My wilting 4-year-old caryopteris is one of 4 plants, all in the same location. The other 3 are fine. It perks up after a heavy rain, but wilts within a day. I do trim it back by 1/3 each spring after new growth begins. Since parts of it seem to be less affected, I trimmed back the wilting parts two days ago. But today, after another heavy rain, several branches were wilting. The leaves are still pale green when they wilt. The entire branch is affected. I did see a few black specks on the tops of the leaves last month, but no more have appeared. I live in zone 5. The plants get at least 8 hours of full sun/day.
This problem has been addressed in a former Ask a Master Gardener question.I am repeating the answer for you below:
Caryopteris is happiest when it has a well drained sandy soil and is in full sun. It is a remarkably drought tolerant plant. It dislikes heavy soils that hold too much moisture. It is a plant that has few disease or pest problems.
This being said, I would think that your plant is suffering from too much moisture being held in your heavy clay soil from all the recent local rainfall. Soils naturally have pore spaces between the soil particles which are used for holding onto enough water for plant use, getting rid of excess water, but also for holding oxygen which allows plant roots to exchange gases. A clay based soil has very small pore spaces that fill up rapidly and hold water leaving little room for oxygen – water does not drain away quickly. This effects the plant roots ability to use the oxygen so the plant basically shuts down its food making and storage process – hence the wilting and leaf drop.
What to do to help this plant out?
Prune out any severley wilting or dead branches. Do not water until the soil dries out again. Ongoing, this plant will be more vigourous and will bloom better if it is pruned back by one third each spring when the new shoots have fully emerged. The addition of organic matter in the form of Compost or Sheep Manure to the clay will help with soil structure, add larger pore spaces, add beneficial nutrients and encourage organisms such as earthworms to take up residence which will help with soil aeration. Any house downspouts that are discharging close to the plant should be directed elsewhere.