Pigweed pollen allergy
Amaranthus
Pigweeds are weedy amaranths that shed wind-borne pollen from summer into fall. Their pollen is reported to cross-react with that of lamb’s quarters and Russian thistle.
What it is
Pigweeds are weedy species of Amaranthus, the amaranth genus. Redroot pigweed (Amaranthus retroflexus) is a typical one. It is also called common amaranth, rough pigweed and careless weed.1 It is native to the United States, Canada and Mexico.3 It is a summer annual of disturbed soil that can grow 0.6 to 1.8 meters (about 2 to 6 feet) tall.2
Its flowers are very small and rarely noticed.3 The male flowers are wind-pollinated and release large amounts of pollen.2 In Yavapai County, Arizona, a related species, careless weed (Amaranthus palmeri), is listed as producing allergenic pollen too.4
Pigweeds have two weedy relatives whose pollen allergens have been identified: lamb’s quarters (Chenopodium album) and Russian thistle (Salsola).5 Oklahoma State University’s plant guides list both in the Chenopodiaceae family.76 Botanists have since extended the amaranth family, Amaranthaceae, to include the Chenopodiaceae.5 Russian thistle is the plant also called tumbleweed.6
Symptoms it causes
Pigweed pollen can cause symptoms when it is breathed in. A Nevada extension fact sheet calls redroot pigweed a common cause of late-summer allergies.3 A University of Florida extension guide lists pigweed among the highly allergenic weeds.8
Hay fever is usually caused by pollen from trees, grasses and weeds, or by mold spores. It can bring a runny or stuffy nose, sneezing, itchy eyes, mouth or skin, and tiredness from poor sleep.9 An allergist can find out which pollens are behind the symptoms, for example with a skin test.9
A 2014 review focused on the Mediterranean area notes that a high degree of cross-reactivity between the pollen allergens of the amaranth family, which includes pigweed, lamb’s quarters and Russian thistle, has been reported for more than 20 years.5
Where and when it grows
Redroot pigweed grows in waste ground, prairies, fallow and cultivated fields, farm lots and gravelly areas.1 It flowers from July through September, and its seeds ripen from August through October.1
Related weeds can have longer seasons. A 2007 extension bulletin for Yavapai County, Arizona, gives high-pollen months there of April through October for careless weed and May through November for Russian thistle.4 The 2014 review notes that plants in this family can rapidly colonize salty, arid soils, and that the first reported cases of sensitization to Russian thistle pollen were in Arizona.5
These weeds matter in some western cities. Among the 10 US cities with the greatest weed pollen exposure in AAFA’s 2025 analysis, pigweed and lamb’s quarters were the primary weed pollens in Las Vegas and San Jose.10
Russian thistle is an annual introduced from Europe. It grows throughout the Great Plains, most often in the western part, and flowers from August to October.6 Lamb’s quarters is an annual of European origin that grows in cultivated fields, gardens, pastures, vacant lots and other disturbed soil.7
Cross-reactions
| Can also react to | What happens |
|---|---|
| Lamb’s quarters and Russian thistle pollen | These weeds are now placed in the same family as pigweed, Amaranthaceae. A 2014 review focused on the Mediterranean area notes that a high degree of cross-reactivity between this family’s pollen allergens has been reported for more than 20 years.5 |
Symptoms it can cause
When to get help
See an allergist if
- Allergy symptoms last for several months of the year
- Allergies bring repeated sinus problems, a blocked nose or trouble breathing
- Over-the-counter remedies don’t control your symptoms
- Allergies or asthma get in the way of daily life
- You often wheeze, cough at night or after exercise, or feel short of breath
Based on ACAAI’s “When To See an Allergist”. General information, not medical advice.
Questions
Is pigweed the same as amaranth?
Pigweeds are species of Amaranthus, the amaranth genus. Redroot pigweed (Amaranthus retroflexus), for example, belongs to the amaranth family and is also called common amaranth or redroot amaranth.1
Is Russian thistle the same as tumbleweed?
Yes, tumbleweed is another name for Russian thistle. It is an annual introduced from Europe that grows throughout the Great Plains, most often in the western part, and flowers from August to October.6
Can pigweed cause late-summer allergies?
It can. A University of Nevada, Reno fact sheet calls redroot pigweed a common cause of late-summer allergies. The plant blooms from summer into fall.3
Sources
- Amaranthus retroflexus (Common Amaranth). North Carolina State University Extension, Gardener Plant Toolbox. Read on October 15, 2026.
- Redroot pigweed, Amaranthus retroflexus. Colorado State University, Department of Agricultural Biology. Read on October 15, 2026.
- A Northern Nevada Homeowner’s Guide to Identifying and Managing Redroot Pigweed. University of Nevada, Reno. Read on October 15, 2026.
- Plants That May Cause Allergies in Yavapai County. University of Arizona Cooperative Extension. Read on October 15, 2026.
- Amaranthaceae Pollens: Review of an Emerging Allergy in the Mediterranean Area. Journal of Investigational Allergology and Clinical Immunology. Read on October 15, 2026.
- Russian thistle. Oklahoma State University Extension. Read on October 15, 2026.
- Common Lambsquarters. Oklahoma State University Extension. Read on October 15, 2026.
- Pollen and Allergies in Florida. University of Florida IFAS Extension, Gardening Solutions. Read on October 15, 2026.
- Hay Fever. American College of Allergy, Asthma and Immunology. Read on October 15, 2026.
- Rise of Weed Pollen and Return to School Ushers in Sneezing Season. Asthma and Allergy Foundation of America. Read on October 15, 2026.