Exploring the Different Ways Trees Are Pollinated

During the spring season of renewal, some trees reproduce with the help of wildlife, while others need only the wind to carry their pollen around the landscape. In this article, we’ll take a look at how trees of various species each form their particular type of seed. Knowing a tree’s behavior can help Nashville residents make decisions about planting locations and long-term yard maintenance requirements so that mature trees can be as happy and productive as possible within the built environment.

What Is Pollination?

Trees reproduce by transferring pollen (a plant’s genetic material) between male and female flowers. The pollen, once received by another flower of the same type of tree, then forms into a seed. Exactly how a tree is pollinated depends on the particular species, and there are two ways a tree can be naturally pollinated: by wildlife and by wind. 

Wildlife Pollination

Flowers, including those on trees, have special characteristics that attract insects, birds, and bats, which transport pollen by flying from flower to flower. The bright petals of a flower attract insects like bees, butterflies, moths, flies, and beetles, as well as larger animals like bats and hummingbirds, which feed on the pollen and/or nectar of a bloom. 

When pollinators engage with a flower, pollen particles (which are large and sticky) cling to the body of the animal, which then visits another nearby flower and leaves behind pollen particles from other blossoms. The plant provides the animal with food, while the animal helps the plant reproduce. 

Many of these relationships are mutually beneficial, where species evolve with dependence on one another. This is one reason why plant diversity is important in the landscape; unique features of pollinators (such as their size, form, and behavior) allow them to interact with certain types of flowers but not others. The presence of a particular flowering food source supports a certain animal species and vice versa.

For example, tulip poplars attract large bees with long tongues due to their deep, cup-shaped flowers. Hummingbirds and bats also visit this species, as well as catalpa and locust trees, both of which have cupped blossoms. Maple, redbud, dogwood, and cherry trees all have little open flowers that attract smaller insects, while the big open flowers of the southern magnolia are primarily pollinated by native beetles.

Wind Pollination 

Tree species whose pollen is spread by the wind don’t need to attract wildlife helpers, so their pollen production strategies are less showy. The flowers of wind-pollinated trees aren’t bright or colorful and may be too small to be appreciated by passersby. 

The pollen grains of these trees are smaller and more lightweight than wildlife-transported pollen so they can float through the air. These are also produced in larger numbers since they aren’t as efficient at matching to a recipient flower as wildlife-spread pollen. As the dust of pollen is cast into the air, the grains become the culprit of seasonal allergies to those with sensitivities (wildlife-spread pollen, on the other hand, is heavier and doesn’t hover in the air to irritate our sinuses). 

Many wind-pollinated trees have inconspicuous pollen clusters that are arranged on dangling green-to-brown catkins (long bunches of tiny flowers without petals). Catkins, found on oak, beech, and birch trees, release pollen that is caught by the small clusters of female flowers of their respective species. Ginkgo biloba trees have catkin-like pollen clusters that are spread by the wind and caught by small stalk-like flowers.

Similarly, conifer trees such as the pine, eastern red cedar, and arborvitae release their windborne pollen into the air to be received by a female cone. The female pine cone is hard, brown, and scaley, while eastern red cedar and arborvitae have tiny, berry-like cones.

Cross-Pollination

Cross-pollination refers to the process by which pollen from one type of plant is transferred to another type of plant. This is a common phenomenon that increases genetic diversity and benefits the broader ecosystem. Most apple trees, for instance, require wildlife-assisted cross-pollination to produce fruit, meaning they need to receive pollen from another apple tree specimen for their flowers to develop into seed-bearing fruit. 

As mentioned above, cross-pollination occurs naturally, but it can also be encouraged by human behavior via grafting and other means. Edible fruit trees are grafted (when a branch of one variety of tree is fused to the rootstock to conserve its fruit’s traits) to ensure that the tree produces its particular variety of fruit. This is because apple trees naturally produce seeds with a wide genetic variety. The seed of a given apple likely will produce a tree that grows a different type of apple, which may not be as big or tasty as edible varieties usually are. Oak trees, while not fruit-producing, have a similar requirement for cross-fertilization and a tendency to hybridize easily with other oaks.

Pollen and Seeds in the Nashville Landscape 

Whether by wind or by wildlife, pollination’s purpose is seed production. When planting a tree, the species’ reproductive behavior is important because it has certain implications for the built environment. The tree’s purpose (ornament, shade, privacy, or fruit) and location will factor into whether the tree has the potential to make a mess and whether the mess causes a problem.

Messy trees that drop a lot of material are usually best situated away from hard surfaces like driveways and sidewalks. Fruiting trees drop their produce and, if not cleaned up, can rot very quickly, get smashed around, and attract pests. Big oak trees drop acorns, which may need to be cleaned up if they are in a part of the yard that gets used regularly. People who get hay fever might avoid planting wind-pollinated species in their yards to decrease their immediate exposure at home, while residents with gardens may want to attract local pollinators with blossoming trees. 

Embrace Trees of All Types

All types of trees are important to contribute to a robust canopy ecosystem! Browse the Nashville Tree Conservation Corps’ tree sale for a range of tree species, both wind- and wildlife-pollinated. 

Consider volunteering with us or donating a tree for us to plant. NTCC is supported in large part by local financial donations, which allow us to do our part for Nashville’s canopy! Stay informed about our work by subscribing to our email newsletter and check out our Linktree for more ways to connect.