When do you trim a ninebark?

You don't "trim back" a Physocarpus (NineBark); you remove no more than 1/3 of the stems (the longest ones) at the base of the plant, at the crown of the root ball (ground level). This reduces the shrub's overal size, reduces density, allows light to reach the interior of the plant and air to circulate, preserves the natural shape of the plant, promotes health and vigor, and does not dimish flowering of the remaining stems. It should actually increase bloom production, assuming the remaing stems have not been previously "trimmed back" !