Poplar, Cottonwood, Aspen (Populus) – a quick guide w/ photos.


A row of yellow green, tall, columnar trees

In this article, with the help of photos, we will learn about poplar tree allergy. In addition, we will also learn how and when their pollen spreads.

The poplar trees (genus: Populus), are closely related to the willow (genus: Salix) and belong to the same family Salicaceae. The species within the genus Populus have common names of poplar, aspen, and cottonwood.

Poplar tree allergy facts and figures (Populus)

Poplar tree allergy profilePopulus
Pollen seasonFeb, Mar
Pollination typeWind-transported; releases abundant pollen in the air
GenderDioecious: Male and female flowers are on separate trees. Only male trees release pollen.
Cross-reactivities with other pollenCross-reactivity within the genus and family – Poplar, aspen, cottonwood, and willow.
Pollen sourceMale catkins that appear in winter before the leaves (See pictures below)
Tree leavesTriangular with round edges, 2 to 4 inches long. Green in summer, yellow in autumn.
Tree shapeColumn-like tall and straight.

How to know if a poplar tree is releasing pollen?

The pollen is released only by male trees, which bloom during winter. The male catkins start to appear during winter before leaves. In fact, a pollen-releasing tree looks pretty bald because it only has dark-colored catkins, but no leaves.

Following is a series of photos of a poplar tree that is releasing pollen. The pictures were taken during the first week of March.

A group of bare trees, which give only appearance of closely packed branches in column-like shape.
Lombardy poplars [Populus nigra var. Italica] – This is how they look when producing pollen.
A closer look of the poplar branches show buds of catkins, which are inconspicuous from the distance
A closer look at poplar tree branches, when it is releasing pollen. Notice that leaves are absent.
Green catkins emerging from red buds are the source of poplar pollen allergy
A close-up of male poplar catkin, which is responsible for producing pollen.

These catkins are the source of pollen. They stay on the tree for four to six weeks and then fall off. Each tree produces pollen for about six weeks each year.

When do poplar trees release pollen?

Poplars and Cottonwood trees release pollen during the months of February and March in the San Francisco Bay Area.

Black and Lombardy poplars are the most common poplar trees in the Bay Area. Although there are a few Fremont cottonwood trees in the area as well, I have yet to observe any aspen trees locally.

Weather conditions can shift the pollen season by a few weeks, each year. This is why it is best to observe the neighborhood trees during winter when the catkins have started to appear.

Even though there are only a small number of poplar trees in the Bay Area, each individual tree produces a large amount of pollen. Therefore, poplars overall produce a moderate pollen count during late winter and early spring.

Depending on the amount of rainfall and weather, the pollen season could differ slightly from one year to the next. This is why it is important to learn about allergy plants and their pollen.

However, if you live in the San Francisco Bay Area, you have an easier way out! I do regular tree inspections and air sampling in the area to provide reliable pollen updates on our website

What does the poplar tree pollen look like?

Poplar tree pollen does not have any pores or furrows. It is round about 30 to 40 microns, with a slightly thickened intine. The exine is flaky and looks like a peanut shell.

a round fuchsin stained pollen with a darker outer ring. This is what causes poplar pollen allergy when inhaled.
Poplar pollen [Populus nigra]. ~400x magnification.

What do poplar trees look like?

Leaves

The trees have green triangular leaves that turn yellow during autumn. The tall and narrow canopy of the poplar is lush and full during summer and fall but becomes bare during winter.

two poplar leaves green, triangular with round edges. One leaf is about 2 inches long and the other about 4 inches
Poplar leaves during summer.
A yellow leaf of poplar
Poplar leaf fallen of the tree during autumn.
A triangular looking green leaf with soft edges and visible veins
Fremont cottonwood leaf [Populus fremontii]

Tree shape and appearance

The poplar trees have a unique tall, upright, and narrow canopy. These are the tallest deciduous trees in San Francisco Bay Area and their bald canopies are easy to spot during winter.

The cottonwood trees are not as columnar as poplar trees and their branches spread out a little more.

Two closely growing, 50 feet tall poplar trees with yellow green narrow upright canopy.
Black poplar (Populus nigra) during early autumn. Leaves are turning from green to yellow.
a bare upright tree with no leaves
Poplar during winter after all leaves are gone.
Lombardy poplar in bloom.  This is when it is most allergenic. There are no leaves but it has male catkins that produces pollen.
A poplar tree in late winter, early spring. This is when it is most allergenic because it is loaded with male flowers.
A lush green tree with a canopy of medium spread and grey ridged trunk which is three feet wide.
Fremont cottonwood tree [Populus fremontii]

Flowers or Catkins

Poplar flowers are thin, 1 to 3 inches long catkins, which are a mix of red and green in color. Mature catkins often turn purple before falling on the ground.

A thin, 3 inches long catkin growing out of red flower buds of poplar.
Lombardy poplar catkin.

Bark and trunk

The tree bark of the mature tree is grey with irregular ridges. Usually, a single trunk rises from the ground, and branches out anywhere at the height of 1 to 10 feet.

Poplar allergy summary

Generally speaking, poplar, cottonwood, and aspen pollen is present in the air during late winter and early spring all across the US. The clinicians usually test for allergies only to one of these species and ask patients to be vigilant about cross-reactivities to other trees among the genus Populus.

To understand cross-reactivities better, read my pollen allergy guide.

Sources

  1. https://plants.usda.gov/
  2. http://dendro.cnre.vt.edu/dendrology/factsheets.cfm

References

  1. Allergy Plants by Mary Jelks, M.D.
  2. Plant identification terminology by James G. Harris and Melinda Woolf Harris (Second Edition)
  3. Sampling and indentifying pollens and Molds by E. Grant Smith
  4. The trees of golden gate park and San Francisco by Elizabeth McClintock PhD.

All pictures, unless otherwise credited to another source, are taken by the author and are copyrighted material. The pollen picture is taken in our aerobiology lab using an Olympus compound microscope. The use of pictures is permitted with a link back to the source page on the internet, or, an attribution to allerma.com on the printed material.

Sudhir Setia

Sudhir is certified by the National Allergy Bureau (NAB) as a pollen counter and identifier. He has been living with Hay Fever for nearly 30 years and studies allergens at his aerobiology lab.

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