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From the Hort Desk

Nothing to Sneeze at: Tree Pollination and How it Impacts You

By Lisa Mason, CSU Extension Horticulture Agent

“Is it true that cities and towns plant only male trees, and male trees tend to cause more allergies for people?” The answer to this question is far more complex than just choosing to plant male or female trees, so let’s explore tree pollination and diversity in the landscape.

This question stems from a theory that female trees produce seeds and thus require more cleanup and maintenance, while male trees require less maintenance because they only produce pollen. However, that pollen can trigger allergies.

Note that tree pollen is only one of the sources of allergens in the air. According to the Asthma and Allergy Foundation of America, grasses are the most common cause of allergies, and ragweed is the top contributor to weed allergies. Other allergy triggers include other weeds, pet dander, dust, and mold. Wildfire smoke has also contributed to poor air quality across the west aggravating allergy symptoms and other health conditions.

Understanding Pollination and Tree Reproduction

Trees produce pollen as part of their reproductive process. To understand tree reproduction, look inside a flower such as a tulip or lily. You can easily see the male and female reproductive organs in the flower.

Pollen is produced by a male reproductive organ called the stamen. The stamen includes the anthers and filaments (see photo below). The female reproductive organ on a flower is called the pistil, which comprises of the ovary, style, and stigma. For trees to reproduce, pollen is transferred from the stamen (male organ) to the pistil (female) organ. These pollination biology concepts apply to other plants, but this article will focus on trees.

Anatomy of a “perfect flower.” Credit: University of Missouri Extension

Wind-Pollinated or Insect-Pollinated?

Pollen is transferred between trees in a variety of ways to facilitate tree reproduction including wind-pollination or insect-pollination.

Wind-pollinated trees produce mass amounts of pollen to increase their chances of the wind carrying the pollen to the appropriate tree to reproduce. Examples of wind-pollinated plants include conifer trees (pines, spruces, firs, etc.), aspens, cottonwoods, ash trees, elms, all grasses, etc. Wind-pollinated plants tend to cause more allergies due to the amount of pollen in the air.

Insect-pollinated plants have a more efficient process of pollination because they rely on insects to transfer the pollen from plant to plant. Pollen grains are generally larger and often are sticky, allowing the insects to intentionally and unintentionally carry pollen on their bodies. Bees are among the most efficient pollinators because they have hairs all over their body that easily carry pollen. Other pollinating insects include flies, beetles, butterflies, and some wasp species. Hummingbirds are also pollinators in Colorado, and around the world other animals such as birds, reptiles, and bats and other mammals pollinate plants. Examples of insect-pollinated trees include lindens, apples, crabapples, cherries, northern catalpas, Ohio buckeyes, Kentucky coffee tree, eastern redbud, etc. Insect-pollinated trees generally do not cause allergies. The flowers on these plants are big, showy, fragrant, contain pollen and nectar, and are visually attractive.

How do Male and Female Trees Play a Role?

To add more complexity, the biological processes vary among trees. Trees are either: 1) monecious, or 2) dioecious.

  • Monecious – trees have male and female parts on the SAME TREE.
    • Perfect/complete flowers – trees have BOTH male and female parts on the SAME FLOWER.
      • Since both male and female parts are on the same flower, the pollen stays near the flowers, and these trees are unlikely to cause allergies. Apple trees have “perfect flowers.” Many of these trees are insect-pollinated or they will self-pollinate. Some “perfect flowers” may also be “incomplete flowers” if they have both reproductive parts, but they lack petals or other flower parts. Elm trees have “perfect flowers,” but lack petals.
    • Incomplete flowers – trees have male flowers AND female flowers on the SAME TREE.
      • Oak trees and black walnut trees have male and female flowers on the same tree. Plants can be wind- or insect-pollinated.
    • Polygamous – trees that have a combination of “perfect flowers” and male and/or female flowers on the plant. Ohio buckeye are polygamous because they have “perfect flowers” and they have male flowers.
  • Dioecious – trees have either male OR female flowers, but NOT BOTH.
    • The male plants produce pollen, and seeds are produced on the female plants after they are pollinated. Dioecious trees are typically wind-pollinated. Male trees tend to cause allergies. Female plants do NOT have pollen. Ash, willows, aspen, and some maples are examples of dioecious trees.
A complete flower cluster known as an inflorescence on an Ohio buckeye tree. Photo: Lisa Mason

Let’s Talk Cottonwoods

Cottonwood trees receive a lot of attention in the spring and summer when cotton is flying in the air. They are an example of a dioecious, wind-pollinated tree. Cottonwoods have male and female trees. The male trees produce pollen around April but not cotton. After pollination, female trees produce capsules full of cotton seeds. The capsules open around June, and the wind will carry the cotton to spread the seeds. A common misconception is that cotton causes allergies; the pollen that is released much earlier in the season is what can cause allergies. When the cotton is flying, the cause of allergies is typically grasses, weeds, and other trees.

A male catkin on a cottonwood tree. A catkin is the flowering spike or flower cluster without petals. Photo: Lisa Mason

How are Tree Species Chosen for the Landscape?

Deciding what trees to plant in a landscape depends on many factors including the purpose and goals of the landscape. In terms of the choosing tree species in a home landscape or municipal landscape, all of the following should be considered:

  • Mature size of the tree,
  • Soil, water, and other growing requirements,
  • Management considerations like pruning and susceptibility to insects or diseases, and
  • Weather and climate adaptations (e.g., exposure to cold, heat).

Choosing where to plant the tree is just as important as choosing the species of tree. Nearby infrastructure like power lines and buildings, space for the tree to grow to its mature size, competition from turf or other plants, potential for exposure to de-icing salts in the winter, and potential damage to the trees from human activity, lawn mowers, etc. are all additional considerations.

Different species of trees can achieve different goals. Some trees are more suitable for wildlife habitat or forage for pollinators. Other trees might provide shade or privacy around a home or building. If allergies are a concern, you might select trees that are insect-pollinated versus wind-pollinated. Seedpods on trees can also provide winter interest. If you prefer less maintenance in your landscape, a pollen-producing tree might be more suitable.

Another critical consideration for tree selection is DIVERSITY in the landscape. We’ve learned from Dutch elm disease, chestnut blight, and emerald ash borer that when we plant many trees of one species, we risk losing all those trees to insects or diseases. By planting a diversity of species in the landscape—no more than 10% of all trees should be of one species—we create a more resilient landscape.

Even though Colorado can be a tough place for trees to grow, we have varieties that do very well. Consult the Front Range Tree Recommendation List to learn more. If you are considering adding a tree to your landscape, do your research to make sure the tree meets your goals. In addition to finding the right place for the tree, make sure the tree is planted and cared for correctly to give the tree the best chance to survive and thrive.

If you need some ideas or inspiration, the next time you are at your local park or public area, take a close look at the trees. What species do you see? Are they wind- or insect-pollinated? Monecious or dioecious? Do you notice a diversity of trees in the landscape?

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