says "March" for one region may mean "early May" for another. Always translate the stages below into your own frost-date window.
Late winter — start under lights, prepare structures
- Start slow crops such as onions, leeks, and some herbs indoors under lights.
- Clean and repair cold frames and greenhouse glazing; check that vents move freely.
- Pre-warm outdoor beds by covering them with clear or black film a couple of weeks before sowing.
Early spring — first protected sowings
- Direct-sow cold-tolerant greens and radishes inside cold frames or low tunnels.
- Begin hardening off indoor seedlings in a cold frame, venting on sunny days.
- Watch for clear-sky overheating; a frame can swing from frost risk at dawn to too hot by midday.
Late spring — transition to the open garden
- After the last expected frost, move warm-season transplants outdoors, keeping row cover handy for late cold snaps.
- Use the greenhouse for heat-loving crops like tomatoes and peppers where the outdoor season is too short to ripen them reliably.
| Stage | Under cover | Main goal |
|---|---|---|
| Late winter | Indoor lights, prepped frames | Get a head start on slow crops |
| Early spring | Cold frames, low tunnels | First hardy sowings, hardening off |
| Late spring | Greenhouse, row cover | Protect tender transplants |
| Late summer | Tunnels, frames | Sow autumn and overwintering greens |
| Autumn | Cold frames, tunnels | Extend the harvest past first frost |
Late summer — sow for autumn while it is still warm
The cool-season crops that carry a garden into late autumn need to be sown while soil is still warm and days are still long enough for them to bulk up. Mache, spinach, hardy lettuces, and many Asian greens fit this window. Getting them established before the light fades is the key to harvesting them under cover later.
Autumn — protect and harvest
As frosts return, move protection back into place. Low tunnels and cold frames over established greens can keep them harvestable well past the point when the open garden has stopped. On the coldest nights, an extra layer of row cover inside a tunnel adds another margin of safety.
Further reading
For broader context on growing under cover, see Wikipedia's season extension article and the public horticultural resources from Agriculture and Agri-Food Canada. Pair this calendar with the guides on greenhouse glazing and cold frames.