The key to preventing browning of boxwood after trimming is water. When you trim a boxwood, you’ve opened up quite a lot of surface area, all of which will start to give off water vapor. If that water is not replaced, the foliage will brown as a result. So after you trim, give your plants a good watering and continue to do so over the ensuing weeks. You can safely trim your boxwoods into mid-summer, but don’t do it beyond that. It can push new growth which will not have time to properly harden off before winter’s cold zaps it.