Water one week and again one day before the move. A cool, cloudy, calm day is best. For perennials taller than 15 centimetres, cut back by one-half before dividing.
Cut straight down around root ball, aiming to include as many roots as you can. Gently loosen root ball, trying to keep it intact; leave as much soil on as possible to lessen transplanting shock.
Inspect plant for obvious places to divide, keeping in mind each new plant needs a balance of top growth and roots. Use trowel to gently spread leaves; starting in centre, cut down to sever first section.
Ensure each section has part of crown, roots and soil intact. After plant has been removed and divided, select only the best and healthiest portions to replant; three to five vigorously growing shoots are ideal. If you can’t immediately transplant divisions, dig them into a shady bed with wet soil over roots to keep moist.
Add compost to bottom of planting holes (mixing well with existing soil) and backfill to a depth that allows divisions to sit at their previous level. Don’t crowd divisions; spread out roots in planting holes.
Fill in hole with more amended soil. Using your hands (not your feet), press down gently around roots to firm and remove air pockets. Create a slight ridge around plant to hold moisture. Water plant twice: let it puddle and soak in, then repeat. Don’t fertilize until plant sends out new growth; then use an organic fertilizer. Don’t let divisions dry out.
Mulch with 2.5 centimetres of compost or leaf mould. (Note, peonies, delphiniums and tall bearded irises hate mulch crowding their stems.) If it’s sunny or windy, protect divisions with small root balls with a tent-like piece of wet burlap or newspaper placed over them for several days.
The two-fork method
Using garden fork, loosen soil around outside of plant clump, as well as underneath.
Place two forks (back to back) in centre of clump. Push forks in opposite directions away from middle—a one-person task with smaller clumps, a two-person job with bigger ones.
Once clump has been separated with forks, some perennials will need further dividing by hand.
Do not disturb
Some plants should be left well enough alone, such as gas plant (Dictamnus) and false lupine (Thermopsis), because they hate to be disturbed. These plants should only be moved when there's no other option. Others have extensive, deep, brittle, delicate or tuberous root systems that make moving them a tricky proposition. These include butterfly weed (Asclepias tuberosa), bugbane or snakeroot (Actaea racemosa), baby's-breath (Gypsophila), balloon flower (Platycodon) and bleeding heart. Oriental poppy has a different life cycle from most perennials. The plants die down in the summer and start to regrow in early fall, so in theory that's the best time to move them, but their fleshy taproots make successful transplanting iffy.
A number of perennials and biennials produce seedlings or plantlets, making it unnecessary to move the parent plants. Among these are hollyhocks, columbines, delphiniums, foxgloves and mulleins.