Plum trees, belonging to the Prunus species, are grown worldwide for their spring blossoms and edible fruit. The mature size of a plum tree is highly variable, ranging from a compact bush to a large, wide-spreading tree. Understanding the factors that influence this final size is important for successful planting and planning space. The ultimate dimensions are determined by the root system the tree is grown on and its inherent genetic makeup.
Rootstock Determines Final Size
The most significant factor controlling a plum tree’s mature size is the rootstockâthe root system and lower trunk onto which the desired fruiting variety is grafted. The rootstock governs the tree’s vigor by controlling the efficiency of water and nutrient uptake from the soil. This control is partly achieved through the transport of growth-regulating hormones from the roots up to the canopy.
Plum rootstocks are categorized into three main size classes. Standard trees, often grown on vigorous rootstocks like Brompton or Myrobalan, can reach a height of 18 to 25 feet with a similar spread. Semi-Dwarf trees, commonly grafted onto rootstocks like St. Julien ‘A’, typically mature at 12 to 15 feet tall. This size balances substantial fruit yield with a manageable height.
The smallest option is the Dwarf tree, frequently grown on rootstocks like ‘Pixy’ or VVA-1. These trees generally top out at 6 to 10 feet tall and wide, making them ideal for small gardens, containers, or high-density planting.
Comparing European and Japanese Plum Sizes
Beyond the rootstock, the specific type of plum (the scion) has an inherent genetic growth habit that influences the final shape and size. The two major cultivated types are European plums (Prunus domestica) and Japanese plums (Prunus salicina). These species possess distinct natural tendencies in terms of vigor and canopy structure.
European plums typically develop a more upright and less vigorous growth habit. This means they are generally easier to manage and maintain a narrower profile, which is beneficial in smaller spaces.
Japanese plums, by contrast, are genetically faster-growing and more vigorous overall. They naturally form a wider, more spreading canopy than their European counterparts, often requiring more space for their horizontal reach. Even when grafted onto the same rootstock, a Japanese plum variety will likely occupy a greater volume of space.
Controlling Tree Height Through Pruning
While rootstock determines the maximum potential size, pruning is the primary tool for actively controlling the tree’s height and spread once established. Size control pruning is most effective when performed during the summer, rather than the dormant winter season. Pruning in late winter or early spring tends to stimulate vigorous new growth, which counteracts the goal of restricting size.
Summer pruning, usually conducted after the fruit harvest, removes actively growing wood, which limits the tree’s overall energy reserves for that season. This technique is useful for maintaining a manageable height and keeping the canopy open to sunlight. The central leader (main vertical trunk) can be controlled by making a heading cut just above an outward-growing lateral branch.
Extreme size management can involve specialized training systems, such as espalier or fan training, which flatten the tree against a support structure like a wall. These systems use precise pruning and tying techniques to severely restrict the tree’s height and width, allowing trees to be successfully grown in very confined areas. Pruning stone fruits like plums should be done when the weather is dry to minimize the risk of fungal diseases, such as silver leaf disease.