Some perennials, such as Primula and Aquilegia, have a loose crown of many stems which can be simply pulled apart by hand without damaging the plant. You can also pull away stems growing at the edge of the plant provided each portion has its own roots.
Other perennials, such as Hosta and Phlox, will need the use of a spade, or two spades, to help divide the roots.
Step by step
Carefully lift the whole crown with a garden fork or, for smaller plants and alpines a hand fork. It may be easier to start by cutting around the crown with a spade.
Plants can be divided as lifted, but it is often easier to first remove lose soil, especially if it is clay.
Shorten tall stems to minimise water loss and transplant ‘shock’. This is especially important when dividing in summer or hot weather.
Carefully prize away healthy sections from the edge of the crown with a hand fork or break them off.
Very large or old crowns may need to be divided with a pair of back-to-back garden forks or even cut into sections with a spade.
Make sure each new section has several good basal buds or shoots.
Replant immediately in well-prepared soil, making the sure they are replanted at the same depth as the plant was originally growing.
Water thoroughly to settle the soil around the roots and again during hot weather for the first few months to aid establishment.