Lift and divide plants if you want to promote growth in the border areas
Some perennial flower plants have a tendency to spread outwards and make ever-widening clumps. They encroach on neighbouring plants while the clump weakens at the centre. Such plants include monarda, helenium, saponaria, helianthus, rudbeckia, lysimachia, phlox, plume poppy, phygelius and aster.
This outward-spreading habit of growth - under or above soil - is natural for these species. Their growing stems seek fresh ground and more nutrients and it can be a way of out-growing pests and diseases because young shoots are more vigorous and resistant to attack.
Lifting and dividing these plants is a way to reduce their spreading while getting better flowers from the replanted vigorous pieces. Division can be carried out at any time between late autumn and spring but only when the soil is not sticky. If plants are mucked into wet soil, they can be set back and even die, and it is a very messy operation.
Divide plants by simply digging out suitable chunks of the younger growth from around the edges, replanting these and discarding the rest. Or lift the whole plant and then chop it up - this can be done with a spade, an old bread knife, or, in some cases, by just pulling the stems apart.
Before replanting, dig over the emptied space, working in some old compost to aid re-establishment - but not too much or the ground will become too rich with feeding. If possible, plants should be planted in a new space, if that suits the look of a border, because plants do better in fresh soil, fresh to them at least.
Another reason for lifting and dividing is to propagate established plants and these need not be spreading kinds. Some plants do not spread outwards rapidly, forming instead a tight clump that slowly expands, such as red-hot poker, dierama and Siberian iris.
These can be lifted and split to make new plants, and perhaps used to replace some of the more aggressive spreading kinds. However, it can be a mistake to replace all the spreaders. The non-spreaders, or less-spreading kinds, tend to appear like dots in a border, although they can be planted in groups, or drifts, to counter this, but often they still look like a group of dots.
The spreading kinds have a floating quality that is very effective in a billowing summer border. Start by looking at the existing border, marking the plants that most need division. Do a few of these each winter.
Only divide summer-flowering kinds in winter or spring. Spring flowering kinds are best divided after flowering or in early autumn. Division and removal offers space to plant new perennial flowers to improve a border.