Birch tree allergy (Betula) – a complete guide w/ photos.


green leaves with saw tooth edges with white and grey bark in the background

In this article, we will discuss birch tree allergies and how and when their pollen spreads.

Birch tree allergy facts and figures

Birch tree (Genus: Betula)Pollen allergy profile
Pollen seasonSpring
Pollination typePrimarily wind transported. Trees release abundant pollen in the air.
Cross-reactivities with other pollenAlder, Hazelnut, Hornbeam (the member of Betulaceae family)
Pollen sourceGreen, thin, cylindrical male catkins that turn yellow and brown upon maturation.
GenderMonoecious: Each tree has both male and female flowers (catkins).
BarkEuropean birch: Silvery white mixed with dark brown or grey bark. River birch: Very flaky, peeling brown, beige, with reddish hues (see pictures below)
FruitThe female catkins when fertilized by pollen mature into very dark brown seedbeds.

How to identify a birch tree (Betula)?

In this article, we will cover three different species of birch trees. By association, you will be able to recognize other species and varieties.

Generally, the Himalayan, European white, Japanese, and Paper birches, all have some form of white barks. In contrast, the River birch has a flaky brown/beige bark. The trees generally have long limp twigs that hang loosely, which have serrated almost triangular leaves with saw-tooth edges.

Birch is a deciduous tree that changes color in autumn before losing its leaves. In this article, first, we will show you how to identify birch tree’s bark, leaves, flowers, and fruit. Then, we will discuss its pollen and how it spreads.

Bark and Trunk

The European white birch (Betula pendula), also known as silver birch always has a silver-white color in its bark. Depending on the variety, it is either smooth, spotty with dark grey, or flaky.

The Himalayan birch (Betula utilis) has mostly white birch.

River birch (Betula nigra) has beige and brown bark, which peels off to show reddish hues on the other side.

white and grey tree trunk
European white birch: Betula pendula
Silver white tree trunk
Himalayan birch: Betula utilis
white and grey tree trunk
Birch species unknown : Betula sp.
three white and grey tree trunks
European white birch with multiple trunks: Betula pendula.
flaky brown and beige tree trunk
River birch: Betula nigra

Leaves

Small leaves arrange on the twig in an alternate fashion. The twigs hang loosely on the tree giving the tree a weeping canopy.

The birch leaves are dark green or yellow-green during summer with serrated (saw-toothed) edges. The leaves turn yellow, or orange in autumn. The leaves are triangular or ovate in shape generally slightly longer than they are wide.

Birch leaves
Birch leaves of three different species from the same neighborhood of California.

Catkins (flowers).

Birch trees have male and female flowers called catkins on the same tree.

The male flowers are shiny green when young and turn yellow-brown when mature. In most species, the catkins take six to eight months of maturation before they release pollen in spring[1].

The female flowers are lighter green and fatter compared to their male counterparts. They turn brown in summer and produce seeds. If not eaten by birds or insects, they turn black and remain on trees for the whole year before falling off.

male and female catkins around one inch long
Birch flowers
male and female birch flower on the same tree
Birch: male and female catkins on the same tree.
a mix of dried-out and old brown-black female catkins and some new green catkins on the same birch tree.
A birch tree with both one year old (circled in red) seedbeds and new catkins.

Trees

Some species and varieties of birch trees grow only 10 feet and some can grow as tall as 60 feet. The trees are generally much taller than they are wide.

Himalayan birch
Himalayan birch: 15 feet tall
20 feet tall River birch
River birch: 20 feet tall
60 foot tall European white birch
European white birch: 60 feet tall.

Where do birch trees grow in the US?

There are dozens of birch species that grow across the US. The two species – European white birch (Betula pendula) and River birch (Betula nigra) pretty much cover all 50 US states[2].

Paper birch and water birch are the other native trees that grow across the US.

How does birch pollen spread?

Birch tree releases its pollen in the air during spring and wind can carry its pollen many miles. The birch tree is very common in the US, so it is likely that you have one nearby.

Every single catkin on the tree is capable of releasing up to six million pollen and each tree is capable of releasing up to 5 billion pollen[R3].

What does birch pollen look like?

Birch pollen looks like a fine yellow powder when collected in a Petri dish. However, each individual grain is invisible to the eye when it’s airborne.

Birch pollen grain is about 20 microns (0.02mm) in size and has three protruding pores with dark circles around them. Betula pollen: Triporate with annulate and aspidate pores.

a round, fuchsin stained, grain with three protruding pores with dark circles around the pores.
Betula: Birch tree pollen at ~400x magnification.

When do birch trees bloom and release pollen?

The birch trees allergy season is spring. In California, they release pollen during March and April. In Calgary, Canada, which has a much colder climate, the trees release pollen during April and May. Therefore, it is fair to assume that the colder East Coast of the US also experiences peak birch pollen season during April and May.

Each birch tree is capable of releasing up to five billion pollen in a year[R3]. However, birch trees do not release the same amount of pollen every year. Some years they release a lot, and in others, they release very little. Our air sampling in Burlingame, California has confirmed this finding.

The pollen schedule for birch can vary by one to two weeks each year depending on the weather and rainfall. For accurate weekly pollen reports, please check out our pollen count page.

How do you know if a birch tree is releasing pollen?

All birch trees have separate male and female flowers on the same tree (monoecious). The mature male flower (catkin) is responsible for producing and releasing pollen and causing birch tree allergies.

The male flower development starts in summer and it matures through winter before releasing pollen in spring. Just before releasing the pollen, the male catkins start showing yellow and brown hues.

If you see yellow-green catkins hanging on a birch tree during spring, it means the tree is releasing pollen. The catkin eventually turns brown and falls to the ground.

The pollen from the catkins on the ground can also become airborne if they are disturbed by wind, humans, or machines like leaf blowers.

young green male catkins on birch tree
Young green male birch catkins
three brown yellow mature catkins hanging from a twigof birch flanked by one green leaf.
Mature birch catkins that are releasing pollen.

Final thoughts

Birch tree allergies are common in the US as well as in Europe. It belongs to the Betulaceae family, which also includes another allergen, the Alder tree. It is common, as is the case for the author, if you react to the pollen of one tree, you react to the other as well[4].

Don’t forget to read about other trees like oak (Quercus), redwood (Sequoia), hackberry (Celtis), sycamore (Platanus), and sweetgum (Liquidambar styraciflua) that bloom alongside birch trees during spring.

Sources

  1. https://www.srs.fs.usda.gov/pubs/misc/ag_654/volume_2/betula/nigra.htm
  2. https://plants.usda.gov/
  3. Molecular analysis confirms the long-distance transport of Juniperus ashei pollen (nih.gov)
  4. https://pubmed.ncbi.nlm.nih.gov/32040996/

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

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.

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.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Recent Posts

Pin It on Pinterest

Share This