Trees produce pollen as a fundamental part of their reproductive cycle. Pollen consists of tiny grains containing the male reproductive cells of a plant, essential for tree reproduction.
The Purpose of Pollen
Pollen serves a singular biological purpose: to facilitate reproduction in trees and other seed-bearing plants. Each microscopic pollen grain holds the male genetic material necessary for fertilization. Since trees are rooted in place and cannot move, pollen acts as the vehicle for transferring this genetic information between individuals. This process, known as pollination, ensures the mixing of genes, which contributes to genetic diversity within tree populations.
How Trees Produce Pollen
Trees generate pollen within specialized male reproductive structures. In flowering trees, pollen develops in the anthers of flowers, while conifers produce pollen within small pollen cones. Once mature, these structures release the pollen grains into the environment.
Trees employ two primary strategies for dispersing this pollen: wind and animal (including insect) pollination. Wind-pollinated trees release vast quantities of lightweight, dry pollen designed to travel long distances through the air. In contrast, insect or animal-pollinated trees produce smaller amounts of pollen that is often heavier and stickier, designed to cling to pollinators. The timing of pollen release is influenced by factors like temperature, humidity, and the tree’s internal biological clock.
Common Pollen-Producing Trees
Trees vary significantly in the amount and type of pollen they release. Wind-pollinated trees are the primary source of airborne pollen that triggers allergies, producing immense quantities of light pollen for successful dispersal. Common examples include oaks, birches, maples, elms, ashes, and cedars (often referred to as junipers). Pines also produce large, visible pollen, which is less allergenic than many other wind-pollinated species.
Conversely, insect-pollinated trees have showy, colorful, or fragrant flowers designed to attract animals. These trees, such as apples, cherries, dogwoods, and basswood, produce less airborne pollen because their heavier, stickier grains are directly transferred by pollinators. Female trees of species where male and female reproductive parts are on separate plants (dioecious trees) do not produce pollen. Selecting female varieties of trees like certain hollies or mulberries can help reduce local pollen exposure.
Tree Pollen and Human Health
When tree pollen enters the human respiratory system, it can trigger an immune response in sensitive individuals. The immune system mistakenly identifies pollen as a harmful invader and releases chemicals, including histamines, to combat it. This reaction leads to symptoms of allergic rhinitis, commonly known as hay fever.
Common symptoms include sneezing, a runny or stuffy nose, itchy eyes, nose, or throat. Some individuals may also experience watery eyes, coughing, or worsened asthma symptoms like wheezing. Tree pollen season begins in early spring, usually from February to April, but can start as early as December or January in warmer climates. Monitoring local pollen forecasts can help individuals manage their exposure during peak pollen times.