The preference for planting male trees in cities is a result of intentional urban landscaping practices. Many non-fruiting street trees are specifically selected to reduce street litter and maintenance costs. The underlying preference for trees that do not drop seeds, pods, or messy fruit has led to a noticeable imbalance in the gender of certain tree species across many urban forests. Understanding this practice requires examining the fundamental biology of tree reproduction and the economic goals of city management.
Understanding Tree Reproduction
Trees reproduce using different biological strategies, which determines whether they can be gender-selected for urban planting. The majority of tree species are classified as monoecious, meaning both male and female reproductive structures are found on the same plant. Monoecious trees, such as oak, pine, and birch, produce both pollen and seeds, making selective gender planting impossible.
A smaller group of trees are dioecious, meaning individual trees are either male or female. The male tree produces only pollen, while the female tree produces the seeds or fruit necessary for reproduction. Common dioecious examples include Ginkgo, Holly, and Ash, where the gender of the tree can be specifically chosen by nurseries and planners.
Since male dioecious trees invest their energy only in pollen production, not in the formation of heavy fruit or seeds, they are often observed to grow faster and live longer than their female counterparts. This difference in energy allocation can influence species selection, favoring the more vigorous growth of male trees. The ability to guarantee a tree’s gender through cloning allows urban planners to bypass the potential disadvantages of the female plant.
The Rationale for Selecting Male Trees
The primary motivation behind the selective planting of male dioecious trees is the simplification of urban maintenance and the reduction of costs. Female trees produce seeds, cones, or fruit that, when dropped onto sidewalks, streets, and cars, create litter. This organic debris requires regular, expensive cleanup, which city maintenance budgets seek to avoid.
For example, the female Ginkgo tree produces a soft, fleshy fruit that emits a foul odor when crushed, leading to public objections and high cleaning costs. By planting only the male clone, urban planners can utilize the species’ desirable shade and hardiness without the unpleasant fruit nuisance. This focus on “clean” streetscapes led to recommendations, such as those found in a 1949 USDA Yearbook of Agriculture, to select only male trees for street plantings.
Nurseries facilitate this practice by propagating specific cultivars, often through cloning, that are guaranteed to be male. These cloned male trees ensure uniformity in growth and a predictable lack of messy fruit, which is a selling point for urban forestry departments. The economic benefit of lowering long-term cleaning costs often outweighs other ecological considerations for city managers.
The Impact on Urban Air Quality
The unintended consequence of this preference for male trees is an increase in airborne pollen in urban environments. Since male trees are the sole producers of pollen and are overrepresented in city plantings, the urban forest becomes a substantial source of allergens. Unlike a naturally balanced forest, where female trees absorb a portion of the pollen for fertilization, a male-biased canopy releases almost all of its pollen directly into the air.
This imbalance contributes to the rising rates of seasonal allergies and respiratory issues in city populations. Studies have demonstrated a clear correlation between areas with high concentrations of wind-pollinated male trees and heightened pollen levels, exacerbating conditions like allergic asthma and hay fever. This concept, termed “botanical sexism,” highlights how landscape planning decisions can negatively affect public health.
Furthermore, air pollution, which is concentrated in cities, can interact with the increased pollen load. Pollutant particulates have been shown to attach to pollen grains, potentially causing them to fragment into smaller, more allergenic pieces. This combination of elevated pollen counts from male-only plantings and urban air pollution creates a more potent mix of allergens, intensifying the health impact on city residents. Cities are now exploring planting more diverse, low-allergy species to mitigate the decades-long consequences of this selective planting strategy.