Sweet alyssum · Philadelphia, Pennsylvania, United States
When to sow sweet alyssum in Philadelphia
Main outdoor window
7 Apr - 5 May
The window this page is built around.
Timed from last spring frost
14 April
In Philadelphia, on your chosen risk.
Ready to pick
27 May - 11 Jun
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 Philadelphia
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
17 Feb - 17 Mar
Eight weeks before your last frost, under cover or on a windowsill.
Sow direct
7 Apr - 5 May
One week before your last frost.
Plant out
7 Apr - 5 May
One week before your last frost.
In flower
27 May - 11 Jun
Sweet alyssum through the year
Against the frost risk in Philadelphia
Why these dates
Sow direct a week before the last frost, or start indoors eight weeks earlier for immediate effect. Shear the whole plant back by half when it stops flowering in midsummer heat and it will flower again within three weeks. Self-seeds into paving cracks and is welcome there.
Takes light frost to about -6C and often keeps flowering after the first autumn frosts have finished everything around it. In zone 9 and warmer it flowers through winter and behaves as a short-lived perennial.
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. Sweet alyssum are timed from 8 weeks before that frost for an indoor start, with planting out 1 weeks before it, and this page simply resolves those offsets against the frost date computed for Philadelphia.
Hardiness is not the obstacle here. Philadelphia is zone 7a, comfortably inside the range this crop is rated for, so an established plant should come through an average winter without protection.
The median frost-free season here is 202 days, against 50 to 65 days to first flower for this crop counted from sowing. That margin is what decides whether a late sowing is worth attempting.
Also in Philadelphia
Other flowers to time
Growing sweet alyssum in Philadelphia
When should I plant sweet alyssum in Philadelphia?
Aim for 7 Apr - 5 May in an average year. If you would rather not gamble on the weather, wait until 16 Apr - 14 May - by then frost has finished in nine years out of ten.
When will sweet alyssum be ready to harvest in Philadelphia?
Expect to start picking around 27 May - 11 Jun, based on 50 to 65 days to maturity from the planting date calculated for this location.
How much frost will sweet alyssum tolerate?
Takes light frost to about -6C and often keeps flowering after the first autumn frosts have finished everything around it. In zone 9 and warmer it flowers through winter and behaves as a short-lived perennial.