I have two false spirea in the shade, planted for screening. I have been timid about pruning them. Both are reaching for the sunlight, one more than the other. This doesn’t bother me, but I know that I need to prune them. When is the best time and how do I do it? Also, we put a barrier in the ground to deter them from spreading (about 5 years ago), but this year there have been new stems coming up beyond the barrier. We keep them clipped. If we prune the original shrubs, will that help to contain the shoots? Anything else we should know? Thanks!
False Spirea (Sorbaria sorbifolia) is a very good shrub for shady areas but it does have a suckering nature which, if not controlled, will take over a large area, particularly if the soil is nice and lose. Generally, you prune this shrub hard in early spring just before the buds start to break. As it flowers around June, you may want to enjoy the plumes and prune a little for shape after it has finished blooming.
The barrier you have placed around it is a very good idea. Pruning this shrub hard, once a year as indicated above, will control some of the suckering and will rejuvenate it for a better form.
Hope this information has been helpful.