Ninebark (Physocarpus spp. and cvs., Zones 3–9)
Because it adds brilliant foliage color to the garden for months, ninebark has become a popular substitute for the invasive burning bush (Euonymus alatus, Zones 4–8). However, many varieties of ninebark will get super large.
When to prune: Shear for shape in late spring or early summer; renovate for size management in late winter.
How to prune: If you just need to shape your ninebark a bit, you can give it a light shearing during the active growing season, after it flowers. This can be done with bypass pruners or with power shears. Just be aware that this can reduce the amount of potential showy fruit you will see in summer and fall.
If you need to do more of a major pruning for size management and want to preserve its natural growing habit (cascading branches that show off abundant flowers and fruit), you will need a different treatment. The key is to remove in late winter—at ground level with loppers or a pruning saw—any stems bigger than a broom handle.
For a totally different look that highlights the exfoliating bark of ninebark, you can choose a bunch of the thickest stems to save. Choose three, five, or seven, and make sure that they aren’t crossing each other and that they have a similar angle coming out of the ground. Clean up any branches and small twigs off the stems as high as you can, leaving some at the top. These will become the trunks of your multistem, small ninebark “tree.” As the tree grows through the years, keep removing the lower small branches and twigs off the large central leader trunks. As they mature, these trunks will have beautiful exfoliating white bark, and the tops will weep over, forming a beautiful flowering canopy. Any limbing up should be done from late spring to early fall.
source: https://www.finegardening.com/article/how-to-prune-ninebark