Answer: Based on your description it is difficult to make a diagnosis, especially when your other plants including some that require similar conditions are doing well. I would expect that possibly two things could have caused the leafdrop, one being simple transplant shock and the other being a change in lighting from where the plants were originally grown at the wholesale nursery to where they are planted in your landscape. For instance, if the foliage had developed in full sun and was accustomed to that and then the plants are placed in a partial sun location, the foliage may thin in response to the relative lack of light. Overwatering can also cause some leaf drop, so take care that your watering is enough to keep the soil slightly moist but never oversaturated.
At this point, I would expect the plants to become better acclimated to their growing location beginning with their new growth this spring. The only suggestion I could offer in advance of that is to run some basic soil tests now and make sure that the pH and fertility are fully suitable for them. Your county extension or possibly local garden center should be able to help you with the tests and interpreting the results. If any adjustments are needed, then you could begin working on that between now and when the growing season starts. Best of luck with your Purple Gems.
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