Answer: Butterfly bushes are usually pest-free so I would look carefully at the growing conditions for the cause of the defoliation. These plants need a full sun location (sun all day long) and although drought tolerant, bloom best with an evenly moist but well drained soil, meaning not a soggy one. During the establishment phase, say the first year they are in the ground, the soil should be kept evenly moist (damp not soaking wet) and you should use your finger to make sure that both the original potting mix and the surrounding soil are moist because sometimes they dry out at different speeds. It is important to keep the surrounding soil damp to encourage the roots to grow into it. Apply several inches of organic mulch to help keep the soil more evenly moist and to keep down weeds. The mulch will also help feed the soil when it breaks down over time. Defoliation can be caused by overwatering and by underwatering. It can also be caused by overfertilization using a granular fertilizer, or by using a liquid fertilizer at the wrong dilution rate or on a very high temperature day, and/or possibly even by chemical applications.
At this point, I would suggest you allow the plant some time to recover. Just water as needed to keep the soil evenly moist, but not dripping wet. (Remember to check down an inch or so with your finger.) It is better to water deeply about once every five days or once a week than every day. Mulch if you haven't already. Try to be patient and allow several weeks for it to try to leaf out and grow. If it can do that, it should still bloom for you. Once it comes back into active growth and the leaves are several inches long, you could fertilize it with a water soluble fertilizer for flowering plants, but not exceed the amounts on the label and stop fertilizing by mid September. You could also apply some compost to the root zone. Good luck with your butterfly bush.
Q&A Library Searching Tips