The aggressively invasive Himalayan blackberry (Rubus armeniacus) and related blackberry species grow new canes from the base of the plant, called the crown, from underground stems called rhizomes, and from the seeds on the berries. Bulldozing or digging up stands of blackberry canes a single time breaks the rhizomes into pieces that keep on growing. Only repeated tilling will keep the aggressively spreading canes under control. Likewise, blackberries suppressed with herbicides also regrow, so applications must be repeated.
Types of Herbicides
Herbicides applied to blackberry foliage are used to control tangles of canes growing alongside roads and in pastures and range land. Licensed professionals are required to apply non-selective herbicides to the soil; those herbicides kill all vegetation in the area that is treated.
Herbicides Applied to Foliage
Agronomists at the University of California, Davis, recommend herbicides containing the active ingredients glyphosate, triclopyr, dicamba or dicamba combined with 2,4-D for application to blackberry leaves. The Environmental Protection Agency (EPA) has registered herbicides containing glyphosate and triclopyr for home use. The EPA has registered glyphosate formulations Aquamaster and Rodeo for use on blackberries growing in riparian sites, those near creeks, streams and rivers. Blackberries should be sprayed until they are saturated, but not dripping.
When to Apply
For effective control, herbicides applied to blackberry foliage must move to the underground rhizomes from which new canes grow. Early in the summer blackberries transport nutrient sugars from the rhizomes upward to the growing shoots. The first-year canes, those not producing flowers, begin moving sugars downward to the rhizomes after the middle of the summer. The second year canes that produce flowers move sugars down to the rhizomes later in the summer after they produce berries.
Apply herbicides in late summer to first-year canes that are growing because the previous year's canes were burned, bulldozed or cut. Apply herbicides early in the fall to a combination of first- and second-year canes. Herbicides applied too early will kill the top of blackberry bushes but will not go to the rhizomes.
Licensed applicators should apply non-selective, systemic herbicides containing the active ingredient tebuthiuron to the soil in early spring when the sugars are moving upward into the buds or in the fall when they are moving from the leaves to the base of the canes.
Treating Dormant Canes
A 1 percent solution of triclopyr ester in a 3 percent concentration of crop oil can be sprayed on dormant blackberry canes in the late fall and winter. Although it is less effective than herbicides applied to foliage, this technique prevents berry pickers from having contact with herbicides.