Posted by: Lisa Mason, Arapahoe County Extension
“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
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.
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.
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.
|A bumble bee visiting a linden tree. Photo: Lisa Mason|
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.
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|
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|
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 trees 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
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 more
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 a variety of species 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?