Answer: Generally speaking, butterfly bushes are somewhat drought tolerant once they are well established. They do however perform better with adquate water, especially when they are young. The soil should be evenly moist butby no means soggy or sopping wet. The best way to tell if you need to water or not is to test the soil with your finger. A layer of several inches of organic mulch such as shredded bark placed over the root zone (but not touching the stem) can help keep the soil cool and moist. You will want to keep the soil moist over an area a bit larger than their current root zones so that the roots can extend outward. Water stress would more likely cause an overall wilting rather than one or two isolated branches. In that case I would look closely for physical damage such as broken branches -- sometimes a stray basketball or the like can cause that. There are occasionally cases of root rot and stem canker on these shrubs that could cause some wilting but that would be pretty unusual.
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