Science Natural Science 11 Allergy-Inducing Pollen Producers By Russell McLendon Senior Writer University of Georgia Russell McLendon is a science journalist who covers a wide range of topics about the natural environment, humans, and other wildlife. our editorial process Russell McLendon Updated September 06, 2019 Photo: Gilles San Martin [CC BY-SA 2.0]/Flickr Share Twitter Pinterest Email Science Space Natural Science Technology Agriculture Energy Plants need love as much as anyone. But unfortunately for us, some plants' very public displays of affection can be hard to ignore. That's because many people — including some 35 million Americans — have severe allergic responses to plant pollen, even though the reproductive specks are otherwise harmless. Allergy testing can tell you which plants' romantic overtures offend your immune system, but then what? Say you're allergic to ragweed, ash trees, timothy grass or mugwort. Where do they grow? When do they pollinate? And what can you do? For answers to these and other itching, burning questions, check out the following roundup of plants that prove the Everly Brothers and Nazareth right: Love hurts. 1 of 11 Juniper Photo: micklpickl [CC BY-SA 2.0]/Flickr This tree often looks like it's on fire — but it's just releasing pollen. Often called "mountain cedar," it's actually an Ashe juniper, one of many allergenic juniper species. Others include the Eastern red cedar, Pinchot's juniper and Western juniper. Habitat: Usually semi-arid regions Description: Small evergreen trees or shrubs with many stems. Most are dioecious, meaning only males produce pollen. Allergenicity: Moderate to severe Allergy season: Varies (e.g., winter for Ashe junipers; spring for Western junipers) Tips: Avoid male junipers, especially near true cedars, since juniper and cedar pollen can cross-react. Also limit outdoor activity from 5 to 10 a.m. during juniper-allergy season, since that's typically prime time for pollen. 2 of 11 Ragweed Photo: Frank Mayfield [CC BY-SA 2.0]/Flickr Ragweed is one of the top seasonal-allergy sources in North America. Climate change has lengthened its pollen season by 27 days since 1995, a trend that's expected to continue. Habitat: Temperate regions around the world, often near farm fields, roads and rivers Description: Lobe-leafed shrubs in upright clumps. All 41 species are monoecious, which means every ragweed plant produces pollen. Allergenicity: Severe Allergy season: August to November Tips: Most trees and grasses have finished pollinating by late summer, but ragweed is just getting started. It releases pollen at dawn, with airborne levels often spiking from 10 a.m. to 3 p.m., depending on wind and dew. (Dry, windy days are worst.) Some natural allergy remedies may help, from butterbur and quercetin to hot peppers and raw local honey. 3 of 11 Orchard grass Photo: Hiroyuki Takeda [CC BY-ND 2.0]/Flickr One of the most prolific cool-season grasses, orchard grass is widely used as a forage crop and as ground cover in orchards. It's also one of the top grass allergens in the world. Habitat: Native to Europe, Asia and North Africa; introduced to U.S. in the 1700s. Prefers cool climates, moist soil and partial shade. Description: Tall perennial grass reaching 3 to 4 feet in height. Grows in bunches. Allergenicity: Severe Allergy season: May to June Tips: Grass pollen doesn't travel as far as tree pollen, so windy days tend to be worse. Avoid the outdoors before 10 a.m., keep windows closed and don't use attic fans, especially if you live near grazing land or orchards. 4 of 11 Hickory (pecan) Photo: Clearly Ambiguous [CC BY 2.0]/Flickr Pecan trees like this one make more than just sturdy timber and nutritious nuts. They're also major sources of allergenic pollen, and they aren't alone: At least 11 other hickory trees also cause severe pollinosis, including the bitter-nut, mockernut and shellbark hickories. Habitat: Eastern and central U.S., Mexico, Canada, China, India and Indochina Description: Tall trees with wide, straight trunks that can reach 100 feet high Allergenicity: Severe Allergy season: Late spring (April to June) Tips: Hickory trees are monoecious, which can make their pollen hard to escape. Fortunately, the large particles don't travel very far and are less prevalent in the afternoon. 5 of 11 Ash Photo: vaprwere [CC BY 2.0]/Flickr A wide array of ash trees cause allergies, including black, blue, green and white ash (pictured). But while they may spur seasonal misery in humans, many U.S. ash trees are suffering more than we are: They're under siege from the emerald ash borer, an invasive beetle introduced from Asia in the 1990s. Habitat: North America, Europe, Asia and North Africa Description: Tall, hardwood trees that grow up to 90 feet high. Most are dioecious. Allergenicity: Severe Allergy season: Varies (winter to spring) Tips: Avoiding male ash trees is usually enough, but a few species are monoecious, and some even have bisexual flowers. Be wary of places where both ash and olive trees grow, since their pollen can cross-react. 6 of 11 Mulberry Photo: fescue [CC BY 2.0]/Flickr Not only do mulberry trees produce tart, nutritious fruit, but they've also been used for millennia to cultivate silkworms. Their role as ornamental trees in U.S. cities has faced criticism, though, since male mulberry trees release heavily allergenic pollen (pictured). Habitat: Temperate and subtropical regions of Asia, Africa, Europe and the Americas. Also widely planted in rural and urban areas. Description: Deciduous, lobe-leafed trees, rarely growing taller than 30 to 50 feet Allergenicity: Severe Allergy season: Spring (April to June) Tips: Like other dioecious plants, male mulberry trees produce the most pollen. But they can be made less prolific if given a "sex change" by top-grafting them with branches from a female tree. Their pollen may cross-react with other Moraceae species, including fig and breadfruit trees. 7 of 11 Oak Photo: Edd Prince [CC BY 2.0]/Flickr At roughly 600 species strong, oaks are some of the most familiar trees on Earth — and some of the most allergenic. Habitat: Northern Hemisphere, from cold latitudes to tropical Asia and the Americas Description: Sturdy trees, up to 150 feet tall. Oaks can be either deciduous or evergreen, but they're all monoecious. Allergenicity: Severe Allergy season: Spring Tips: There's little hope of escaping oak trees, and since they're monoecious, you can't just dodge the males. Be sure to at least follow general pollen-avoiding advice, such as keeping windows closed and limiting outdoor activity from 5 to 10 a.m. in the spring. Natural remedies like astralagus root, raw local honey or spicy foods may also offer some relief, but don't expect any miracles. 8 of 11 Ryegrass Photo: Bob MacInnes [CC BY 2.0]/Flickr Ryegrass is often grown in lawns, but most of its pollen comes from pastures, wild areas and roadsides, since it usually has to be a foot tall to flower. Not to be confused with rye, a grain crop like wheat, ryegrass is one of the most potent and widespread of all grass allergens. Habitat: Native to Europe, Asia and North Africa; cultivated worldwide Description: Single-stemmed tuft grass Allergenicity: Moderate to severe Allergy season: June to August Tips: If you have a ryegrass lawn, keep it mowed (or at least less than a foot tall). Beyond that, avoid unidentified grass pastures in summer, and consider staying indoors from 5 to 10 a.m. with the windows closed. 9 of 11 Birch Photo: saaby [CC BY-SA 2.0]/Flickr Birches rank second only to oaks among allergenic trees, according to Pollen.com's Pollen Library. They form fast-growing forests that quickly colonize new habitat, becoming increasingly allergenic as they multiply. Habitat: Europe, Asia and North America (especially east of the Rocky Mountains) Description: Tall, narrow trees growing 60 to 100 feet high, often with scaly, papery bark Allergenicity: Moderate to severe Allergy season: Spring Tips: Birches are at their worst in large numbers, but if you're allergic, actual birch trees may not be your only problem: Proteins in many raw fruits and vegetables resemble birch pollen enough to trigger allergies in some people. These include almonds, apples, cherries, peaches and walnuts, but fortunately the proteins can be denatured by cooking. 10 of 11 Timothy grass Photo: Dean Morley [CC BY-ND 2.0]/Flickr Named after Timothy Hanson, an American farmer who popularized it in the 1720s, timothy grass is now widely grown as a forage and hay crop in the U.S. It's also one of the worst early-summer grass allergens, behind only orchard grass, according to Pollen Library. Habitat: Native to Europe and Asia, and adapted to cool, humid parts of North America Description: Perennial grass with flat leaves, grows 2 to 4 feet tall Allergenicity: Severe Allergy season: June to July Tips: As with other grasses, timothy grass pollen can be controlled by mowing the plants before they flower. The species isn't heavily grown in urban areas, but it does cross-react with pollen from other grass species. 11 of 11 Wormwood Photo: H. Zell [CC BY-SA 3.0]/Wikimedia Commons The Artemisia genus contains an army of allergenic plants, most of which are called wormwood, sagebrush or mugwort. According to Pollen Library, they may be the top overall source of pollinosis after ragweed and grasses. Artemisia absinthium (pictured) is also the main ingredient in absinthe. Habitat: Temperate climates around the world, usually in dry or semi-dry habitats (including much of the Western U.S.) Description: Aromatic, bitter herbs and shrubs, varying in height from 1 to 10 feet Allergenicity: Severe Allergy season: Midsummer to fall Tips: Ragweed gets more attention, but midsummer and fall are also when Artemisia plants erupt with romance. Watch out for multiple species blooming close together, since one study found "very strong in-vitro cross-reactivity" within the Artemisia genus.