Wikipedia
Blackberry winter is a colloquial expression used in south & midwest North America, referring to a cold snap that often occurs in late spring when the blackberries are in bloom. Other colloquial names for spring cold snaps include "Dogwood winter," “Whippoorwill winter,” "Locust winter," and “Redbud winter.” The different names are based on what is blooming in particular regions during the typical spring cold snaps. Another colloquialism for these spring cold snaps is "Linsey-Woolsey Britches winter," referring to a type of winter long underwear which could be put away after the last cold snap. The Blackberry winter term may have arisen to describe the belief that a spring cold snap helps the blackberry canes to start growing.
"Blackberry Winter" is also the name of a song written by Edith Lindeman and Carl Stutz. It became a back-door million-seller as the B-side of Mitch Miller's recording of The Yellow Rose of Texas, a #1 hit in the U.S. in 1955.
Usage examples of "blackberry winter".
This was probably winter's last hurrah, Sarah thought-blackberry winter, the cool spell that came right after the blackberry bushes had bloomed.