According to Wikipedia, "In South & Midland North America, Blackberry Winter is referred to as a period of cold weather as low as -6 °C (20 °F) in late spring when the blackberries are in bloom. This is generally in the month of May and is known to last approximately up to 14 days.
Prior to technological advances in meteorology, farmers would use such terms to know when to plant certain crops. Blackberries must have a cold snap to set buds on the blackberry canes. It comes with a somewhat less severe return of a continental polar air mass after the maritime tropical air masses have begun to dominate the weather. So, there will be a cold snap when the blackberries bloom, called Blackberry Winter and it is traditionally the opposite of Indian Summer in terms of seasonal shifts."
And "In Tennessee dogwood winter and blackberry winter are two separate occasions. Dogwood being first and blackberry a couple of weeks later."
This is usually one of the last cold snaps to hit us and we always wait until May 5th to plant flowers. Today we have had the first day of Blackberry WInter 2012! It began at 36 degrees this morning and spitting snow! The blackberries are definitely blooming, and Dogwood Winter was about 2 weeks ago...so we're right on time!