Oldtimers also knew that blackberries (Rubus fruticosus) need a cold snap to set buds on the blackberry canes, so as sure as night follows day, there will be a cold snap when the blackberries bloom, called Blackberry Winter. 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.[1] In some areas, a late cold snap occurs with the blooming of the locust trees (Robinia pseudoacacia) usually before the dogwoods bloom or the redbuds (Cercis canadensis). So you have Locust Winter, and Redbud Winter happening after the first flush of warm spring days and before Dogwood Winter and Blackberry Winter.

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Note added at 1 hr (2011-05-18 11:48:47 GMT)
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Every gardener knows there are vagaries of weather following the first warm days in early spring, but not all gardeners know their names, or that they have names, and why. Old-timers in the Appalachians know there are several named "winters" following winter, and Blackberry Winter is perhaps the best known. There is also Dogwood Winter, Locust Winter, Whippoorwill Winter, Redbud Winter and even Linsey-Woolsey Britches Winter.