Critical Spring Temperatures for
Tree Fruit Bud Development Stages 

The March 27 issue of the Brattleboro Reformer included an article, “Warm temps threaten state’s apple crop” (http://www.reformer.com/ci_20262150/warm-temps-threaten-state-rsquo-s-apple-crop?IADID=Search-www.reformer.com-www.reformer.com).    

We’re into April now, but it seems that the weather’s cooled down, and apple growers are a bit less anxious than they were in March. Just what does it take for cold weather to damage fruit buds?  That depends to a great extent on the development stage(s) of the buds.  The table below, from Mark Longstroth of Michigan State University, provides photos of shoot development stages and temperatures (in degrees F) required to kill 10% or 90% of normal fruit buds.  The old standard temperature is the lowest temperature that can be endured for 30 minutes without damage.

Differences in microclimates and varieties generally prevent from all buds being vulnerable to low temperatures at any given point in time. In late April and early May of 2010, Vermont apple orchards suffered minor losses across the state when temperatures dropped to 28 degrees F during first pink and full pink.