Cosmos · Edmonton, Alberta, Canada
When to sow cosmos in Edmonton
Main outdoor window
20 May - 3 Jun
The window this page is built around.
Timed from last spring frost
13 May
In Edmonton, on your chosen risk.
Ready to pick
29 Jul - 18 Aug
If you plant at the start of the window.
The average last frost. Half of all years are later than this.
Step by step in Edmonton
Each window is a range, not a single day. Earlier in the range buys you a longer season; later in it buys you a safer one.
Start indoors
15-29 Apr
Four weeks before your last frost, under cover or on a windowsill.
Sow direct
20 May - 3 Jun
One week after your last frost.
Plant out
20 May - 3 Jun
One week after your last frost.
In flower
29 Jul - 18 Aug
Cosmos through the year
Against the frost risk in Edmonton
Why these dates
Sow direct around the last frost date. Pinch young plants once to produce a bushier, more productive plant.
Frost-tender at both ends, but flowers freely until the first autumn frost stops it outright.
Seed packets and extension guides express timing as weeks either side of the last spring frost, because that is the only reference point that travels. Cosmos are timed from 4 weeks before that frost for an indoor start, with planting out 1 weeks after it, and this page simply resolves those offsets against the frost date computed for Edmonton.
The median frost-free season here is 137 days, against 70 to 90 days to first flower for this crop counted from sowing. That margin is what decides whether a late sowing is worth attempting.
Also in Edmonton
Other flowers to time
Growing cosmos in Edmonton
When should I plant cosmos in Edmonton?
Aim for 20 May - 3 Jun in an average year. If you would rather not gamble on the weather, wait until 29 May - 12 Jun - by then frost has finished in nine years out of ten.
When will cosmos be ready to harvest in Edmonton?
Expect to start picking around 29 Jul - 18 Aug, based on 70 to 90 days to maturity from the planting date calculated for this location.
How much frost will cosmos tolerate?
Frost-tender at both ends, but flowers freely until the first autumn frost stops it outright.