Perennials that have finished flowering need to be cut back now. This will make the garden look much tidier and will discourage diseases from attacking old growth. There are some perennials that have finished flowering but their foliage is still green and attractive. You can leave them until they will be blackened by the frosts and only then cut them back, because cutting everything down can leave unsightly gaps in the borders, so you should avoid this until as late in the autumn as possible.
You can put in the compost heap any soft growth cut down but also semi-woody growth can be composted as long as the material is shredded. Shredding will speed up the composting process because else the process will take longer to break down on its own. A more usual practice is to burn these materials but shredding for composting them is better. If the material is clean and diseases free you can chose to put it in the compost heap to rot down further or to use it as a mulch on the borders in spring.
While the soil is still warm and moist you can continue planting new herbaceous perennials. Before planting, prepare the soil well by incorporating plenty of organic matter and adding some bonemeal. Perennials will last and beautify your garden with very attractive displays for many years if they are propagated regularly. This can be done by dividing them up and replanting, collecting seeds and sowing them or by taking cuttings and growing them on.
When adding perennials into your garden consider planting them in groups of three or more plants, as they always look best this way. If your budget is limited you can buy one of each of the varieties you want to have in your garden and then propagate them yourself using one of the methods shown above. For some perennials it might take up to two or three years until you will obtain the number of plants you want.
Mid autumn is also the right moment to lift and divide overgrown clumps of perennials. The older clumps can be easily recognized as all the young and vigorous growth is towards the outside of the clump and the center is bare. You can divide these clumps from now until spring as long as the soil is not frozen or too wet. Late-flowering perennials are best left until spring before dividing them.
Lift the clumps and separate larger ones with two forks pushed into the center of the clump back to back, pushing the handles apart to separate the roots. Smaller pieces can be separated out by hand. Replant new pieces into their new places after adding some organic matter into the soil. Water them well after planting.