Different Perennial Plant Root Systems
To divide a plant whose roots form offsets (small plants growing at the base of a larger one), snap the connection between any of the sections to obtain a piece with ample roots and three or more growing points (or “eyes”). Some denser clumps may have to be cut apart.
Plants that form offsets include asters (Asterspp. and cvs., USDA Hardiness Zones 4–8), coneflowers (Echinacea purpureaand cvs., Zones 3–9), hostas (Hostaspp. and cvs., Zones 3–8), tickseeds (Coreopsisspp. and cvs., Zones 4–9).
Some perennials have roots that run on or just below the surface of the soil. They form new crowns and roots when they reach open spaces or make contact with the soil. If you cut between any of the stems as you would cut a piece of sod from a lawn, you will have a division with its own stems and roots.
Plants with surface roots include bee balms (Monardaspp. and cvs., Zones 4–9), black-eyed Susans (Rudbeckiaspp. and cvs., Zones 3–9), creeping sedums (Sedumspp. and cvs., Zones 3–9), and creeping speedwells (Veronicaspp. and cvs., Zones 3–8).
Plants that have taproots can be divided by using a sharp knife to slice down the length of the root. Every piece that has at least one eye, some of the taproot, and a few side roots is a viable division.
Plants that have taproots include balloon flowers (Platycodon grandiflorusand cvs., Zones 4–9), butterfly weeds (Asclepias tuberosaand cvs., Zones 4–9), cushion spurges (Euphorbia polychromaand cvs., Zones 4–9), and Oriental poppies (Papaver orientaleand cvs., Zones 4–9).
Underground running roots
Underground running roots can develop suckers as they grow beyond the shade of the mother clump. These suckers can be cut away from the main plant, or you can dig up the main plant and cut away any piece with an eye or sucker already forming.
Plants with underground running roots include hardy geraniums (Geraniumspp. and cvs., Zones 4–9), Japanese anemones (Anemone × hybridacvs., Zones 4–8), ostrich fern (Matteuccia pennsylvanica, Zones 3–8), and plume poppies (Macleayaspp. and cvs., Zones 4–9).
Woody perennials often form roots when stems rest on the ground or are buried by gradually accumulating mulch. Make a new plant by simply cutting between the rooted stem and the mother plant.
Plants that have woody roots include candytufts (Iberisspp. and cvs., Zones 5–9), euonymus (Euonymusspp. and cvs., Zones 4–9), lavenders (Lavandulaspp. and cvs., Zones 5–10), and sages (Salviaspp. and cvs., Zones 5–10).