Choose roots that are pencil thick
Shrub roots can get quite large and woody, but the best ones for cuttings are those approximately as thick as a pencil. These are young, vigorous roots that are more likely to send up new shoots. With perennial roots, thicker is better. I use a sharp pair of pruners to make a straight cut at the end of the root closest to the parent plant. At the far end, I make a diagonal cut. This helps me maintain the root’s original orientation, critical to the production of new roots and shoots. I always avoid cutting off more than one-third of the roots because this may eliminate too much of the plant’s stored energy.
I then take the long pieces of root I removed and cut them into sections 3 to 6 inches long, making sure to cut the ends closest to the plant straight and the ends farthest from the plant at an angle. The optimal length is 3 to 6 inches because it ensures that there is enough energy in the cutting and, in some cases, enough dormant buds to produce roots and shoots.
After taking the cuttings, I replant the mother plant or cover the exposed roots. Then I water the area thoroughly to remove large air pockets in the soil and settle the roots back into their home.