The shrubs to prune now are the ones that bloom on new stems (current year’s growth). Those are the plants that will bloom in the summer and fall. Prune in March; its best to prune before the plants’ leaves emerge. It is important that you do this, as you only get abundant flowers and fruit on the new wood. If you don’t prune, the plant will get leggy and unproductive. Hydrangea, dogwood, ninebark, spiraea are examples.
Hold off on pruning lilacs, rhododendron, forsythia serviceberry, and other shrubs that bloom in the spring or early summer. If a plant blooms in the spring (February through June), it blooms on old stems (last year’s growth). If you prune now, you’ll sacrifice this year’s flowers because they set up its buds the previous year. Prune right after blooming; you have a window of 4-6 weeks to prune before the new buds are set. A good rule of thumb is to have pruning done by the 4th of July.
Wait to prune evergreen shrubbery such as junipers, yews, boxwood and arborvitae, until you see new growth. Then you can cut them back as long as you don’t remove all the new growth. These plants will keep growing all summer, and you can prune them again in early to mid-July if you want to limit their size. Wait to shorten the new growth of spruce and pines as well.