When you want to propagate a healthy and successful grapevine, take cuttings of the vine and start a new plant. However, it's not as simple as clipping a stem from the grapevine and sticking it in the ground. To root grape cuttings, you'll need a few supplies and time for the cutting to learn to become the entire plant where once it was just a part of the whole. You'll want to get cuttings in winter, when the plant is dormant, and grow them inside your home or greenhouse.
Clip the cuttings with sharp, clean hand pruners. These cuttings should be pencil-thick with three to four sets of buds on them on a vine that is ripe, not green. Trim the cutting so each end has a bud at the clipped surface with no cutting being longer than 1 foot.
Mix equal parts of potting soil and compost, and add it to your pot to fill it only halfway. Place the pot into the plastic bag with the pot centered in the bottom of the bag.
Dip the bottom of a cutting in a small bowl of water and then into the hormone rooting powder. Push the cutting 1 inch down into the soil of your pot. Repeat to place up to six cuttings per pot, spaced out evenly.
Fill the rest of the pot with the soil-compost mix and water the soil to dampen it without leaving the pot resting in standing water. Pull the bag closed around the pot and use a rubber band to hold it shut.
Move the cuttings to a warm place or window where it will be kept from direct sunlight. Wait for the cuttings to start to grow and leave them undisturbed until the cuttings are 9 inches long sticking out of the soil.
Open the bag during this period only to add more water if the pot is drying out, or to remove any cuttings that look diseased or are turning black. Otherwise, the bag should remain closed to keep a moist, warm environment around the cuttings.
Transplant the 9-inch cuttings to their own pots and give them direct sunlight and water them enough to keep the soil damp. Give the plant a support to climb and allow them to grow to 4 feet tall before planting outside.