The White Mulberry tree (Morus alba) frequently causes confusion due to its variable appearance and widespread presence. This species is not native to North America but has established itself firmly across the continent. Understanding its ecological role requires clarifying its origin, classification, and how it affects local plant communities.
The Status of White Mulberry (Morus alba)
The White Mulberry is an exotic species, native to China and parts of India, where it has been cultivated for millennia. It was intentionally introduced to North America in the 17th century to establish a silkworm industry, as its leaves are the preferred food source for silkworms (Bombyx mori). Although the silk industry failed, the trees thrived and escaped cultivation.
Today, Morus alba is widely considered a problematic non-native species throughout the United States and Canada. While some regions classify it as strictly invasive, others refer to it as highly naturalized or weedy. Regardless of the local designation, the tree is recognized as disruptive due to its rapid spread and ability to colonize disturbed areas, such as roadsides, old fields, and forest edges.
Identifying Features
Identifying Morus alba can be challenging because its leaves display high variability, known as polymorphism. A single branch may contain unlobed, heart-shaped leaves next to others with two, three, or five deep lobes. This highly variable shape is a distinctive identification clue.
The leaves typically have a bright green, noticeably shiny, smooth upper surface. In contrast, the native Red Mulberry (Morus rubra) has a duller, rougher texture. Despite the tree’s common name, the fruit is often confusingly colored, ranging from white to pink, red, or deep purplish-black when ripe. The bark of mature Morus alba is commonly gray with shallow, irregular furrows, sometimes exhibiting an orange or reddish-tan tint.
Mechanisms of Ecological Disruption
The White Mulberry is highly successful because it is extremely tolerant of diverse environmental conditions. It quickly colonizes habitats, including dry, poor soils and urban environments, often outcompeting native tree saplings that require specific conditions. The species is a prolific seed producer; its fruit is eagerly consumed by birds and mammals, which widely disperse the seeds through their droppings, contributing to its rapid spread.
The most significant ecological threat posed by Morus alba is its tendency to hybridize with the native Red Mulberry (Morus rubra). The two species readily cross-pollinate, creating fertile hybrid offspring. This process, known as genetic pollution or genetic swamping, dilutes the gene pool of the native Morus rubra, which is already endangered or threatened in several states. The hybrid progeny often display more vigorous traits closer to the introduced White Mulberry, placing the native species at a strong disadvantage and threatening its long-term genetic viability.
Control and Management Strategies
Effective control requires persistent effort because the tree vigorously re-sprouts from roots and stumps after being cut. For small seedlings and young saplings, manual removal by hand-pulling or digging them out is an effective mechanical control method, provided the entire root system is removed. This approach is most practical for new infestations.
For larger, established trees, simple cutting is insufficient and must be paired with chemical treatment to prevent re-growth. The cut-stump method involves immediately applying a systemic herbicide, such as glyphosate or triclopyr, directly to the freshly cut stump surface. This allows the chemical to be absorbed and translocated into the root system to kill the entire plant. Post-treatment monitoring is necessary for several years, as the site’s short-lived seed bank (viable for one to two years) can produce new seedlings that must be addressed.