AT this time of year, just when plants are entering their dormant phase, I always look at the overcrowded clumps of perennials dying down and decide what I need to lift and divide next year.

Dividing plants involves digging them up, splitting them into pieces which each have roots and growth shoots or buds, and then replanting them. The method stops many herbaceous plants becoming overcrowded, untidy and invasive.

Many perennials, including delphiniums, heuchera, hostas, anemones, phlox and cranesbill geraniums, benefit from regular division to maintain their health and vigour, and require no special conditions when the new divided sections are replanted.

Other plants benefit from division every three to four years although some, like asters, are best divided annually.

A few, including red hot pokers and peonies, hate being moved so only lift and divide them if you want to propagate them.

The best time to divide plants is autumn, when they are dormant, and spring, when they are just starting into growth.

Grasses, including bamboos, should be divided in early spring, while perennials which flower in spring and early summer, such as lily-of-thevalley, epimediums and rhizomatous bearded irises, should be divided as soon as they have finished flowering.

TOP TIPS

There are various ways to divide plants.

* Those with tough roots which are difficult to prise apart, such as hostas, are better off being dug up and then cut into thick slices with a knife like you would a cake, making sure good roots are present in each slice.

* Other plants can be lifted carefully with a garden fork, working away from the crown centre to limit root damage, then split into two portions across, each with some roots, shoots or buds. Larger pieces with more roots are likely to flower sooner than small ones.

* Evergreen and perennial ornamental grasses should be left until spring, when they’ve had a chance to recover from all the energy they’ve expended making their feathery plumes in autumn.

* Cut down the foliage of evergreen grasses in spring and divide them, while bamboo stems can be cut down to 30cm (12in). Å Perennials such as daylilies (hemerocallis) have masses of fibrous roots and large crowns which are difficult to pull apart, so use two forks thrust back-toback in the centre and work the handles outwards in opposite directions to loosen the clump.

* Plants with fleshy roots, such as delphiniums, should be cut with a spade or knife. Divided clumps should ideally contain about three to five shoots.

* Divide primulas into rosettes, asters into single-rooted shoots and bugles into single rooted plantlets. Clumps of epimediums should be teased apart into separate plants using your hands or a hand fork, replanting on mild autumn days, while crocosmia and crocus clumps can be dug up and the new corms which are resting on the original ones can be eased off and replanted up to 10cm deep.

* When replanting, add compost or well-rotted manure to the planting hole on a new site, or a balanced fertiliser if you are replanting the divisions into the same hole. They should be replanted at the same depth in flowering positions and watered in well.