Ninebark flowers mid to late spring. It flowers on old wood and the flower buds for the following year are formed in the summer. You should keep this in mind when pruning. So, it’s best to prune ninebark right after it flowers. But, you can do light pruning anytime to clean up scraggly branches. In the fall, the leaves will get even more colorful before they drop off. Then, you can really enjoy the colorful, exfoliating bark. Ninebark is truly a year-round plant.