September - the ideal time for planting a perennial garden, but care must be taken to shield the plants from the few remaining days of scorching heat and they must be well watered.
September is also the time for deadheading (removing spent flower heads) and pruning back. Deadheading can stimulate a second bloom, improve the overall plant appearance and prevent self seeding. Seed production can weaken roots and cause the plants leaves to appear dead or dying. Deadheading can prevent the plant from looking dry and unhealthy.
If your perennial garden is recently established, year 1 plants need little deadheading. The real work begins in year 2. The method used to remove dead flower heads can be dependent on the species; however the main thing to look for is new growth or new flowers. When there is no clear way to deadhead down to a lateral flower or leaf, then the choice is somewhat subjective. Generally, when seed pods outnumber flowers then it is time to prune off the dead flowers.
Some perennials such as Bearded Iris, Oriental Poppies and Peonies can be divided now.
Plants will show signs of needing division such as smaller blooms or branches that appear “leggy” and require staking or you may simply want to divide to create more plants.