Planting in september
September and October are fine planting dates for perennials here in Oregon. Depending upon which kind you plant will determine whether or not it survives the winter. For instance, tender perennials won't survive; half-hardy perennials may survive with protection; and hardy perennials will be fine.
I suggest you contact the Yamhill County Master Gardeners to ask them about the plants you are interested in growing. They will need to know the names of the perennials you want to plant as well as where in the county you live.
Contact information for the Yamhill County Extension Service - 2050 Lafayette Ave, McMinnville, OR 97128. Phone 503-434-7515 to ask when the Master Gardeners are in.
Or email the specifics of your question to the Master Gardeners at firstname.lastname@example.org.