By September I am looking at the garden with the most critical eye and see only problems. It's the month I move plants around and chuck some out. So its fortunate that September and autumn generally, is a good time, (arguable the best) for planting, shrubs and herbaceous plants of all types; the soil is still warm and the inevitable autumn rain will bed in the new plants.
I know that spring is the popular time for planting but Autumn is the best time, especially as we often get such very dry spells in spring. So I plant and move around, and I know I am become obsessed when I find myself moving several plants merely a few feet one way or another. As a plant mature sometimes it outgrows it's spot, or dominates, or don't do very well and so I take stock. I have dug out two shrubs, moved one; binned one. There comes a point when a plant which hasn't done very well or is in the wrong place isn't going to get any better, be ruthless .
September is also a good time for planting spring bulbs, more another time, and Alliumss; I love Alliums and not just the lovely mauve ones which are so familiar.
This is Allium flavum which is stately growing to around .5 metre and it flowers for weeks and weeks, during the summer from June on wards.
Late summer flowering Alliums look just great with grasses for more information about grasses and Alliums as a planting combination follow the link