New England asters and Boltonia are September bloomers but need a lot of room to look their best. Be sure to plant these at the back of your bed or border and give each plant an area about five feet wide in which to spread out.
The shrubs in my garden bloomed for just a couple weeks in the spring but their upright stems provide structure year round. I have a ninebark Diablo with dark purple leaves all summer at the back of one garden. My white and purple phlox are beautiful against the dark leaves of Diablo and I have used it as a filler in flower arrangements since June.
Clethra (also known as summersweet) is still in bloom now and some weigela have pushed out a second bloom. Weigela is covered with flowers in spring but often produces a modest flower show in late summer as well.
A couple of years ago I planted my annual flowers late by accident but I enjoyed the effect so much that now I do it on purpose. Cosmos, zinnia and calendula are just some of the annuals that are quick to flower from seed. I plant these in mid June so theyll be at their peak in late summer rather than peaking in mid summer with the bulk of my other flowering plants.
Ornamental grasses also provide a lot of interest with their beautiful leaf texture and seed heads but they need a whole article of their own to do them justice. For now I just urge you to consider working these into your border as well.
So by incorporating a variety of plants with beautiful flowers, perennials with attractive foliage, shrubs and reliable annuals in your bed or border, there will always be something interesting to look at, any time of year.