Oregon gardeners prune shrubs for two main reasons: to keep the shrub healthy and to keep the shrub at a desired size and looking neat. In general, all Oregon gardeners should always prune during the dormant season, once frost danger has passed but before the shrub begins growing again for the spring. This generally means March or April in Oregon, when low temperatures will be near or above 40 F. However, spring-flowering shrubs can be pruned once they've dropped their blossoms, since pruning during dormancy would reduce their flowering.
Check over the branches on your shrub, looking for any that are dead, diseased or damaged. Dead branches will feel light to the touch, while diseased and damaged ones show discoloration, scarring or other physical imperfections. These branches need to be removed for the health of the shrub whenever you notice them.
Snip off the unhealthy wood at its intersection with the trunk, using lopping shears for branches larger than 3/4 inch in diameter. Use anvil pruners for smaller ones. In between each cut, spray the pruning tools with a disinfectant spray. This helps guard against the accidental spread of disease while pruning. Clean your tools again after all the unhealthy wood has been removed, then dispose of the wood in a garbage bin.
Head back shrubs that have grown too tall or wide. Use anvil pruners and work branch by branch. Determine how tall you want to leave the shrub, then strip back growth to that point. Make your cuts at an angle rather than straight on, and cut back to just above a bud so that the shrub can branch out from its bud. Heading back is one type of pruning and is used to control the outward appearance and size of your shrub.
Thin out old growth to encourage new growth. You can remove up to one-third of the old growth in a year. To thin out wood, look for woody stems on your shrubs that produce few leaves or flowers. Then trim this growth off at the base or at a Y-intersection using your pruning tools. This is the second major pruning technique.
Thin out the interior canopy of the shrub to increase air circulation, which promotes shrub health. Remove branches that cross or rub against other branches. Trim off weak shoots that do not produce leaves.
Avoid cutting any shrub back to bare wood. Always leave a couple of buds so the shrub can continue to grow.