CORVALLIS, Ore.—It can take years to eradicate a large, prickly patch of blackberries, because even after the plants are gone, many of their seeds remain in the soil.
"Blackberries can remain an issue also because of their tremendous and effective dispersal potential," according to Andy Hulting, weed control specialist with the Oregon State University Extension Service. "Birds and animals disperse the berries—and therefore seeds—over long distances, allowing blackberries to reinvade an area."
Nevertheless, with good timing and dedication, property owners can reduce a sprawling blackberry thicket to a few manageable stragglers.
Contrary to the notion that the blackberry is as native to Oregon as rain, this hardy relative of the rose was introduced by the famous Luther Burbank. The western European blackberry he introduced in 1885 as "Himalayan giant" has become a giant problem. A single blackberry cane can produce a thicket six yards square in less than two years and has choked out native vegetation from Northern California to British Columbia.
"Several control methods work well as long as anyone going to battle against blackberry vines is armed with the benefits and drawbacks of the most common methods," Hulting said.
Digging up or plowing under can eliminate existing plants but also create an ideal seedbed for the next generation of plants not completely killed by tillage. Planting a perennial such as grass in the area provides competition with new weed seedlings, and the soil surface is no longer disturbed to bring up new seeds. This is the best practice for long-term control.
Goats or mechanical mowing both work by removing the leaves so the plant can't turn sunlight into food. The root eventually starves. Both goats and mowers must be brought back often, however, and both have the same drawback: They also mow down everything else in their path.
Another problem with goats is that they will eat only around the edges of a patch. "A lot of people find inventive ways to get goats to the center of the patch, such as mowing pathways or placing boards that goats can walk on or smashing down canes so the entire patch can be grazed," Hulting said.
Effective herbicides are available and used to control blackberry throughout the year. Each has different use rates and application restrictions depending on the intended use area; always refer to the product label for specific instructions for use on blackberry. Specific herbicide use instructions for blackberry are summarized in the Control of Problem Weeds Chapter of the online version of the Pacific Northwest Weed Management handbook.
Contrary to some popular misinformation, it is usually best not to cut down blackberry plants prior to treatment with herbicides unless the plants are too big to reach with spray equipment, Hulting said. Cutting down the plant reduces the leaf area, and the plant may not take in enough herbicide to kill the large root.
No matter if you've grubbed, chopped or sprayed, after you're rid of your blackberry plants, don't forget to plant hardy alternative vegetation that can crowd or shade out new blackberry seedlings.
"You can't treat a patch of blackberry and then walk away," Hulting said. "The control methods can take several years, at least, to eradicate a large patch. Don't take a break and let the blackberries regain their strength."