Answer: While it is important to keep a transplanted shrub watered, you also want to be careful not to overwater it. Check and make sure the soil is moist, and don't water until it begins to dry out a bit. You goal is moist but not sopping wet.
Usually it is a good idea to trim back a transplanted shrub somewhat, primarily to compensate for any lost roots abut also to allow the plant time to become established in its new location. Since the plant was cut back hard last fall, you have basically already done that.
At this point, you might want to wait and see. In a few weeks it will be clear that some of the stems are not going to make it or are going to be very weak and they can be removed or tipped back as needed to make the plant look a bit better. In the meantime, a root ehancer or a top dressing of compost might be useful, but the most important thing is to kepp an eye on the water when the weather turns hot.
In terms of ongoing care, it is better to wait until late spring to trim back butterfly bushes. The reason for this is that the branches help to protect the base of the plant during the winter and late spring freezes. Annual pruning or trimming usually consists of first removing any winter kill (which may be the entire top of the plant in a bad year), then pruning as needed for shape. An extremely overgrown plant can be cut off very short to rejuvenate it, but this drastic pruning is not necessary most years, especially in cooler climates.
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