The idea of a tree being regulated or banned may seem unusual, but it is a reality for the mulberry tree in certain communities across the United States. While the genus Morus includes several species, the specific plant most often targeted by these restrictions is the non-native White Mulberry (Morus alba). These prohibitions are highly localized ordinances aimed at addressing a specific public health concern related to the tree’s biological characteristics and widespread planting.
The Health Concern: Excessive Pollen Production
The primary reason for the prohibition of the White Mulberry is the immense volume of highly allergenic pollen produced by the male trees. Mulberry trees are severe allergens, and their fine, wind-borne pollen significantly contributes to respiratory illnesses in urban areas. The male flowers release pollen with a rapid, explosive mechanism, allowing the microscopic grains to travel great distances.
This feature makes the male White Mulberry a public health hazard, especially in dry, arid climates where the pollen remains suspended longer. The concentration of this airborne material during the spring pollination season can exacerbate conditions like asthma and allergic rhinitis. Studies have linked the planting of these trees to increased rates of emergency room visits related to breathing difficulties. The pollen grains are smaller than those of many other trees, allowing them to penetrate deeper into the respiratory system upon inhalation.
Legislative Response and Geographical Restrictions
The restrictions on planting mulberry trees are typically found in city and county ordinances, not state or federal law. These municipal codes often classify the species as a noxious pollen producer or a public nuisance. The bans are most common in the arid Southwestern United States, where the trees were historically favored for their fast growth and ability to provide shade.
Cities like Tucson, Arizona, were among the first to implement such measures, banning the White Mulberry in 1984. Other municipalities, including Las Vegas, Nevada, and parts of the Phoenix and El Paso metropolitan areas, followed suit. These ordinances were a direct response to public health initiatives aimed at improving air quality and reducing the strain on local healthcare services caused by seasonal allergies. The regulations generally prohibit the sale or planting of new mulberry trees, though existing trees are often grandfathered in.
Differentiating Male Trees, Female Trees, and Edible Varieties
A significant nuance in these regulations is the difference between male and female mulberry trees, which relates to their reproductive biology. The species is often dioecious, meaning individual trees are either male or female. The male trees produce the vast amount of pollen that triggers health concerns but yield no fruit.
Conversely, the female trees produce edible fruit but release very little pollen, as their flowers are structured to receive pollen rather than shed it. Ironically, many “fruitless” mulberry cultivars planted in the mid-20th century were selected to be exclusively male to avoid the mess of fallen berries, inadvertently creating the allergen problem the bans now address.
The non-native White Mulberry (Morus alba) is distinct from the native Red Mulberry (Morus rubra). The native species is rarely the target of these bans but faces a threat from hybridization with the more aggressively spreading White Mulberry.