Walnut pollen allergy (Juglans) – a quick guide w/ photos.


a close up of a walnut tree showing compound green leaves and upper trunk.

In this article, with the help of photos, we will learn about walnut pollen allergy (Genus: Juglans, Family: Juglandaceae). In addition, we will also learn how and when their pollen spreads.

Walnut pollen is routinely used in most allergy test panels in the US. Therefore, it is an important allergen.

Walnut pollen allergy facts and figures (Juglans)

Walnut tree (Juglans)Allergy profile
Pollen seasonSpring
Pollination typeWind-transported; releases abundant pollen in the air
GenderMonoecious: Separate male and female flowers appear on the same tree.
Cross-reactivities with other pollenPecan and Hickory
Pollen sourceGreen, loosely hanging male catkins (see picture below)
Tree leavesEnglish Walnut: Large compound leaf with 5 to 7 leaflets. Each leaflet can be 6 to 18 inches long. Black walnut can have 15-17 leaflets.
Tree fruitA round green nut, which turns brown on maturation. Not all trees produce fruit.

Walnut leaves

The leaves are compound and generally have an odd number of leaves. English walnut has 5 to 7 leaflets, the largest of which can be 18 inches long. On the other hand, California black walnut has 9 to 15 leaflets, but are closer to 10 inches in length.

The leaflets are generally ovate, longer than they are wide.

On the left a compound green leaf with 17 leaflets, next to it is a thin six inches long green male catkin of California black walnut; On the right a larger compound leaf with only 3 leaflets of English Walnut
Leaves and catkins of Walnut (Juglans)

When do walnut trees release pollen?

Walnut trees release pollen during spring in the San Francisco Bay Area, generally, during the month of April.

There are only a small number of walnut trees in the Bay Area. For example, in the San Francisco city out of approx. 180,000 street trees, only 33 are walnut trees. In comparison, the city has 1200 oak trees.

Despite their small population, walnut trees are capable of producing large pollen loads. As a result, I still catch plenty of walnut pollen during spring air sampling.

The pollen is released by male flowers, which are loosely hanging green and yellow catkins. Each mature catkin is about 4 to 6 inches long.

If you see some of these catkins on the tree and some strewn on the ground under the tree, it is a sign that the catkins are mature and releasing pollen. If you have allergies to walnut pollen, it would serve you well to keep an eye on your local trees’ pollination cycle.

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

three green slender beads like young catkins of black walnut tree
Black walnut catkins – Still not mature enough to release pollen. [Juglans nigra]
a narrow long green catkin of walnut. It is about six inches long and just a quarter inch in diameter at its thickest point. This is the source of walnut pollen.
English walnut catkin on the ground under the tree (Juglans regia)
About a dozen compound green leaves with 15-17 leaflets each and about a dozen very thin and 6 inches long green beady male catkinsnches long
California black walnut in spring bloom [Juglans hindsii]

What does walnut pollen look like?

Walnut pollen has several protruding pores, but only on one side of its spherical shape. The pollen is 35 to 40 microns in size. Heteropolar, periporate, annulate and aspidate pores.

two views of walnut pollen, which shows about eight aspidate and annulate pores.
Walnut pollen at ~400x magnification. Equatorial and polar views.

To see the pollen of other plants and trees, visit our pollen library.

What do walnut trees look like?

The walnut trees are generally small to medium-sized, ranging from 15 to 30 feet. Their large compound leaves distinguish them from most of the common trees of the San Francisco Bay Area.

The English walnut bark is silvery gray with ridges.

A green round canopy with a silvery gray trunk that branches out in two.
English walnut tree (Juglans regia)
A tree with dark brown, deeply ridged, stout trunk that rises straight for 8 feet before branching out into a thin, spread out canopy.
Juglans nigra – A Black walnut tree in early spring bloom. It will grow lusher in a few weeks from this stage.
A silvery gray bark on a two feet wide tree trunk that branches into three trunks at about 5 feet height
English walnut tree trunk (Juglans regia)
A dark brown stout trunk that is at least 3 feet wide. It branches out at 5 feet.
Juglans nigra: The trunk of a black walnut tree.

Key takeaway

Walnut tree pollen of the genus Juglans belongs to the family – Juglandaceae. Cross-reactivity is common in this family. As a result, people who have allergies to walnuts could potentially react to the pollen of hickory and pecan.

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

Sources

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

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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