Whether mulberry trees are invasive depends entirely on the specific species of the genus Morus being discussed. North America hosts both a native species and a highly successful non-native relative. The perception of the tree as a threat stems almost exclusively from the ecological behavior of the introduced species. Understanding the distinction between these two types is the first step in assessing the threat they pose to local environments.
Defining the Contenders: Native versus Non-Native Species
The primary native species in eastern North America is the Red Mulberry (Morus rubra), which historically occupied moist, mature forest habitats. This species is currently in decline and is rare or threatened in many parts of its range. The invasive reputation belongs to the White Mulberry (Morus alba), a species originally from China.
The White Mulberry was first introduced to North America during the colonial period to establish a domestic silk industry, as its leaves are the exclusive food source for the silkworm. The Morus alba was extremely successful, escaping cultivation and spreading rapidly due to its hardiness and adaptability. Consequently, the non-native White Mulberry is now a widespread and aggressive invader across the United States and Canada, while the native Red Mulberry struggles to survive.
Mechanisms of Ecological Spread
The White Mulberry’s success as an invasive species is rooted in its ability to quickly colonize disturbed areas such as roadsides, forest edges, and old fields. It exhibits traits typical of a pioneer species. Its ability to thrive in a wide range of conditions, tolerating varying soil types, drought stress, and even salt, allows it to establish where many native species cannot.
A single female White Mulberry tree can produce a massive quantity of fruit, generating up to twenty million seeds. The fruit is highly attractive to birds and other wildlife, which consume the berries and disperse the seeds over wide distances in their droppings. This prolific seed production and efficient dispersal ensure the rapid colonization of new habitats. The White Mulberry also exhibits a faster growth rate and higher seedling survival than the native Red Mulberry, giving it a reproductive advantage.
Identification and Threats to Native Ecosystems
Distinguishing the invasive White Mulberry from the native Red Mulberry can be challenging, but subtle differences exist in their leaf and bark characteristics. The leaves of the native Morus rubra are typically larger, dull, and have a rough, sandpapery texture on the upper surface, with soft hairs underneath. In contrast, Morus alba leaves are often smaller, glossy, and generally smoother. Their shape can vary greatly on a single branch, ranging from unlobed to deeply lobed.
The most significant threat posed by the White Mulberry is genetic pollution of the native gene pool, rather than simple competition for resources. Morus alba readily hybridizes with the native Morus rubra, and the hybrid offspring are often genetically more similar to the White Mulberry. This process, known as genetic assimilation, is slowly eroding the genetic integrity of the native population. Pure Red Mulberry trees are increasingly rare, compromising the long-term survival of the true native species.
Management and Control Strategies
Control efforts focus on the non-native White Mulberry and its hybrids, aiming to prevent seed production and spread. Manual removal is effective for small seedlings and young plants before their root system becomes established. Larger trees require more intensive management to prevent vigorous resprouting from the stump or roots.
The cut-stump method is an effective technique for mature trees. It involves cutting the tree down and immediately treating the fresh stump surface with a systemic herbicide, such as a 25% solution of glyphosate. This treatment is absorbed by the roots and prevents the tree from regenerating. Property owners should carefully identify any mulberry tree before removal; if a native Red Mulberry is confirmed, it should be protected as a conservation measure.