Arborvitae (Thuja) are popular evergreen conifers often used for hedges and privacy screens. Propagation by cutting is a reliable method for reproducing these plants, as it creates an exact genetic clone of the parent. This vegetative propagation technique bypasses the genetic variability that occurs when growing from seed. Successfully rooting an arborvitae cutting requires selecting the correct plant material and providing a stable, high-humidity environment for a few months.
Timing and Selecting the Cuttings
The optimal window for taking arborvitae cuttings is in the late fall or early winter, typically October through December, when the parent plant is semi-dormant. The wood is classified as semi-hardwood or semi-ripe, meaning it is mature enough to survive but retains vigor to initiate root growth. This timing allows the cutting to focus energy on developing a callus and roots, rather than supporting new foliage growth.
The cuttings should be sourced from healthy, current-season growth, selecting side shoots about 4 to 6 inches in length. The “heel cutting” method is effective, involving gently stripping the side branch away from the main stem so a small sliver of older wood (the heel) remains attached. This heel contains growth hormones and meristematic tissue, which improves successful root formation. Remove the scale-like leaves from the lower one-third to one-half of the stem to expose the rooting area and prevent foliage from rotting in the medium.
Preparing the Rooting Environment
Creating a sterile and well-aerated rooting environment is essential for successful propagation. The rooting substrate should be a light, porous mix that holds moisture without becoming waterlogged, as excess water leads to fungal growth and stem rot. A popular blend consists of equal parts peat moss and perlite or coarse sand, offering the necessary balance of aeration and water retention.
Containers need ample drainage holes to prevent excess water accumulation, which is detrimental to the developing root system. Before planting, dip the prepared base of each cutting in a rooting hormone, which contains synthetic auxins like IBA to stimulate adventitious roots. Ensure the hormone adheres fully to the heel and the exposed stem below the foliage line. Carefully insert the cuttings into the pre-moistened rooting medium, taking care not to rub off the hormone.
Maintaining Cuttings Until Root Development
Maintaining the precise environmental conditions necessary for root development is crucial. Cuttings lack a root system to draw water, so they must be kept in a high-humidity environment to minimize water loss through transpiration. This is achieved by covering the container with a clear plastic dome or bag, which keeps the ambient humidity near 100 percent.
The cuttings require bright light but must be shielded from direct, intense sunlight, which can quickly overheat the enclosed environment and damage the foliage. Placing the containers near an eastern window or under shade cloth provides the necessary indirect light for photosynthesis. Maintaining a stable, cool temperature, ideally between 60°F and 70°F, encourages root growth while discouraging new shoot growth. A robust root system can take anywhere from three to six months to develop.
Hardening Off and Transplanting
After several months, successful rooting is confirmed if a gentle tug on the cutting offers resistance, indicating a network of new roots has formed. Once roots are established, the young plants must undergo “hardening off” to acclimate them from the high-humidity, protected environment to standard outdoor conditions. This transition is necessary because the new root system is susceptible to shock from wind, direct sun, and lower humidity.
The hardening off process should be gradual, starting by removing the humidity dome for a few hours daily and slowly increasing exposure over one to two weeks. Initially, place the plants in a shaded, sheltered outdoor location, progressively introducing them to longer periods of light and ambient humidity. Once acclimated, they are ready to be transplanted into a larger container or their final location. When transplanting, handle the new roots carefully and water the young arborvitae thoroughly to settle the soil around the root ball.