Blackberries if left unchecked can quickly spread in a pasture and reduce the amount of grazeable acres. The same competitive characteristics which make blackberries relatively easy to grow in a home or commercial setting make them a persistent foe in your pasture or rangeland.
Blackberry is a perennial, thicket-forming shrub which is very invasive in our area. Each plant has a large lateral-growing root system that can sprout and produce additional plants. The rhizomatous root system is perennial, while the aboveground canes are biennial (living for two years). The first year, the canes emerge and grow rapidly; the second year, the canes bud and produce flowers and fruit. The canes subsequently die after fruiting. This root system is what makes them so competitive and difficult to control.
Mowing can be somewhat effective in slowing the spread of thickets until chemical controls can be applied. Mowing can also help reduce the size of old thickets and remove dead old canes and encourage more new canes to sprout, which can be helpful when coupled with chemical control. Mowing can reduce the size of old, established thickets and in some cases make herbicide application easier. Application of herbicides soon after mowing yields inconsistent results because there is not sufficient leaf area to take in enough herbicide to kill the roots of the plants. Be sure to let plants have plenty of leaves and be actively growing before applying herbicides.
Timing of herbicide application is key in obtaining effective control. Blackberry is most sensitive to herbicides when blooming in late spring and in the fall prior to frost. Applications made soon after emergence from winter dormancy or during fruiting are generally less effective. It is also important that the plants are not drought-stressed at the time of herbicide application. The recent rains and stage of growth make this an ideal time to apply chemical controls.
Currently, several herbicides list blackberry on their label. The most effective herbicides are those which contain metsulfuron or triclopyr ester (Remedy Ultra, others). PastureGard HL (triclopyr + fluroxypyr) and triclopyr ester (Remedy Ultra, others) can safely be applied when blooming, but retreatment the following year will probably be required to achieve control near 100%. These herbicides cause rapid blackberry defoliation and are effective at controlling other weed and brush species.
Products that contain metsulfuron also provide consistent control of blackberries, however in comparison to herbicides that contain triclopyr they slower acting and may take two or three months to show significant control.
In a study done near Altamont, Kan. over 15 years ago a herbicide containing triclopyr and a herbicide containing metsulfuron were both applied about in Mid-May with both providing good control, with the herbicide containing metsulfuron being more economical.
Complete blackberry eradication is probably not possible but acceptable results will likely require multiple applications and/or tactics. When relying solely on herbicides to control these species, it is best to spray when blooming or in the fall prior to frost. If a mowing strategy is employed, at least six months of active regrowth should occur prior to herbicide application, and at least six weeks should pass after herbicide application before removing dead canes.
An excellent K - State publication with specific recommendations for controlling blackberries as well as other weeds and brush species is available at https://live-ccms.pantheonsite.io/sites/default/files/SRP1132.pdf or do an internet search for SRP 1132 2017 Chemical Weed Control.