The ecology of freezing tolerance

Spring is a very critical period. Plants on open ground may warm under solar radiation (temperatures above 20 °C are possible anytime). If plants took such daytime warming as the arrival of the growing season and gave up their dormant (hardy) state, the next cold spell would kill them.
Therefore such plants control their dormancy by a temperature independent "internal calendar" driven by photoperiod (the relative length of night vs. day). Such plants use "photoperiodism" as a safeguard against becoming active at the wrong time.
In contrast, snowbed plants are opportunists, they become active whenever the release from snow and temperatures permit.

1 - Photoperiodism prevents the breaking of plant dormancy at the wrong time.