Plants respond to the warmth that accumulates over many days by breaking dormancy and starting to grow. If this growth occurs too early in the year, a late season cold snap can do real harm.
The effects of temperature on plants can be estimated by adding up growing degree day units, also called heat sums. In mid-February, heat sums were well ahead of normal in all of the southeastern US. West Virginia, Delaware, and parts of Maryland and Pennsylvania were feeling the early heat. Will this lead to an early break in dormancy and increased risk of damage from frost or late season cold?
Two resources to help monitor the situation:
- The Integrated Plant Protection Center of Oregon State University has maps of degree days and differences this year from the normal buildup of degree days (updated daily).
- The National Phenology Network monitors degree days and also watches for the first appearance of leaves to judge when spring has arrived. See their map of whether leaf out this year is ahead of or behind schedule.