Answer: Plants in containers have a disadvantage when they sit in full sun during the summer months - sunshine hitting the container directly can overheat the roots and virtually cook them. Another problem with plants in pots is that the soil dries quickly and air pockets can form in the soil. To counteract each of these problems, either sink the pot into the soil or group other pots around to shade the roots; watering thoroughly by immersing the pot in a larger container of water every 2-3 weeks will help drive out air pockets around the roots.
It shouldn't harm your ninebark to remove the dead foliage.
Best wishes with your ninebark!
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