When should I prune evergreen shrubs?

FAQ
Question: 

When should I prune evergreen shrubs?

Answer: 

Prune narrow-leaved evergreen shrubs, such as juniper and yew, in early April before new growth begins.  Light pruning may also be done in mid-summer.  Avoid pruning narrow-leaved evergreens in fall.  Fall pruned shrubs are more susceptible to winter injury.  

Only a few broad-leaved evergreen shrubs, such as boxwood and rhododendron, can be successfully grown in Iowa.  Boxwoods often experience dieback in severe winters.  Prune out dead growth in early spring (early to mid-April).  Light pruning/shearing (to shape plants) can be done in spring and early summer.  Do not prune boxwoods in late summer as late season growth may not have sufficient time to harden before winter.  Rhododendrons generally require little pruning.  The best time to prune rhododendrons is immediately after flowering.