16 species profiles covering trees, grasses, and weeds
Quercus
Oak trees are one of the most prolific pollen producers in North America. A single oak tree can produce billions of pollen grains in a season. The yellow-green dust you see coating cars in spring is often oak pollen. Oak pollen grains are relatively small (20-30 micrometres), allowing them to travel hundreds of miles on wind currents.
Betula
Birch pollen is one of the most allergenic tree pollens worldwide. It contains the protein Bet v 1, which is the primary allergen responsible for birch pollen allergy and is also the protein behind many food cross-reactions. A single birch catkin releases about 5.5 million pollen grains, and a mature tree can have thousands of catkins.
Acer
Maple trees are early-season pollinators, often among the first trees to bloom in late winter. While less allergenic than oak or birch individually, their early timing catches many people off guard. Box elder (Acer negundo) is actually a maple species and is the most allergenic of the group.
Pinus
Pine pollen is dramatic — you can literally see yellow clouds of it — but it's actually less allergenic than most other tree pollens. The grains are very large (45-65 micrometres) with distinctive air bladders that help them travel long distances. Because the grains are so large, they're less likely to penetrate deep into the respiratory tract. However, the sheer volume of pine pollen can irritate anyone.
Ulmus
Elm trees are very early pollinators, sometimes blooming as early as January in southern regions. American elm was once the dominant street tree in the United States before Dutch elm disease devastated populations. Despite reduced numbers, elm pollen remains a significant allergen, particularly because it pollinates when few other trees are active.
Fraxinus
Ash trees produce moderate amounts of allergenic pollen. They are related to olive trees and share significant cross-reactivity. Ash pollen contains the allergen Fra e 1, which is structurally similar to olive pollen's Ole e 1. The emerald ash borer has severely impacted ash populations in recent years.
Juniperus
Mountain cedar (Juniperus ashei) and other junipers are notorious allergens, especially in Texas and the southwest. 'Cedar fever' is a well-known phenomenon in central Texas where mountain cedar pollen causes intense allergic reactions in winter. These trees release massive amounts of pollen — visible as clouds of 'smoke' rising from the trees on dry, windy days.
Populus
Cottonwood trees are famous for their fluffy white seeds that fill the air in late spring, but this 'cotton' is not the allergenic part. The actual pollen is released weeks earlier and is invisible. Many people blame the cotton fluff for their allergies, but their symptoms are usually caused by grass pollen that peaks at the same time the cotton is flying.
Alnus
Alder is an early-season pollinator closely related to birch. In the Pacific Northwest, alder is the dominant allergenic tree. Red alder (Alnus rubra) is the most common species in western North America. Alder pollen contains allergens that cross-react significantly with birch.
Poaceae
Grass pollen is the most common cause of allergic rhinitis (hay fever) worldwide. The major allergenic grasses include Timothy, Bermuda, Kentucky Bluegrass, Ryegrass, and Fescue. Unlike trees that bloom for a few weeks, grass pollen season can last for months. Grass pollen grains are moderately sized and highly allergenic — the allergens are concentrated on the surface of the grain and are released immediately on contact with mucous membranes.
Phleum pratense
Timothy grass is the most studied allergenic grass and is used as the standard reference for grass pollen allergy testing. It originated in Europe but is now widespread across North America as a pasture and hay grass. The allergen Phl p 1 is the primary protein responsible for reactions and is the basis for most grass pollen immunotherapy products.
Cynodon dactylon
Bermuda grass is the dominant allergenic grass in the southern United States. It has a longer pollen season than northern grasses and can pollinate from spring through fall in warm climates. Bermuda grass is unique among grasses because it has limited cross-reactivity with northern grass species — it belongs to the Chloridoideae subfamily while most northern grasses are in the Pooideae subfamily.
Ambrosia
Ragweed is the single most significant cause of fall allergies in North America. A single ragweed plant can produce up to one billion pollen grains per season. The pollen is extremely lightweight and can travel up to 400 miles on wind currents. Ragweed pollen contains the allergen Amb a 1, which causes reactions in approximately 15-26% of the U.S. population.
Artemisia vulgaris
Mugwort is a widespread weed whose pollen is a major allergen in late summer and fall. It's in the same family as ragweed (Asteraceae) and the two cross-react significantly. Mugwort allergy is particularly notable for causing celery-birch-mugwort syndrome, a complex food allergy pattern. The pollen contains the allergen Art v 1, which triggers reactions in 10-14% of pollen allergy sufferers.
Plantago
Not to be confused with the banana-like fruit, Plantago is a common broadleaf weed found in lawns and disturbed areas worldwide. English plantain (Plantago lanceolata) is the most allergenic species. It produces small, inconspicuous flower spikes that release moderate amounts of wind-dispersed pollen. Plantain allergy is often underdiagnosed because the plants are so unassuming.
Urtica / Parietaria
Nettle family plants (Urticaceae) are significant pollen producers that are often overlooked as allergens. Pellitory (Parietaria) is particularly important in Mediterranean climates and coastal areas. These plants produce small, inconspicuous flowers that release lightweight pollen in enormous quantities. The sting of stinging nettle is caused by histamine and formic acid in the plant's hairs — completely separate from the pollen allergy.