Plum trees, members of the genus Prunus, may or may not have thorns. While most modern orchard trees are thornless, the presence of sharp, woody structures depends entirely on the specific species and its origin. Whether the tree is a wild native specimen or a highly cultivated variety dictates its physical defenses. This variation results from both selective human breeding and ancient evolutionary pressures.
Thorned Versus Thornless Plum Varieties
Most of the plums found in commercial orchards, such as the European plum (Prunus domestica) and the Japanese plum (Prunus salicina), have been selectively bred over generations to be thornless. These cultivated varieties offer large, sweet fruit and lack the defensive structures of their wild relatives. However, thorns frequently appear on older, wilder species, including the American plum (Prunus americana), which is native to North America, and the Damson plum (Prunus insititia), a subspecies of the European plum.
A common encounter with plum thorns occurs not on the main branches but on the rootstock of a grafted tree. Fruit growers often graft a desired, thornless variety onto the hardy root system of a different plum species, such as Blackthorn (Prunus spinosa). If a shoot, known as a sucker, sprouts from the thorny rootstock below the graft line, it will display the aggressive, thorny characteristics of the wild base plant. These suckers must be pruned immediately to prevent the thorny, wild growth from overtaking the cultivated portion of the tree.
The Evolutionary Reason Thorns Develop
The development of thorns is an adaptation that serves as a mechanical defense mechanism against herbivory. Thorns are essentially modified branches or short, sharp stem tips, hardened and pointed to deter large browsing mammals. In the natural environment, this structural deterrent protects vulnerable stems and leaves from animals like deer and rabbits.
This need for robust protection is much greater for wild plum species than for their cultivated counterparts. Wild varieties must survive intense competition and predation in their native habitats, explaining why species like the American plum are consistently thorny. Cultivated trees are protected by human maintenance, allowing breeders to select for traits like high fruit yield and thornlessness.
Key Physical Traits for Identification
Identifying a thorny plum tree requires observing the thorn’s structure along with other key botanical features. Plum thorns typically manifest as short, stiff, pointed side branches found along the twigs. These specialized branches are distinct from the sharp prickles of a rose because plum thorns are woody extensions of the stem itself, not outgrowths of the bark.
Beyond the thorns, several other traits confirm the tree’s identity as a Prunus species. The bark of young plum trees is often smooth and reddish-gray, marked by prominent, horizontal lenticels (small pores used for gas exchange). The leaves are generally oval or elliptical with finely serrated edges. If the leaves or bark are crushed, they often release a faint, characteristic scent of bitter almonds.