Perennials and shrubs have a much easier time getting established if planted now. We have ample time until the first frost to transplant, divide and put new plants in the ground. The warm soil and cooler days, promote faster root development. Also, at this time of the year, the product of photosynthesis is being translocated down towards the root system in preparation for winter and dormancy, this facilitates root development for new plants. We also tend to have a bit more rain in fall, at least more that in the last couple of months, we hope!
Peonies, Irises and all kinds of bulbs should be planted now. Peonies specially are fuzzy about being moved in spring so you will want to get them in the ground now. You should also plant bulbs in pots now for forcing indoors in time for the holidays.
I like to take advantage of the local nursery plant sales and pick up as many good buys as I can. Last year I was still planting in early November with great success. Everything I put in the ground made it through the winter and seemed to have gotten a leg up over all other plants I put in the spring.
Here is a great little find. This little Corydalis is loaded with delicate purple flowers I planted it only three weeks ago… a real gem!
One of the most delightful things about a garden is the anticipation it provides. ~W.E. Johns, The Passing Show