Nothing says summer like the taste of sun-warmed ripe blackberries. Unfortunately, for some blackberry-lovers, moving north of the United States Department of Agriculture's Plant Hardiness Zone 5 meant giving up a favorite fruit. In recent years, however, breeders have worked to extend blackberry hardiness, and several varieties claim to be hardy into zone 3, where winter lows can run from minus 30 degrees F to minus 40 degrees F. Sun-catching strategies plus winter-weather protection allows home gardeners to try growing blackberries as far north as parts of Canada.
Ask Woodstock Nursery in Wisconsin for information on Darrow and Stenulson blackberries. Although blackberry plants are listed under raspberries, Woodstock assures customers that these are neither a blackberry-raspberry cross nor a black raspberry, but true blackberries. Stenulson is the Woodstock development of a local wild strain, which the nursery notes grows well in its zone 3 microclimate.
Choose Illini Hardy blackberries, advises Bachman's Minnesota Nurseries. Read the descriptions of hardiness zones and suitable plants under "Hardy Plants for 'The Lake.'" Note that the link cited in references will take you directly there or to the full library of Garden Care sheets--there appear to have been recent changes in site organization. Look for "Hardy Plants for 'The Lake'" under "Minnesota Gardening" and learn about good companions for your zone 3 blackberries.
Nova Scotia Berries
Investigate Balsors Hardy blackberries from Nova Scotia, carried by the Canadian branch of Vesey's, for large, sweet late-summer berries. The major heading says zone 4, but the full span of hardiness zones listed runs from 3a to 8b.
Doyle's Patented Berry Bush
Check out Doyle's thornless blackberries, patented by Doyle's Thornless Blackberries in Washington, Indiana. Garden forums speculate whether these bushes, which originated in Alberta, can grow in zone 3. Doyle's merely notes that, at minus 20 degrees F, plants require covering with plastic or a temporary greenhouse. Claims for yield suggest a Doyle's plant produces what 10 others would bear but the site makes no definite hardiness statement.