Crops are grouped by how much cold they take, and growing degree days predict how fast they mature. Both decide when it is safe to plant out.
Hardy crops survive
light frost to about −2 °C
Tender crops die at
0 °C
Hardening off
7 – 14 days
GDD base, cool-season
4.5 °C
GDD base, warm-season
10 °C
Check local frost dates before planting anything tender.
Harden seedlings off over 7 – 14 days of increasing exposure.
Treat peas and lettuce as hardy; tomatoes and basil as tender.
Use growing degree days from the right base temperature to forecast maturity.
Watch soil temperature as well as air temperature before sowing.
Every figure above comes from these pages.