Training a multi-stemmed shrub into a single-trunk tree, often called a “standard,” is a specialized horticultural technique. This practice transforms a dense bush into an elegant, small-scale tree structure featuring an upright, clear stem topped by a rounded canopy. Gardeners use this method to create focal points, elevate flowers or foliage to eye level, or introduce a formal element where a full-sized tree is impractical. This maximizes aesthetic impact in smaller spaces like courtyards or patio containers.
Characteristics of Trainable Shrubs
Successful conversion requires selecting a shrub with appropriate growth habits and physical resilience. Suitable candidates have strong, flexible wood that can withstand structural pruning and support the eventual canopy. Plants that naturally exhibit an upright growth habit are easier to train. The plant must also be highly tolerant of heavy pruning, which is necessary to remove competing stems and establish the single trunk.
Many popular flowering shrubs are excellent choices for this technique.
- Panicle hydrangea (Hydrangea paniculata) responds well to severe cutting and blooms on new wood.
- Lilacs (Syringa) and crape myrtles (Lagerstroemia indica) are frequently trained due to their naturally strong stems and beautiful flower display.
- Deciduous shrubs like Rose of Sharon (Hibiscus syriacus) adapt readily, often creating a tidy, lollipop-like form.
Shrubs that produce minimal basal suckers are generally preferred because they require less ongoing maintenance. Suckering shrubs, such as certain viburnums, can be trained but demand much more frequent attention to maintain the single-trunk form.
Structural Pruning for Tree Form
The initial transformation requires focused structural pruning, which should begin when the plant is young. First, identify the single, straightest, and most vigorous stem to serve as the future trunk, known as the leader. All other surrounding stems and competing shoots must be removed completely, cutting them cleanly to the ground. This ensures the plant directs all energy into the chosen leader.
Once the leader is selected, it must be supported immediately with a sturdy stake driven parallel to the stem. Secure the stem to the stake at multiple points using flexible tie material. This prevents movement that could damage the growing layer and encourages a perfectly vertical growth pattern. As the leader grows upward, gradually remove small lateral branches along the lower portion of the stem, a process called limbing up. Leave a few leaves on the stem initially, as they are necessary to support trunk caliper growth.
Allow the leader to grow until it reaches a height slightly above the desired trunk height, typically between three and six feet. At this point, the growing tip is cut back, or “headed,” to a bud just above the preferred height where the canopy will begin. This removal of the terminal bud disrupts apical dominance, encouraging the lateral buds below the cut to sprout and develop into the branching structure of the canopy.
Long-Term Maintenance of the Standard
Long-term maintenance focuses on preserving the clear trunk and shaping the canopy. The most persistent task is managing suckers and adventitious shoots that attempt to grow from the base or along the main trunk. These shoots must be rubbed off or pruned immediately, as they represent the shrub’s natural tendency to revert to its multi-stemmed form. Allowing this basal growth to remain will quickly compromise the integrity of the standard shape.
Annual pruning of the canopy is necessary to maintain density, size, and a balanced form. The timing of this pruning is determined by the shrub species:
- Shrubs that bloom on new wood (e.g., panicle hydrangeas) are best pruned in late winter or early spring.
- Shrubs that bloom on old wood (e.g., lilacs) should be pruned immediately after spring flowering to avoid removing the following year’s buds.
When pruning the canopy, make cuts to encourage outward growth and remove any branches that are crossing or growing inward. The supporting stake should only be removed once the trunk has developed sufficient girth and strength, typically after several years.