It is the
best way to multiply perennial border phloxes, because it does not
transmit eelworm, a pest that infests their top-growth. Root cuttings of
variegated plants will produce all-green plants.
Cuttings are normally taken during the plant’s dormant period in
midwinter, to minimize disturbance and injury. Always choose a healthy
plant. Lift small plants out of the ground completely with a fork and
replant immediately after taking the cuttings. With larger plants,
scrape away enough soil to expose the roots, and then replace and firm
the soil at once to avoid destabilizing the plant. Never remove more
than a few roots from each plant.
Most plants propagated this way have thick roots, which are planted
upright. Plants such as phlox and Primula denticulata have thin roots,
which are rooted horizontally in trays.
About six months after planting a root cutting, leaf growth may appear
before plants have had a chance to make good root systems. Wait until
you see roots near the holes at the bottom of the pot before transplanting.