American sweetgum allergy – a complete guide w/ photos.


Red and green leaves and spikey fruit of sweetgum tree

In this article, we will discuss the Ameican sweetgum allergy (Liquidambar styraciflua) and how and when this tree’s pollen spreads.

American sweetgum (Liquidambar styraciflua)Pollen allergy profile
Pollen seasonSpring. Generally, Mar-15th to Apr-15th.
Pollination typeWind transported. Each tree releases a lot of pollen into the air.
Cross-reactivity with other pollenOther trees of genus Liquidambar but American sweetgum is the only tree commonly found in the US. The other species, e.g. Chinese sweetgum, are mostly found in Asia.
Pollen sourceGreen male catkins (flowers) that grow in the shape of an erect conical inflorescence (see pictures below.)
GenderMonoecious: The tree has separate male and female flowers on the same tree.
FruitSpikey green, golf-ball-sized fruit that eventually turns very dark brown (see pictures below.)
BarkThe bark is grey or dark brown with deep vertical or slanted ridges (see pictures below.)
LeavesStar-shaped generally with five lobes, but three or seven lobes possible (see pictures below.)

How to identify American sweetgum (Liquidambar styraciflua) trees?

American sweetgum, also known as redgum or star-leaved gum, is widely grown as a street tree in California and pretty much in all states east of Texas[1]. The tree is native to the United States and is particularly beautiful in autumn when leaves turn yellow, red, or purple.

A sweetgum tree, or Liquidambar styraciflua, can be easily identified by its spiky golf-sized fruit and five-lobed star-shaped leaves. The young fruit is green in color during spring but changes to dark brown by winter.

Sweetgum is a deciduous tree and it loses its leaves in autumn but the spikey fruit remains on the tree through winter.

Fruit

Since the American sweetgum keeps its fruit on the branches year-round, it is a good tree identification marker.

The American sweetgum tree’s fruit is the of a golf ball and is covered with pointed spikes. It is green in color when young but it turns dark brown upon maturation.

During winter, the tree loses its leaves but the dark brown spikey fruit remains on the tree.

Leaves

The star-shaped, serrated leaves of the American sweetgum tree generally have five lobes but three or seven lobes are possible. Each leaf blade can be 3 to 8 inches long.

The leaves are green during spring and summer and change to yellow, red, and purple during autumn.

The sweetgum leaves can be confused with maple’s, but, the fruits of the two trees are very different so if you are not sure about leaves, use fruit for the identification (discussed in the next section)

bark and trunk.

The American sweetgum tree bark has deep verticle or slightly slant ridges. The color of the bark ranges from gray to dark brown.

A single trunk generally rises straight up 8 to 10 feet before branching out. The trunk is 1 to 4 feet wide.

Gray tree trunk with ridges
Sweetgum bark and trunk – gray.
dark brown bark with ridges of sweetgum
American sweetgum bark and trunk – dark brown.

When do the American sweetgum trees release pollen?

American sweetgum releases pollen during early spring. In California, it is among the first trees to release pollen in March. In cooler climates of the East Coast, the tree releases pollen in April.

The tree releases most of its pollen from mid-morning to early afternoon. Our air sampler in Burlingame, California catches most of the pollen between 11:00 am and 2:00 pm between March-15th and April-15th.

The tree has a relatively short pollen release cycle of three to four weeks.

The pollen schedule for the American sweetgum can vary by one to two weeks each year depending on the length and severity of the preceding winter. If you live in the San Francisco Bay Area and suffer from sweetgum allergies, we do regular air sampling and tree inspections to provide you with reliable pollen updates.

How does American sweetgum pollen spread?

Sweetgum allergies are caused but it’s wind-transported pollen. During spring, the tree releases a copious amount of pollen in the air and wind can carry the pollen many miles.

The tree generally has a very short pollination cycle of three to four weeks, but it does release large amounts of pollen in that short period.

Sycamore (Platanus) tree also releases pollen pretty much at the same schedule as sweetgum.

What does American sweetgum pollen look like?

The male flowers fall on the ground upon maturation and you can see pollen in the form of a fine yellow powder spilling out of it. However, the individual grains of pollen are invisible to the eye when they are airborne.

The pollen grain is spherical, about 40 microns in diameter (0.04 mm), and has 18-20 pores. Liquidambar styraciflua pollen: Periporate with freckles on the pores.

a dark pink stained pollen with several pores
American sweetgum pollen at 400X magnification.

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

How big are sweetgum trees?

An American sweetgum tree can grow to 75 feet tall and 45 feet wide but most street trees are trimmed and do not grow past 50 feet tall.

A green tall tree
American sweetgum in summer (60 feet tall)
green, yellow, and red leaves on sweetgum tree
American sweetgum in autumn (25 feet tall)

What do American sweetgum flowers look like?

The American sweetgum pollen is produced by its male flowers (catkins). Fortunately, these flowers have a very short life – they start developing in winter, release pollen in March and April and then fall off the tree by the end of May.

Each conical-shaped inflorescence is about one inch long and contains about 10 small ball-shaped green flowers.

The flowers are easy to spot in spring because they appear before the new leaves. The tree at this stage only has old fruit and new green catkins.

The female flower is a small ball-shaped flower that hangs loose on the branches. When successfully pollinated, it turns into the spikey round fruit that stays on the tree for over a year before it falls off[2].

A one inch long male catkin with about a dozen tightly packed round balls of anthers
The source of pollen: American sweetgum male flower.
a round spiky green flower hanging on a tree
American sweetgum female flower.

How to know if an American sweetgum tree is releasing pollen?

A bare looking tree without any leaves, but has some specks of green catkins and mature black fruit visible.
American sweetgum tree in bloom. The pollen releasing male catkins appear on the tree before leaves.

If you suffer from sweetgum allergies, you can learn about its pollination cycle to know when to take precautions. When the male catkins are green and glossy during late winter, they are still not mature enough to release pollen. However, when they turn a little yellowish-brown and powdery, they are mature enough to be releasing pollen.

Each catkin is capable of releasing millions of pollen into the air!

If you see some catkins on the tree and some under the tree with yellow dust around them, it means the tree is releasing pollen in large quantities.

A mature cakin has yellow brown color and it is now a loosely packed set of balls made of anther and is almost two inches long.
A mature male flower (catkin) of American sweetgum on the ground. Each catkin is capable of releasing over one million pollen.

Once the pollination has started, all-male catkins eventually fall off the tree in three to four weeks. However, the pollen from catkins on the ground can still become airborne when disturbed by leaf blowers or street cleaners.

If you are allergic to the sweetgum pollen, take precautions until the catkins from the ground disappear either by cleaners or rains.

You can learn a lot more about pollen allergies and how to avoid them in my pollen allergy guide.

Do all American sweetgum trees release pollen?

During the early spring, when the American sweetgum trees blossom, each tree will have both male and female flowers on the same tree. The male flower is responsible for releasing pollen. This means all healthy American sweetgum trees will produce pollen if they are mature enough to do so.

Luckily the sweetgum tree has a short pollen release cycle of three to four weeks. It starts in mid-March and concludes by mid-April.

The summary of American sweetgum pollen allergy.

Although American sweetgum allergies are not common, the tree releases large quantities of pollen in the air during spring. However, some clinics and pollen forecasting websites ignore this pollen. Thus, the allergenicity of this pollen is not as well established as some other spring pollinating trees like birch (Betula), Sycamore (Platanus), Oak (Quercus), Mulberry (Morus), Walnut (Juglans), Alder (Alnus), Ash (Fraxinus), and Hackberry (Celtis).

Sources
  1. https://plants.usda.gov/DocumentLibrary/plantguide/pdf/pg_list2.pdf
  2. https://naturewalk.yale.edu/trees/altingiaceae/liquidambar-styraciflua/american-sweetgum-46
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 identifying pollens and molds by E. Grant Smith

We study trees and their pollen by doing regular tree inspections and air sampling. 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.

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