Quick Guide to Dividing your Perennial Clumps
Many hardy perennials need dividing regularly to maintain health and vigour; division is also the easiest method of increasing perennials and no special conditions are needed to look after the new plants.
Division involves lifting plants and splitting them into a number of pieces, each complete with roots and growth buds or shoots. This method is easy and reliable, and stops many herbaceous plants becoming overcrowded and losing vigour as they would over time if they were left undivided. Regular division, usually every three to four years, is usual, but vigorous perennials such as asters are best divided annually.
When to divide
Divide plants when they are dormant or just starting into growth. Autumn and spring are the best times, and spring is preferable if the soil is heavy, cold and wet, and if the plants are rather tender. Grasses. including bamboos, should be divided in early spring, when growth has just started. Perennials that flower in spring and early summer, such as epimediums, convallaria (lily-of-the-valley), and rhizomatous bearded irises, are best divided immediately after flowering.
Different methods of Division
Lift plants carefully with a garden fork, working away from the crown centre to limit root damage, and split them into portions about 5-8cm /2-3in across depending on the plant, each with some roots and some shoots or buds. Larger divisions, with more shoots and roots, flower sooner than smaller pieces, which may take a year or two to recover. Discard the centre of the clump, retaining young outer parts for replanting.
Ensure all divisions have plenty of root (which must not dry out) ; if there are only a few roots then trim the leaves to reduce moisture loss. Evergreen grasses benefit from leaf reduction when dividing, and stems of bamboos and grasses of a similar habit, can be cut down to 30cm / 12in. However, many herbaceous plants will be dormant so excess foliage is not a problem.
Work organic matter, such as garden compost or well-rotted manure, into the planting hole. If replanting into the same site then add a handful of balanced general fertiliser. Replant divisions to the same depth in flowering positions, and water them well to ensure they establish quickly.
Many perennials such as daylilies (Hemerocallis) form large clumps or crowns with a mass of fibrous roots. The crown is often difficult to pull apart, so use two forks thrust back-to-back through the centre, and pull the handles apart. Repeat to divide the clump still further, using hand forks, or cut away sections, after shaking off loose soil, with a sharp knife.
Plants with woody crowns such as hellebores, or fleshy roots, such as delphiniums, need cutting with a spade or knife. Aim to produce clumps each containing three to five shoots. Divide rhizomatous, bearded Iris into fans of leaves immediately after flowering. Cut the rhizome from the main clump and dust the wound with sulphur. Trim the leaves to about 15cm / 6in high and plant the divisions with the upper surface of the rhizome exposed.
Split asters into single, rooted shoots. Divide primulas into rosettes, ajuga into single, rooted plantlets.