The Thuja green giant grows to a stately 50 feet tall when planted by itself and 20 to 30 feet when planted in a group or privacy fence. Being a tree listed in the public domain, any gardener is free to propagate the green giant. Grow green giant from cuttings taken in the winter to ensure a perfect clone of the original plant.
Wait for the green giant shrub to go dormant before taking cuttings. Choose sections of this year's growth for better propagation results. Cut several 6- to 8-inch sections from the shrub with pruning shears. Remove the bottom foliage of stem cuttings to reveal at least 3 inches of the stem.
Prepare the growing tray by wetting down the soil with warm water. Do not over saturate the potting soil but dampen it slightly. Fill the tray with the soil. Make pre-formed holes for the green giant cuttings with the dibble or pencil.
Pour 2- to 3- tbsp. of rooting compound into a baggie or small container. Dip the bare sections of the stem cuttings into the container of rooting compound and shake off the excess. Place the cuttings into the growing tray and tamp down the dirt to remove any air pockets. Air in the soil breeds bacteria which kills green giant cuttings.
Insert wooden skewers into either end of the growing tray so the plastic does not come into contact with the green giant cuttings when you put it over the tray. Slip the grow tray into the plastic bag or cover with plastic wrap. Secure the bag with a rubber band or use cellophane tape for the plastic.
Place the growing tray on a heating mat, set at 70 degrees, in a bright location out of direct sunlight. Propagation can take up to 3 months for green giant cuttings. During this time, keep the soil moist but not wet. Do not use a mister bottle as conifers prefer their needles and foliage to remain dry.
Monitor the green giant cuttings and remove any which die or develop mold. After about a month, give the cuttings a gentle tug. Resistance means roots have developed. Remove the plastic and continue to grow until the roots are fully established.
Transplant the tender green giant plants into separate containers and grow out to the desired height. Transplant outdoors when weather permits.