Herbaceous perennials are hardy plants that dies down in autumn. They do not by but merely die back to its roots. These stay alive through winter, and re-emerge, with signs of life in Spring again. The roots will be raring-to-go and the plant will put on a huge amount of healthy, fresh growth in a season. So very quickly they will fill out in the flowerbed from nothing.
Every winter the foliage will die down leaving dead stems behind. Cut these back to 10cm, 4 inches, to tidy up the bed. Anything that looks unsightly can be removed. If certain plants still have reasonable nice foliage this can be left in situ to be removed later in a second trimming when they do become unsightly. Leaving dead stems in place over the winter allows small insects to use these as homes over the winter.
Herbaceous plants will stay in situ for many years, so if there are any perennial weeds in the ground these can become a real problem, so removal of these at this time is a good idea, taking care to remove as much of their roots as possible.
With the plants trimmed back and the weeds removed now is a good time to feed the bed and dress it ready for the next season. Perennials’ quick and lusty growth means they are hungry plants and can deplete the soil quickly, so enrich it with compost or well-rotted manure. Adding a layer of well-rotted manure or chicken pellets or similar is great to keep the bed performing well during the summer. Finish your Autumn/Winter clean up by adding an optional layer of bark mulch to help keep out the weeds for the following season.
After a few years, flowering power can decrease, but you can rejuvenate the plants by digging them up, splitting them and replanting. This is done simply by removing the plant and putting the spade through the plant roots and breaking it up into 2 or 3 pieces and replanting.