As soon as you can bury part of the stem.
Simple Layering
Most plants with low growing branches or stems, such as vines and woody shrubs, take well to simple layering. Use a dormant branch in early spring or a mature branch in late summer.
Bend a flexible, low-growing branch to the ground and place it in a small hole about 4-inches deep. Remove leaves and side-shoots from the portion of the branch that will be buried and cover it with soil. You may need to place a rock on top of the soil to hold the branch underground. It is important to leave at least 6-12 inches of the branch tip out of the soil and stake it upright to keep it growing straight — this will be the top portion of your new plant!
Usually, the bend in the buried portion of the branch is enough to encourage rooting, but by scraping, or wounding, the bark on its underside, you can help speed rooting along. Keep the layered area moist and free of weeds and within a season or two a root mass will have developed. Cut the layered section from the plant and it’s ready for transplanting.