Divide and conquer

12:05pm Thursday 25th October 2007

VAL BOURNE revives the traditional art of dividing perennial clump-formers

I have been busy moving plants from a holding bed in order to make way for new paths in one of the vegetable patches. They had made large clumps, particularly the summer-flowering geraniums and astrantias. Rather shockingly the central heart of each plant was a dry, peaty ball, probably due to last summer's drought. This dead centre must effect the vigour of the plant so I've been pulling these vast clumps apart and discarding the dry centres and keeping the new vigorous outer pieces. I must have potted up 30 small pieces from each clump into soil-based John Innes. Next year I should have lots of young plants and these will eventually be far more vigorous than the congested clumps that were already past their best.

Yearly division of herbaceous plants used to be the norm when I was younger. The summer-flowering herbaceous was split up in September and the autumn-flowering plants (like asters, phloxes and monardas) were split every spring. Both by the traditional method of placing two forks back to back and levering them apart. But this practice died out and now many gardeners never divide their perennial clump-forming plants at all. Yet most would benefit from division every third or fourth year.

Your soil will influence how and when you divide your plants. Those on well-drained soil rarely lose plants split in autumn. Those on heavy soil risk losing small divisions and, if you are on clay or waterlogged soil, you are better to divide from Epiphany onwards - from January 6. However, the weather must be taken into account, never lift or plant anything in deep frost.

Certain plants prefer yearly division and these include phloxes and larger sedums. Phloxes tend to die away in the centre and they can suffer from eel worm. Splitting them up and moving them to a new site keeps them vigorous. Large sedums have a habit of splaying at the crown and regular division keeps them tighter. You can also chop the sedums back hard in May and this makes them produce superb foliage in late summer - followed by late flowers in autumn.

Heleniums - those sumptuous soft daisies in warm colours - can be divided in autumn as long as the divisions are large. However, spring is the easier time and it's possible to replant single rosettes into enriched soil. You can either add garden compost or water the ground well and then add a controlled-release fertiliser like osmocote.

If you are unsure about whether to divide or not look at the stems from the base. Lots of stems means lots of root. However, a few stems often means that the plant has a deep tap root. Tap-rooted plants are best propagated by root cuttings taken at the turn of the year. Oriental poppies, verbascums, eryngiums and acanthus can be propagated like this. The trick is to remember which is the top of the root cutting and which is the bottom.

Dig the plant up and look for good roots - about the thickness of a small diary pencil. Using a sharp knife, cut some of the the roots away, leaving at least 2in on the original plant. Then wash the cuttings gently - keeping the top upwards. Cut them into 2in lengths and place them vertically, or at a slight angle, into a tray of gritty compost so that the top is just slightly proud. Cover with grit. Some plants root very easily, others take longer.

You can also collect thin, wiry roots from Japanese anemones and phlox and lay them on the soil surface, before covering them with grit.

Editor's choice