The rose of Sharon or Althea shrub is a member of the hibiscus genus. Known for showy, large, late blooming flowers that resemble gathered crepe paper with a long central stamen, Hibiscus syriacus shrubs are favorite landscape centerpieces for summer and fall interest. Keeping them well pruned enhances their shape and ensures continuous sets of those gorgeous flowers into the late days of the growing season.
Rose of Sharon blooms on current season's growth, or new wood, so pruning is best done in early spring before its appearance. Pruning late in the spring will remove bloom buds and provide a disappointing lack of flowers for that season.
General Pruning Tips
When pruning suspected diseased shrub material, make sure tools are kept clean between cuts. Dip tools into a solution of water and bleach to prevent the spread of infection to healthy branches. Dispose of diseased or dead material with care away from the base of your shrub and your compost heap. Diseased material can overwinter and infect other plants if not destroyed.
Heading cuts often are responsible for creating a top-heavy-looking shrub with a sparse interior and leggy base stems. To properly head off branches, choose varying lengths of shoots and cut at a 45-degree angle close to the base branch rather than topping the branches at the same level.
When done properly, thinning cuts will provide a well-formed shrub that is uniformly healthy and blooms well. Cut back branches clear to a main base branch or to the ground, at various positions around the shrub, to allow more light into the center and create a better overall shape.
Spread over three seasons of growth, the renewal pruning process effectively re-shapes and invigorates a mature or overgrown shrub. Begin in the first season by cutting back one third of the oldest canes to ground level, removing dead or diseased wood at the same time. Remove the next third of oldest canes in the second season, and the final third during the last season of the process. At this point, your shrub will be smaller but much healthier. Spreading out these severe cuts also ensures some blooms and show each season while the shrub is being maintained.