Though not native to the Pacific Northwest, naturalized wild blackberries grow on waste patches of land throughout the region. Gardeners in Washington spend more effort removing and controlling the Himalayan blackberry than growing this bramble fruit. Improved varieties offer the Washington gardener larger fruit size, higher yields and even thornless harvests.
Till the berry bed a year in advance if possible. Early tilling helps control weed growth and allows amendment of soil. Work in plenty of rotted organic matter. Growing a cover crop such as annual ryegrass and tilling it under increases soil tilth. Test the soil and choose fertilizer accordingly. Till in half the recommended amount of fertilizer before planting. Save the rest to use as side dressing when spring growth begins.
Plant blackberry varieties known to do well in your part of Washington. Local growing conditions vary widely from western to eastern parts of the state. Seattle blackberry varieties may not survive winters east of the Cascades. Dewberries produce well west of the mountains but more than one variety is required for pollination. Self-pollinating Cherokee blackberry matches well to conditions in eastern Washington.
Dig planting holes 4 inches deep in the center of the new bed. Plant in late winter. Keep canes vertical. The different varieties of blackberries grown in Washington require different spacings--use the distance appropriate to the cultivar. Vigorous trailing dewberries need about twice the space as cultivars with upright canes. Typical spacing ranges from 4-8 feet. Leave 10 feet between rows.
Set posts 15 feet apart in the row if growing trailing varieties. Drive support stakes into the ground at the end of the row. Angle the supports at 45 degrees away from the trellis. String two support wires--the top wire at 5 feet and the bottom wire at 3 1/2 feet. In Washington, erect varieties that are correctly pruned don't need trellising.
Train new trailing canes by wrapping the canes loosely around both wires in a spiral. Tie the canes to the wires with wire ties. If growing erect cane blackberries, select three or four of the strongest canes from each plant and prune the rest back to the ground in midsummer. Tip-prune the remaining canes to 3 feet in height.