Ask Mr. Smarty Plants

We are not sure where the Himalayan blackberry got its name; likely some commercial nursery was looking for a name that would evoke thoughts of clear mountain air and fresh, pure fruit. They probably did not want the customer to think of the thorny thickets that reproduce aggressively, take over stream beds and shade out more desirable native vegetation. We found the plant identified with three different scientific names, Rubus procerns, Rubus discolor and Rubus armeniacus, none of which are native to North America.

Your state has been particularly unlucky with this plant. See this website from Sequoia and Kings Canyon National Park Invasive and Non-Native Plants - Himalayan Blackberry.That site details the problems and has some suggestions for holding it at bay, if not destroying it. Ways of distinguishing this aggressor from less invasive native blackberries are listed, with pictures. 

Another site with information about the Himalayan blackberry in the Northwest is this one from Washington State University The Ten Most Un-Wanted Pests Himalayan Blackberry.

Both sites agree that persistence is about the only way to actually get rid of the plant. Constantly cutting off the canes at the ground will eventually starve the plant, but it can sucker from stems lying on the ground, and quickly spreads. One suggestion is to cut off the canes close to the ground and immediately paint the raw stump with an appropriate herbicide. It should be painted within 5 minutes of the cut, in order to get the material started to the roots before the cut begins to heal over. Another idea is to cut off the canes and then grub out the roots. Keep an eye out all the time for fresh suckers and yank them off the minute they are spotted. Don't spray herbicides as the spray can easily drift to a more desirable plant, and certainly never spray around berries that might be eaten.