Assuming you are transplanting them from their nursery pots to a container or the soil, there is a phenomenon called “transplant shock,” which may occur shortly after you transplant a plant into a new environment, especially if the new environment has very different conditions (more light, more heat, less water, etc.). The plant may drop its flowers or some of its leaves in order to prioritize producing new roots. It sounds like this is what’s happening with yours, made worse by the heat of summer. After it is planted and settles in, the phlox should produce new flowers. Do yours?
Alternatively, it could be that the phlox is not used to so much sun and heat. It may have lived a more sheltered life in the nursery, so perhaps try keeping it in partial sun for a week or so when you first bring it home, then move it into full sun.
Assuming you are transplanting them from their nursery pots to a container or the soil, there is a phenomenon called “transplant shock,” which may occur shortly after you transplant a plant into a new environment, especially if the new environment has very different conditions (more light, more heat, less water, etc.). The plant may drop its flowers or some of its leaves in order to prioritize producing new roots. It sounds like this is what’s happening with yours, made worse by the heat of summer. After it is planted and settles in, the phlox should produce new flowers. Do yours?
Alternatively, it could be that the phlox is not used to so much sun and heat. It may have lived a more sheltered life in the nursery, so perhaps try keeping it in partial sun for a week or so when you first bring it home, then move it into full sun.