The ‘Green Giant’ arborvitae (Thuja standishii x plicata) is a popular, fast-growing evergreen conifer valued for its dense, pyramidal form and ability to create a quick privacy screen or windbreak. This hybrid, which can grow three to four feet per year, is also noted for its resilience against common pests and diseases. Creating new plants from an existing ‘Green Giant’ is achieved most effectively through vegetative propagation, specifically by taking stem cuttings. This method ensures the new plant retains the exact genetic characteristics of the parent, unlike propagation from seed, which can introduce variability.
Selecting and Preparing Cuttings
The most opportune time to harvest propagation material is in the late summer or early fall, when the current season’s growth has begun to “semi-harden.” This means the wood is no longer soft and pliable, but it has not fully matured into hard, dormant wood. This maximizes the cutting’s energy reserves while minimizing water loss. Select cuttings from healthy, vigorous shoots, aiming for a length of six to eight inches.
The most successful cuttings are taken with a small piece of older wood, known as a “heel,” still attached to the base. This heel contains a higher concentration of natural growth hormones and pre-formed root initials, which improves successful root formation. To achieve this, gently pull a lateral side branch downward from the main stem, allowing a small strip of the parent branch bark to tear off with the cutting. Use sharp, sanitized pruning shears or a knife to clean up any ragged edges on the heel to prevent disease and promote a clean wound for rooting.
Preparation continues by removing the foliage from the lower two-thirds of the cutting, exposing the stem where the roots will emerge. This reduces the surface area for water loss and prevents submerged foliage from rotting in the rooting medium. For an added boost, gently scrape two thin, shallow vertical strips of bark from the base of the cutting. This encourages callus tissue formation and subsequent root development.
Establishing the Rooting Environment
Once the cuttings are prepared, applying a rooting hormone is beneficial, as it introduces auxins that stimulate root growth and improve the success rate. Use either a powder or liquid formulation, dipping the prepared base of the cutting into the hormone before planting, ensuring only the bottom tip is coated. To avoid contaminating the entire supply, pour a small amount of the hormone into a separate disposable container for dipping.
The rooting medium must be sterile and possess excellent drainage while retaining enough moisture to prevent the cutting from drying out. A common composition is a 50/50 mixture of sphagnum peat moss and perlite or coarse sand. This blend provides the necessary aeration for new roots to form without becoming waterlogged, which can lead to fungal diseases and rot. The medium should be pre-moistened to the consistency of a damp sponge before the cuttings are inserted.
Environmental controls determine the success of the rooting process, with humidity being a primary factor. Since cuttings lack a root system to absorb water, a high-humidity environment is required to reduce moisture loss from the foliage. Achieve this by placing a clear plastic dome or bag over the planting container, creating a miniature greenhouse effect. The container should be placed in a location that receives bright, indirect light, as direct sunlight can overheat the enclosure and scorch the foliage. Maintaining a stable temperature, ideally around 70 degrees Fahrenheit, accelerates the cellular processes necessary for root initiation.
Ongoing Care and Transplanting
After the rooting environment has been established, the cuttings require consistent monitoring to ensure the medium remains lightly moist and the humidity level is maintained. The initial rooting period typically extends for eight to twelve weeks. Periodically check for root development by gently tugging on a cutting; resistance indicates that roots have formed and are anchoring the new plant.
Once a healthy root system is confirmed, the new plant must undergo a gradual process of “hardening off” before being transplanted. Hardening off involves slowly acclimating the young arborvitae to the lower humidity and stronger air currents of its final outdoor environment. This is accomplished by progressively increasing the time the humidity dome is removed over a period of one to two weeks, starting with just a few hours a day. This process minimizes the shock that occurs from a sudden shift in environmental conditions.
The rooted cuttings are ready to be moved into a larger, individual pot with a standard, well-draining potting mix or directly into a protected nursery bed. If transplanting into a pot, the young arborvitae can be grown there for an entire season to build strength before being planted in the landscape. If rooting occurred in the late fall, young plants must be protected over their first winter. This can be done by placing them in a cold frame, an unheated garage, or by heavily mulching the pots to insulate the root ball.