Dividing your perennials
QTo fill empty spaces in the garden, I plan to divide some large clumps of perennials. Is spring or autumn the best time to do this – and when making large groups, how many plants per metre should I use?
AHardy herbaceous perennials such as Lysimachia clethroides, Michaelmas daisies and Sedum spectabile, are plants that die back in winter and grow again in spring. Versatile and long-lived, they eventually spread into large clumps with all the healthy flowering growth around the outside and a bare patch in the middle. Fortunately, these perennials take kindly to being forked up (usually while dormant), divided into chunks and replanted. This is good news when you have spaces to fill, especially as larger swathes and repetitions of favourites like hardy geraniums, agapanthus and daylilies are easy on the eye.
Most gardeners lift and divide summer-flowering perennials between
March and May, or wait until early autumn when the weather is cooler and rain has moistened the soil. Knowing the soil and climate of your plot will help when deciding which to choose. In drier regions on thin, sandy soil, dividing and moving plants in spring can mean extra watering while plants settle during their first season. Autumn might be a better choice, especially as light soils don’t usually suffer from winter waterlogging.
In wetter regions with clay soil, dividing and replanting in autumn is possible. However, get the work done promptly, so roots can grow into wellaerated soil before winter rains, and make slightly larger divisions than usual. If there is any chance of waterlogging, consider mounding beds or raise them slightly so that damaged roots are sitting just above the general lie of the land.
Mulching newly planted perennials while soil is damp is a good way to enrich soil and conserve moisture. I suspect most of your plants will take well from divisions made now. If you leave some alone, don’t forget basal cuttings from new spring growth root easily. 2
Established agapanthus from borders or pots form a solid mass of thick, entangled roots best separated by cutting between growths using an old long-bladed kitchen knife (see also page 50). 3
Late-flowering actaea can be divided using two forks pushed in back to back. The handles, prised apart, separate the roots. Create portions about 6in (15cm) wide. 4
Clumps of perennials like these New England asters respond well to division by knife. As long as the soil is not saturated, cut portions heal quickly and most take well.