Some browning after planting or after a hard winter is common in young evergreens. Both come down to water: a root ball that dried out, or winter sun and wind pulling water from the leaves while the roots sit in frozen soil. Water deeply through fall until the ground freezes, and wait for spring growth before you trim, since buds under brown tips can still be alive. Mild winter bronzing on boxwood almost always greens up in spring. Narrow black streaks on green boxwood stems are different, so send us photos.