The question of whether blackberries are invasive often confuses people who simply see them as a source of delicious fruit. This confusion is understandable because the plant genus Rubus is large, containing hundreds of species. Some Rubus species are native and beneficial, while others are aggressive ecological threats. The difference between a garden variety and a noxious weed lies entirely in the specific species and its origin.
Defining the Threat: Native Versus Non-Native Species
The vast majority of invasive blackberry issues are caused by the non-native species Rubus armeniacus, commonly known as Himalayan or Armenian Blackberry. This species was introduced to North America in the late 19th century for its large, appealing fruit. An invasive plant is non-native to an ecosystem and causes environmental or economic harm upon introduction.
Native Rubus species, such as Pacific blackberry (Rubus ursinus) or Thimbleberry (Rubus nutkanus), integrate naturally into the local environment. They do not exhibit the aggressive characteristics that allow the non-native species to dominate. The Himalayan blackberry, originating from Armenia and Northern Iran, possesses a biological advantage that allows it to rapidly spread and outcompete indigenous flora.
The distinction is crucial because native blackberries support local wildlife, while the non-native species actively reduces biodiversity. This single species is the primary culprit behind the formation of dense, impenetrable thickets found across many regions, particularly the Pacific Northwest.
Visual Identification of the Invasive Species
Identifying the invasive Himalayan blackberry relies on several distinct physical features. The plant produces stout, rigid canes that are sharply five-angled, or pentagonal, in cross-section. These arching canes can grow exceptionally long, up to 12 meters, and are covered with large, stiff, reddish, hooked prickles.
The leaves are palmately compound, meaning the leaflets radiate from a central point. First-year canes typically display five leaflets, while second-year, flowering canes often have three. A particularly telling characteristic is the color difference between the leaf surfaces: the top is dark green, but the underside is noticeably paler, often described as white-felted or silverish. Mature berries ripen relatively late, from mid-summer through late August.
Ecological Impact and Mechanisms of Spread
The success of the invasive blackberry is due to its highly effective reproductive strategies. The plant spreads both by seed and through vegetative means. Seeds are produced in massive quantities—up to 13,000 per square meter—and are dispersed widely by birds and omnivorous mammals.
Vegetative spread occurs through tip-rooting, where the stem tip of long, arching canes touches the ground and forms a new plant. This mechanism, combined with the ability to sprout from root fragments, allows a single plant to quickly colonize a large area. The resulting dense thickets create a monoculture, displacing native plants by aggressively outcompeting them for sunlight, water, and soil nutrients. This dense growth prevents the establishment of native tree seedlings, halting forest regeneration and restricting the movement of larger wildlife.
Effective Management and Removal Strategies
Controlling or eliminating the invasive blackberry requires a persistent, multi-year approach due to its extensive root system and vigorous re-sprouting capacity. Mechanical removal is most effective on small infestations, focusing on digging out the entire root crown, the main nodular mass at the base of the plant. Failure to remove all root and cane fragments will result in immediate re-sprouting.
For larger thickets, repeated mowing or cutting of the canes is necessary. This must be done frequently, at least four times per year, to deplete the root system’s stored energy reserves. Simply cutting the plant down without follow-up treatments is not effective. Cut canes should be piled and dried on an elevated surface, as they can root if left in contact with moist soil.
Chemical control offers a solution for extensive patches, particularly using herbicides like glyphosate or triclopyr. The optimal time for application is late summer to fall (September to October), when the plant is actively transporting sugars down to the roots for winter storage. Applying the herbicide during this period ensures the chemical is carried deep into the root system, maximizing the kill. After removal, the area must be monitored closely for new seedlings and re-sprouts, and replanting with vigorous native species will help suppress future growth.