Q.Troubleshooting Butterfly Bush
I have a Butterfly bush which was planted last spring. Thank g-d it returned this season. However, The leaves have started to Yellow at the bottom working their way up. Towards the top the leaves are speckled yellow. The plants have been fertilized and properly watered.
Certified GKH Gardening Expert
This appears to be a magnesium deficiency. This is very common in acidic soils, but can be a sign of actual deficiency as well. Dolomitic lime will usually correct this, and bring it to a stable and close to neutral pH.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-care.htm