The purple asters were received from the landscape architects many years ago. They grow about 3 to 4 feet tall (1 to 1.2 m), and form a large clump that needs support when reaching full height. The flowers are dime-sized in clusters along the top of the plant, and attract a certain orange & brown butterfly in late summer. The plant grows stalks with leaves during the summer, and begins blooming in late summer, continuing into fall. This aster overwinters and multiplies easily, forming what looks like a bush. I divide it in the spring and fall, and always take pieces to the neighborhood Plant Swap Brunch. It had white fuzzy 'cotton' bugs one year, but otherwise has been pest free.
As recommended, the stems were always pinched off at about 6 inches (15 cm), and then again at 12 inches (30 cm) in height during their growth to produce more side shoots and thus more flowers. As an experiment, last year I decided to let them grow without pinching off. The results: the plant bloomed earlier and was through earlier, was taller, had fewer flowers, and because there were fewer stalks for fellow support and was taller, it more easily fell over. Lessons learned.