By Ken Johnson
Early spring often brings unpredictable weather, from 80-degree days to temperatures well below freezing. While many plants are adapted to withstand cold temperatures when dormant, they become far more vulnerable once they begin to emerge from dormancy. While early-emerging plants such as bulbs and tree fruit can tolerate cold temperatures, if temperatures drop too low, they can be damaged.
Dormancy and early growth
Plants enter dormancy in the fall, triggered by short days and freezing temperatures, as a way to protect themselves from harsh winter conditions. This type of dormancy is called endodormancy. Plants will remain dormant until they are exposed to cold temperatures for a sufficient amount of time (chilling requirement). This helps prevent the plants from ‘waking up’ during warm spells during the winter.
However, once plants accumulate enough chilling hours, usually temperatures between 32°F and 45°F, they will leave endodormancy and enter ecodormancy. During ecodormancy, the plants remain dormant while temperatures remain cold, but once the weather begins to warm (into the mid-40s for tree fruit), they are triggered to break dormancy, buds begin to swell, and growth resumes.
As plants move from endodormancy into ecodormancy and eventually into active growth, their cold tolerance declines. Buds, flowers, and young shoots become increasingly vulnerable to cold temperatures as development progresses. This means that unseasonably warm late-winter weather can push plants into active growth prematurely, leaving them vulnerable to later freezes.
Source : illinois.edu