I have 3 spirea bushes. They are kind of big ( approx 3 to 4 feet across and wide) Think they were planted quite some time ago. I have lived in the home for 2 years. The inside of the bushes (sort of hidden by the outside leaves) they have a lot of dead branches. ALOT A friend told be to cut them down totally 6 to 8 inches from the ground. Is this the proper way to prune these ? Should it be done in the fall or spring ? Thank you.
Hennepin County Minnesota
If your spirea bloom in the spring, They are likely Spiraea prunifolia or Spiraea x vanhouttei and they bloom on "old wood" or the previous year's growth. These should be cut back after blooming in 2016. The exception would be to do renewal pruning, meaning cutting them to the ground as your friend suggests. This will result in few flowers (if any), but will reduce the shrub size. Note that if your spirea are "bridal wreath" spirea (S. x vanhouttei) and you do renewal pruning, it will take a number of years before they have the arching, drooping flowering branches.
If they bloom in summer, they are likely a cultivar of Spiraea x bumalda or Spiraea japonica and they bloom on "new wood" or new growth that emerges in spring from the base of the plant. In this case, you should prune these types of spirea now or this winter.
Selective pruning - removal of up to about 1/3 of the largest branches and heading back (shortening) other long branches - can be done, but the shrubs may not have very good form for a few years. Here is an Extension publication on pruning trees and shrubs that may be helpful: http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/pruning-trees-shrubs/