The Thuja Green Giant, formally known as Thuja ‘Green Giant’, is a fast-growing hybrid evergreen that resulted from a cross between the Western Red Cedar (Thuja plicata) and the Japanese Thuja (Thuja standishii). This cultivar is valued for its rapid growth rate, making it a popular choice for dense privacy screens and hedges. Propagating the Thuja Green Giant from cuttings offers home gardeners a cost-effective method to produce genetically identical plants, ensuring the new trees maintain the parent’s desirable traits. The most reliable technique involves rooting semi-hardwood stem cuttings.
Timing and Necessary Supplies
The optimal time for harvesting cuttings is late summer or early fall. This period is when the current year’s growth has begun to mature, or “harden off,” but is not yet fully dormant, classifying the material as semi-hardwood. Taking cuttings during this time ensures the plant tissue is mature enough to survive while still having enough metabolic activity to initiate root growth.
Several supplies are needed for successful propagation. You will need sharp pruning shears or a clean knife, which should be sterilized with an alcohol solution to prevent the transfer of pathogens. Rooting hormone, typically containing the synthetic auxin Indole-3-butyric acid (IBA) in a concentration of around 0.8% or higher, is required to stimulate root development. Containers with proper drainage holes and the chosen rooting medium complete the preparation checklist.
Harvesting and Treating the Cuttings
Cuttings should be taken from healthy, disease-free, semi-hardwood branches that are approximately pencil-thick and 6 to 8 inches in length. The most effective method for harvesting is the “heel cutting” technique, where the side branch is gently pulled downward and away from a larger branch. This detaches the cutting with a small strip of the older wood (the “heel”) still attached to the base.
The presence of this heel is thought to concentrate rooting hormones and growth factors, improving the rate of callus and root formation. Remove all foliage from the lower one-third of the stem. This prevents rotting when buried in the damp rooting medium, which could introduce fungal pathogens. Immediately after preparation, dip the cutting into the rooting hormone powder or liquid, ensuring the heel and the exposed cambium tissue are fully coated before planting.
Creating the Rooting Environment
The rooting medium must provide adequate moisture retention and superior drainage to prevent waterlogging and rot. An ideal, sterile mixture consists of equal parts by volume of peat moss and coarse perlite or sand. This ratio creates a porous structure that allows for necessary air exchange around the cutting while maintaining a slightly acidic environment.
After planting the treated cuttings into the pre-moistened medium, focus on creating a high-humidity environment. Thuja cuttings require consistently high humidity to reduce transpiration and prevent the foliage from drying out before roots can form. This is achieved by placing the containers inside a clear plastic dome, a plastic bag, or a makeshift mini-greenhouse structure.
Maintaining an ambient temperature between 55°F and 70°F encourages root development. The cuttings should be placed in a location that receives bright, indirect light; direct sunlight can rapidly overheat the enclosed environment and scorch the foliage. Bottom heat, provided by a propagation mat set to about 70°F, can stimulate the formation of a callus, the mass of cells from which roots will eventually emerge.
Post-Rooting Care and Transplanting
Rooting typically takes between two to four months, with success often visible when new, green growth appears at the tips of the cuttings. You can confirm the development of roots by gently tugging on a cutting; resistance indicates a solid root system has formed. Once rooted, the young plants must undergo a gradual process known as “hardening off” before they can be exposed to normal outdoor conditions.
Hardening off involves slowly acclimating the rooted cuttings to lower humidity and intense light over one to two weeks. This is accomplished by progressively opening the humidity dome or plastic covering and increasing the duration of exposure to ambient air each day. This step helps prevent shock and desiccation when the protective environment is fully removed. The new Thuja Green Giants should be transplanted into larger pots or their permanent location in the garden during the following spring, allowing the newly established roots to benefit from a full growing season.