Japanese maples (Acer palmatum) are prized ornamental trees, celebrated for their delicate leaf shapes and vibrant seasonal colors. A common feature on nearly all commercially available specimens is a noticeable knob or scar near the base of the trunk. This mark is the graft union, the physical point where two different parts of maple tissue were joined to grow as a single tree. Grafting is the primary method used to propagate these beautiful trees, ensuring the unique characteristics of specific varieties are reliably reproduced.
Why Grafting is Essential for Cultivar Preservation
The primary reason for grafting Japanese maples is to preserve the unique traits of named varieties, known as cultivars. When a Japanese maple cultivar produces seeds, the resulting seedlings rarely inherit the parent’s distinct ornamental features. This genetic variation means planting a seed from a tree with brilliant red leaves often yields a generic green-leaved maple with unpredictable growth characteristics. Grafting is a form of asexual propagation, effectively creating a genetic clone of the desired parent plant. A small section of the cultivar is physically attached to a separate root system, ensuring the new tree’s top growth will exhibit the exact traits of the parent, including leaf color and growth habit.
Understanding the Anatomy of the Graft
The process of grafting involves joining two separate plant components to form a single, unified tree. The top section, a cutting taken from the desired cultivar, is called the scion. The scion determines all the above-ground features, such as the branch structure and leaf characteristics. The bottom section, which provides the roots and lower trunk, is called the rootstock. For the graft to be successful, the cambium layers of the scion and the rootstock must be aligned and held tightly together. The cambium is the thin, actively dividing layer of tissue beneath the bark, and its alignment is necessary for the vascular systems to fuse and transport water and nutrients.
Controlling Growth and Vigor
Beyond genetic preservation, the choice of rootstock controls the mature size and overall vigor of the tree. Growers intentionally select rootstocks for their inherent characteristics, often choosing a fast-growing, hardy strain of Acer palmatum that establishes a strong, durable root system. This vigorous foundation supports the less-robust ornamental cultivar, driving its growth. Grafting also allows for the production of compact or dwarf varieties by utilizing a rootstock that naturally restricts the overall growth rate. By pairing a specific cultivar with a rootstock known for limited growth, commercial nurseries can produce smaller trees suitable for container gardening and restricted landscape spaces. This manipulation of vigor provides a standardized, durable plant that might otherwise struggle on its own roots.
Practical Care: Managing Rootstock Suckers
A unique maintenance requirement for grafted Japanese maples is managing rootstock suckers. Because the rootstock is genetically distinct from the scion, it retains its own growth potential. Occasionally, the rootstock will attempt to sprout branches, or suckers, from below the graft union. These suckers display the generic, non-ornamental characteristics of the rootstock, often being vigorous and green-leaved, even if the scion has red foliage. If left to grow, these shoots compete with the desired cultivar for water and nutrients, diverting energy that can weaken or completely overtake the ornamental top growth. The suckers must be removed immediately and cleanly by cutting them off flush with the trunk to prevent the rootstock from dominating the tree.