There are two clumps of perennial tall white Phlox beside the house, and the previous owners planted them in an area that only gets morning sun. These are blooming mostly in September. I thought perhaps if they were moved to the south facing side where they would get sun all day long, they would perform better, and perhaps bloom earlier. But I'm wondering if it's OK to transplant them now before they bloom, and may not suffer much if I take most of the root ball and water well? Or would it just be best to wait? Also is it true with this type of plant, they will grow from root cuttings? Thank you.
Yes, P. paniculata will bloom earlier in full sun (July-August) depending on cultivar. It might be better to wait until early spring to move your plant. Let the plant finish the growing season undesturbed in order to accumulate more energy before it goes dormant. Root cuttings might work but juvenile shoots are much easier to propagate.
The perennial varieties, P. paniculata and others, do not come true from seed. They revert mostly to magenta colors, and only rarely produce worthwhile plants. P. paniculata can be divided either in spring or fall, and cuttings taken in early summer root easily. For large numbers of plants, root cuttings can be used. Dig the entire plant in the fall, and cut off all the large roots except those close to the crown. The crown can then be replanted. Cut the roots into pieces about two inches long, and plant hem in flats of rich, sandy soil, a half-inch deep and mulch heavily or store in a protected cold frame for the winter. By late June they should be ready to transplant. If you want only a few plants, an easier way is to leave the original clump in place. Cut all around it severing the outside roots with a sharp spade, but leave the roots in the ground. This, too, is best done in the fall. The following spring, the roots that were cut off send up new little plants which can be dug and transplanted in early summer. Good luck to you. - Millet (1,261-)
This is a tough plant, and you can move it now, or anytime, without much risk of killing it. You will, however, probably sacrifice much or most of the flower display for this season, if that is a concern. You should also be able to divide the clump, either by digging it up and splitting it into pieces, or simply by driving a garden spade down through the clump and extracting some goodly chunk of it (along with, of course, as many roots as you can get). These operations are ideally performed in spring or fall. But if you're like me, you might do well to just get the job done when the inspiration is upon you.
