Are these cultivated blackberries (ex Colombia Star) that you want to spray to control an insect or disease? Or, is this an invasive blackberry that you want to spray with an herbicide in order to control the spread? If the former, then you need to know what pest you are spraying for. Not every pesticide will control every pest. If this is an invasive blackberry (ex. Himalayan blackberry) that you want to control, then it depends on which herbicide you are using. Triclopyr works on Himalayan blackberry in early June and through the growing season. Glyphosate is only effective after flowering/fruiting and into the fall. Late summer and early fall herbicide applications are most effective for killing Himalayan blackberry plants. For more on controlling Himalayan blackberry, refer to the following: https://solvepestproblems.oregonstate.edu/weeds/himalayan-blackberry