The cool, cloudy weather in May or June, when blackberries bloom, is disagreeable - a "blackberry winter." But "blackberry summer" is another name for that last bit of fine weather in early October. Blackberries were identified with weather centuries ago because they were so numerous. "Plentiful as blackberries," Shakespeare wrote in "Henry IV." Part of the rose family, blackberries are a common wild fruit in England and an abundant producer of fruit. A single plant can yield five gallons of berries.
SOURCE: 'The Encyclopedia of Word and Phrase Origins,' by Robert Hendrickson.