Begin by choosing the plants you want to keep. Blackberries that grow in full sun and good-quality soil generally produce the largest, best-tasting berries. To limit the future spread of your blackberry patch, make sure it's in a place where you can mow around the edges. A mowing margin provides easy access for berry picking while limiting the blackberries' spread. You can get rid of the plants you don't want by spraying them with a woody brush-killer herbicide. Be sure to cover nearby cultivated plants-including the lawn-with plastic to prevent damage by the herbicide. Some blackberry root buds may survive and send up shoots the next season, but you can eliminate them by chopping them down with a sharp hoe. If you prefer not to use an herbicide, you could cut off the blackberry vines at ground level. Mow or chop off any shoots that re-sprout.