The Approved Method The method used depends a lot on your skill level, the plant you are working with and the time available. Plants with fragile, brittle roots require tender, careful handling. Plants with stronger root systems require equal care, but more skill in separating the plant into usable pieces that are viable new plants.
The official method is to insert two forks back to back through the centre of the plant and ‘tease’ it apart. However this requires that you own two forks, and most gardeners resort to the crude but tried and true, and quicker method of chopping the plant into divisions with a spade.
Chopping can result in unusable plant fragments, severed by accident. Potential plants lost. Make sure that you chop cleanly and don’t hack at the plant or you will damage a lot of root and stem tissues, increasing the chances of failure and the risk of disease. Make sure that any severed body parts are removed from the scene and into the compost pile, as rotting root segments risk disease infecting your new plant family.
If the plant is very dense, as is the case with hostas, hellebores and hemerocallis, use a hose to wash off the soil and expose the roots, pulling sections apart where you can. If the plant still resists division, use a clean knife to cut it cleanly into sections, pulling apart where you can.
Hemerocallis (day lilies), hostas and tuberous iris, such as Iris sibirica, can be tough to split. Anenome japonica clings determinedly onto its cousins.
Discard old woody sections and any fragments of root or tissue left during the process. Always replant or pot up immediately to prevent root sections drying out. Plant only to the original soil depth and water in well, cover with a light mulch to protect emerging growth buds and shoots from frost.
Leave Them Be Just as there are people that hate to move, so there are plants that perform better if we leave them alone! Paeonies, hostas, aconitums, hellebores and kniphofias prefer to grow ever bigger and more spectacular and take time to re-establish after division. Other plants, such as dicentra, have very brittle roots and are best left in place, where they will quietly grow larger and more gorgeous.
Leave these plants alone unless you want to increase your stock or are redesigning a planting, and then divide into decent sized sections, and with care, or they will ‘sulk’ and take several seasons to give of their best and flower again.
Aftercare As with anything newly planted, new divisions require a little more TLC than usual. To help them recover from the trauma of being ripped apart make sure that you mulch well and don’t let them become too dry, or waterlogged for that matter, until well-established and making strong growth.
Particularly with those plants that resent disturbance, don’t fiddle with them but let them get growing – so if you are not sure of their eventual home pot them and then the next move is easier on everyone.
Plants marked '*' are quite toxic
Always wear long sleeves, trousers and gloves, and clean your tools carefully afterwards. Watch for euphorbia sap in eyes and against bare skin.
