I have had my stokes asters for 5-6 years mostly in the same place for the last 3 yrs. I have divided the larger ones to fill out the area where I wanted them - edging a driveway. They have bloomed profusely in the past, but this year only 8 or 10 of the 20 plants bloomed - and that sparsely. They have ALL developed from one pot I bought ~ 5-6 years ago and now each is as big as the original one was -- so I guess I am doing something right, but I want more blooms. See photo from 2015. I began to wonder if they don't bloom well every year -- but I expect they are asking to be divided. I have another question too! I have many hydrangea bushes and they are becoming more blue. I would like to add lime to make the soil less acid so they would be more pink or lavendar, but wonder how and WHEN to do it.
Blossoms could be affected by environmental reasons, soil nutrients and simply age of the plants.
To keep Stokes aster flowers growing strongly from year to year, scratch 3 tablespoons of slow-release 10-20-10 fertilizer into the soil around each plant in the spring. Water the fertilizer in right away to start the nutrients working their way down to the roots.