The ‘Royal Purple’ Smoke Tree (Cotinus coggygria ‘Royal Purple’) is admired for its dramatic, deep reddish-purple foliage that holds its color throughout the growing season. This large shrub or small tree adds significant ornamental value, especially when summer brings hazy, plume-like flower clusters that give the plant its common name. Though generally low-maintenance, thoughtful pruning is necessary to shape the plant, manage its size, and maximize its signature color display. The approach depends entirely on whether you prioritize the purple leaves or the delicate summer ‘smoke’.
Understanding Pruning Timing
The timing of major pruning significantly impacts the health and subsequent display of the smoke tree. The ideal window for performing substantial structural cuts is during the dormant season, specifically in late winter or very early spring. Pruning before the new buds swell minimizes stress and encourages vigorous regrowth. This timing is also beneficial because the plant’s structure is visible without leaves, allowing for precise shaping.
Pruning should be avoided in late summer or autumn because stimulating new growth leaves tender stems vulnerable to winter frost damage. Minor maintenance, such as removing dead, diseased, or damaged wood (the three Ds), can be performed immediately upon identification to prevent the spread of pathogens. Major shaping or rejuvenation cuts must be reserved for the dormant season.
Techniques for Structural Health and Size Control
Maintaining the overall structure of the smoke tree involves specific types of cuts aimed at promoting air circulation and managing its naturally rangy growth habit. The primary technique is the thinning cut, which involves removing an entire branch back to its point of origin, such as the trunk or a main lateral branch. These cuts open the canopy, allowing sunlight and air to reach the interior, which helps reduce the risk of fungal disease. Thinning cuts are most effective when removing older, unproductive wood or stems that are crossing and rubbing.
Heading cuts are used to encourage a fuller, denser growth habit on a specific branch. This method involves cutting a branch back to a healthy bud or a side branch pointing in the desired direction of new growth. When making any cut, cut cleanly just outside the branch collar—the swollen area where the branch meets the parent wood—to facilitate proper wound closure. When managing the height or spread of a mature shrub, remove no more than one-third of the total wood in a single year to prevent excessive stress.
Another important structural practice is the removal of suckers that emerge from the base or root system. These vigorous, non-cultivar shoots divert energy from the main plant and should be pruned off as close to the ground as possible. For those training the smoke tree into a small, single-trunk tree form, continuous management of these lower side shoots is necessary to maintain the desired clear trunk.
Specialized Pruning for Maximum Foliage Color
The ‘Royal Purple’ cultivar offers the option of a severe pruning method, often referred to as coppicing or stooling, specifically to enhance leaf size and color. This technique exploits the plant’s natural response to trauma, forcing it to produce vigorous, juvenile growth that exhibits the deepest purple hue. The foliage that emerges after a hard cut is noticeably larger than leaves produced on older wood.
Coppicing involves cutting all stems back severely to a low framework, typically within 6 to 12 inches of the ground, during the late winter or early spring dormant period. This drastic reduction of the plant’s above-ground mass stimulates the root system to push out numerous long, straight shoots rapidly. The result is a dramatic, shrub-like form covered in large, deeply colored leaves.
This method requires accepting a significant trade-off concerning the plant’s flowering potential. Since Cotinus species flower on wood that is at least one year old, the vigorous new shoots produced by coppicing will not bear the signature “smoke” plumes that summer. Gardeners must choose whether they prioritize the intense purple foliage or the airy flower clusters. If the smoky plumes are desired, a milder, structural pruning approach should be used instead of coppicing. Hard pruning should only be performed on established plants, typically those that have been in the ground for three to five years.