Cutting Stems and Roots
Certain plants don't grow from seeds. Named cultivars like "David" phlox must be cloned (vegetatively propagated) to get a plant with all the exact qualities of its parent. This is done by rooting sections of stems or sprouting chunks of roots. Clump-forming plants can be divided into several pieces, and some stems can be rooted while still attached to the mother plant.
Cuttings from Stems
To propagate stem cuttings, select a mature plant that is in a stage of active midsummer growth. Prepare a container filled with rooting medium. It should be at least 3 to 4 inches deep, filled with 2-1/2 inches or more of rooting medium. Clean, coarse builder's sand, a mixture of half perlite and half peat moss, or half perlite and half vermiculite are good choices. Fill the container with the moistened medium, then let it settle and drain for a half hour.
Take stem cuttings in the morning. Using a sharp knife, cut off growth tips just above the node, or the point where a leaf or side shoot attaches to the main stem. Each of the cuttings should be between 3 and 6 inches in length and have 4 to 6 nodes. The stem tissue should be easy to cut through.