10 invasive plants that can also trigger allergies


10 invasive plants that can also trigger allergies

Every spring, many people think of allergies. Common allergens are plants or weeds known as noxious weeds. Federal, state and local authorities legally classify weeds or noxious plants as harmful to people, wildlife and agriculture. Many noxious plants or weeds are also considered invasive, meaning they are not native to the local ecosystem or even the United States.

Humans often spread invasive species, albeit unintentionally. For example, large ships can carry aquatic organisms in ballast water and small boats can carry them on their propellers. The seeds of plants used for ornamental purposes can spread, allowing them to grow in the wild.

From spring to fall, plants release tiny pollen grains that fertilize plants of the same species. Most flowering plants that spread their pollen by insects do not cause allergies. Weeds, grasses and trees are the main sources of pollen that can cause allergies. A pollen allergy is called allergic rhinitis, more commonly known as hay fever.

Wyndly compiled a list of 10 invasive species and described their impact on seasonal allergies, citing data from the US Department of Agriculture and the Invasive Species Atlas. Plants such as ragweed, which is native to the entire continental United States, are not included.


Dalmatian toadflax

– Scientific name: Linaria dalmatica
– Habitat of origin: Mediterranean region
– Introduced in the United States: late 1800s or early 1900s

Dalmatian toadflax and yellow toadflax grow along the roadside, in grasslands and among crops. Toadflax has an allergen rating of 4 out of 10, making it a moderate allergen. As they crop up in many commonly traveled places, these plants can make hay fever worse for passers-by.

Toadflax was originally imported into the United States for ornamental purposes, but it can also be used to dye fabrics. Yellow toadflax has also been used as a folk remedy for liver disease. Both species of toadflax are perennials and start growing early in the spring. Toadflax is very competitive with other plants for space and light and can spread quickly, so it gradually suppresses the growth of native plant species.



– Scientific name: Sorghum halepense
– Habitat of origin: Mediterranean region
– Introduced in the United States: early 1800s

Johnson’s grass is a moderate allergen and was originally introduced to the United States as a forage crop. Johnsongrass spreads quickly in different habitats including forests, fields and wetlands. It forms dense colonies, which can inhibit the growth of agricultural crops and prevent tree seedlings from taking root. It spreads rapidly at any time of the year, affecting allergies throughout the year.

Johnsongrass causes millions of dollars in lost farm income each year in the United States by invading crops. For example, it can reduce the yield of sugar cane fields in Louisiana by up to 50%. It can also serve as a host for insects, fungi, roundworms and viruses that damage crops.

kosmos111 // Shutterstock

Russian knapweed

– Scientific name: Rhaponticum repens
– Native habitat: Eurasia
– Introduced in the United States: early 1900s

Russian knapweed was accidentally introduced to the United States via contaminated alfalfa seeds from the Turkestan region of Central Asia. These plants are part of the daisy family, which makes anyone prone to daisy allergies also vulnerable to knapweed pollen.

Russian knapweed invades open land and crowds out native plant species, including animal fodder. This plant can quickly get out of hand, causing infestations that deteriorate the quality of the plants for grazing animals.

Grazing animals generally avoid eating Russian knapweed because of its bitter taste. If eaten in sufficient quantities by horses, Russian knapweed causes a neurological disorder called nigropaldal encephalomalacia, or chewing disease. Horses with chewing disease have difficulty eating, drinking and swallowing.

ioanna_alexa // Shutterstock


– Scientific name: Elymus repens
– Native habitat: Eurasia
– Introduced in the United States: 1600

Quackgrass grows in open, disturbed areas where natural vegetation has been removed to expose the underlying soil. This plant can quickly become a rhizome infestation, which involves plant roots invading the soil. Cutting the grassy tops is not enough; the root system of the plant must also be eradicated. It is commonly found in pastures, roadsides and gardens. It was introduced to the United States by accident, probably via contaminated seeds.

Quackgrass blooms in early summer, when it sheds pollen and contributes to hay fever. This plant competes with agricultural crops and native plant species. It forms dense brush that limits the regeneration of woody plants and trees. It also inhibits the restoration of cropland and pasture, reducing the availability of soil moisture and important nutrients.


Absinthe Absinthe

– Scientific name: Artemisia absinthium
– Habitat of origin: Europe, northern Asia and northern Africa
– Introduced in the United States: 1840s

Wormwood wormwood, also known as mugwort, was originally introduced to the United States for use as an antiseptic liniment. This plant begins to grow in late April and early May, blooming only in July or August. Those who suffer from spring and summer seasonal allergies may not be spared from absinthe wormwood, especially since this plant is perennial.

The leaves give food a sage flavor and are used to make vermouth. Like Russian knapweed, wormwood is toxic to horses. It also taints the milk of cows that have eaten it.



– Scientific name: Imperata cylindrica
– Habitat of origin: East Africa (uncertain)
– Introduced in the United States: Louisiana in 1912 and Florida in the 1930s

One of the world’s worst invasive species, cogongrass causes ecological and economic damage to forests, agriculture and natural ecosystems by infesting native wildlife. Cogon grass is wind pollinated, which means it can spread quickly because it is carried in the air we breathe. It blooms between March and May, in time for spring hay fever.

Cogongrass was accidentally introduced into the United States as a packing material for oranges shipped from Japan to Alabama in 1912. It has also been tested in Mississippi and Florida as possible fodder and for its erosion control potential .



– Scientific name: Albizia julibrissin
– Habitat of origin: from Iran to Japan
– Introduced in the United States: 1700

Mimosas are smaller trees, less than 50 feet tall, that often grow with multiple trunks. They can invade any disturbed area, such as stream banks and roadsides. Mimosas are difficult to remove due to the long-lived seeds that grow in long pods and sprout in late summer. Native to Asia, mimosas were introduced to the United States in the mid-1700s. The trees are widely used for ornamental purposes.

Bees and butterflies love mimosas, spreading their pollen from plant to plant. This helps the plant spread and can also release allergens into the air and onto other insect-pollinated plants.

rwtrahul // Shutterstock

Palmer’s amaranth

– Scientific name: Amaranthus palmeri S.Watson
– Native habitat: southwestern United States
– Introduced in the United States: reported in Virginia in 1915

Palmer’s pigweed is a serious allergen and is one of the most widespread and economically devastating weeds in the Southwestern United States. The plant’s pollen is spread by the wind, which can make allergies worse. Palmer’s pigweed likely spreads on soil through contaminated seeds: recent infestations in the Midwest have been traced to contaminated seeding mixes from the conservation reserve program.

Palmer amaranth thrives in hot weather and tolerates drought well, making it more resistant to extreme weather conditions than most native plants. It affects corn, cotton and soybean plants. Some populations of Palmer’s amaranth are even resistant to herbicides including glyphosate.


giant reed

– Scientific name: Arundo donax
– Native habitat: probably East Asia
– Introduced in the United States: early 1800s

A prolific pollen producer, the giant reed is a moderate allergen and has been introduced worldwide for erosion control, ornamental use, and reed production for paper, pulp, and musical instruments . It grows in dense stands, with many plants per acre, replacing native vegetation. This displacement of vegetation leads to changes in food resources and the infestation of giant reeds reduces wildlife habitats, especially those of amphibians and birds.

The giant reed is a member of the grass family, generally considered a major source of allergies, and releases its pollen between July and October.

Bew Nantaphong // Shutterstock


– Scientific name: Alternanthera philoxeroides
– Native habitat: South America
– Introduced in the United States: reported in Alabama in 1897

Alligatorweed is a mild allergen that grows thickly in shallow, slow-moving water, ditches, marshes and ponds. These mats inhibit the growth of native aquatic vegetation, slow or prevent water flow, increase sedimentation levels and inhibit water sports including boating, fishing and swimming.

Plants release pollen all year round and live for years. Alligatorweed was introduced to the United States from its native South America in ballast water, which is kept in ships’ holds to provide stability and improve maneuverability at sea.

This story originally appeared on Wyndly
and was produced and distributed in partnership with Stacker Studio.

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