Ask an experienced gardener how to propagate phlox, and s/he will say to divide an established clump. However, there are other methods: root cuttings, stem cuttings, seed. Division or cuttings will yield new plants identical to the parent plant; seeds often do not breed true due to hybridization. Late spring or early summer is the best time to do division or cuttings.
To divide, dig up and separate 2/3 of an established clump. Divide into two or three smaller clumps and plant each in a location with full sun or partial shade, and rich, humusy, moist but well-drained soil. Established clumps should be divided every 2 to 4 years.
To take root cuttings, dig up the edge of an established clump to expose the fleshy roots. Make 2-to-4-inch-long cuttings with a sharp knife; be sure to keep them oriented properly. The end nearest the parent plant must be upward, or the cutting won’t root. Plant them in a container filled with equal amounts of sterile potting soil and peat moss that has been well-watered and allowed to drain. Space the cuttings about 2″ apart, making sure they are oriented correctly.
Stem cuttings should be 3 to 6 inches long and include from 2 to 6 leaf nodes. Cut the stem with a sharp knife at a 45º angle just below a leaf node. Remove flowers, buds and lower leaves. Plant the cuttings in a rooting medium of equal parts coarse garden sand and vermiculite or perlite that has been well-watered and allowed to drain. Place cuttings upright in the medium with the lowest leaf node just above the surface.
All cuttings need a humid environment and should be kept moist, but don’t over-water them. Once your new plants are established, they can be transplanted outdoors.
Growing from Seed
To grow from seed, start the seeds indoors in pots about 8 weeks before the last frost. When danger of frost has passed, transplant seedlings to the garden.