The wild Himalayan blackberry is one of the most difficult weedy plants to control in the Pacific Northwest. Blackberry vines are so difficult to control because established plants spread by nodes – or small underground buds – that grow off established blackberry vine roots. No matter how much of the plant you remove, you’ll miss a few nodes that will soon develop into new vines. Blackberry vines are able to root from the tips of stems at certain times of the year. In early autumn, near the end of the spring-summer growth period, long vines arch over and wherever they touch the ground the tips of the vines take root, creating new plants that will sprout up the following spring. Blackberries are also spread by seeds. Birds eat the fruit and the seeds pass through their digestive tract and are deposited in their droppings.
So how do you control a plant that has so many ways to survive? Cut the vines back to ground level, especially during the spring when the plant is most actively growing. Removing the aboveground part of the vine keeps the plant from manufacturing the sugars it needs to sustain vigorous growth. Cutting vines back continually will eventually kill the plant, although it may take some time.
Brush killer containing Triclopr is readily absorbed by blackberry leaves. The herbicide is translocated through the stems to the roots and kills the entire plant. One application is never enough to completely eliminate blackberries. Some regrowth will occur and repeat applications will need to be made as new growth develops.