Answer: Butterfly bushes should be pruned in early spring. In my experience waiting until spring to prune it allows the branches to provide a little bit of winter protection to the lower part of the plant. You would cut them back to remove winter damage, then possibly a bit more to restore a somewhat symmetrical look. Some gardeners routinely cut them back to about twelve inches tall. This stimulates branching and helps create a denser plant in the coming season. It blooms on new growth of the year so do not prune it in the summer or you will be limiting blooms.
Partial sun can mean different things. Full sun is full direct sun all day, or for a minimum of about six hours including the hour of noon. Afternoon sun is more intense than morning sun. If a plant is a full sun lover that tolerates partial sun, give it as much sun as you can. If a plant is a shade lover that tolerates partial sun, give it morning sun -- not afternoon sun. Usually plants listed for partial shade will do best with a couple of hours of direct morning sun and bright dappled light the rest of the time, but no hot afternoon sun. It is by trial and error, so you may need to experiment and transplant if you have to. I hope this helps.
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