Root Cuttings
Root cuttings are best taken when the plant is dormant and the roots are chock-full of carbohydrates.
Take 1 to 4-inch long cuttings from younger root growth that is about 1/4 to 1/2 inch thick. Cut straight through the end of the root nearest to the stem and cut the other end at an angle. This way you will remember which end is the top (the straight cut) and which is the bottom (the diagonal cut). Roots will not grow if you plant them upside down.
Store cuttings in a moist rooting medium at 40°F. After three weeks, remove the cuttings from storage and bury them upright under 2-3 inches of soilless potting mix. Place the container in a plastic bag and put the whole thing somewhere with bright, indirect sunlight. When roots become established and weather permits, harden the new plants off and transplant them outside. Learn more about transplanting and handling plants here.