By October the garden can look a bit of a mess, collapsing perennials, dying leaves and debris. Autumn is the time to clear up on the garden and one decision is which plants to cut back?
I suggest those with unsightly faded leaves, such as Delphinium, geraniums, are best cut back, but others with ornamental seed heads such as poppies, Allium, grasses are best left alone for now.
Cut right back to ground level: Peonies, Leucanthemum, Nepeta (Cat Mint) Delphinium, Hardy Geraniums, Phlox.
Slightly less hardy perennials such as Penstemons are best left with the top growth in place. It provides some winter protection and should not cut back until the spring to give some cover for the plant from the winter weather.
After clearing and cutting back the borders, which often seems to take into November at least, it's a good time to spread a mulch to help the more tender plants through the winter and to improve the organic structure of the soil.
More information about mulching a border.