The cause of most seasonal allergies is pollen, but not all pollen is created equal. Certain trees and plants produce more pollen than others, and at different times of the year. Oak pollen is produced in the spring (and for much longer than other trees), ragweed pollen is produced in the fall, and grass pollen is produced in the summer. So “seasonal” allergies can last for a massive part of the year, all because of pollen.

Plants produce pollen to reproduce, and the way they pollinate depends on the plant.

“Sometimes plants have male parts in one part of the tree and female parts in another part of the tree, sometimes they’re in the same flower and they can self-pollinate,” Biology Professor Mary Heskel said.

There are many ways for plants to pollinate, and not all are through self-pollination.

“Some are wind pollinated, some are pollinated by bees, some are pollinated by bats, some are pollinated by ants,” Heskel said.

However, just a few types of plants produce most of the pollen that causes seasonal allergies.

“Some plants produce so much pollen that it is just in the air because of the quantity they produce, like oak pollen and ragweed,” Heskel said. “Little amounts of that pollen are leading to fertilization and leading to a new organism so it’s just out there.”

Seasonal allergies are more than just a nuisance; they can have serious public health effects.

“Asthma is a really big [problem],” Economics Professor Gabriel Lade said. “That leads to really big medical costs. If you have allergy-induced asthma that can be a problem. 

The US Department of Agriculture talks about how allergies are linked to respiratory health, viral infections, emergency room visits, and even children’s performance in school.

Allergies have been around forever but what has changed is that they’re getting worse.

Led by William Anderegg of the University of Utah School of Biological Sciences, researchers found that allergy seasons in North America are starting 20 days earlier, last 10 days longer, and produce 21% more pollen than they did in 1990.

There are two main causes for worsening seasonal allergies: global warming and changing types of trees that get planted. Global warming has caused an earlier spring and a delayed fall in many climates. This causes plants to produce pollen for longer and release more of it.

“Spring is starting earlier in [areas] affected by climate warming, so in seasonal areas where you have a spring and a fall,” Heskel said. “Which extends the time when pollen is both produced and released for many plants, not just trees.”

A study by Yingxiao Zhang and Allison L. Steiner, researchers at the University of Michigan, found that increased amounts of carbon dioxide led to more pollen production. If current carbon dioxide production is not curbed, pollen production will triple in certain areas.

Unfortunately, stopping global warming is not something individual cities can do. However, cities do have a role to play in worsening seasonal allergies as well as with what kind of trees they plant.

“Generally, there is some kind of city entity in charge of deciding what type of trees are planted and where, usually funded by taxpayers,” Environmental Studies Professor Anika Bratt said.

However, the issue is, some types of trees are split, with one version producing pollen and another producing flowers and fruit. It’s this division that cities have focused on, with almost every city exclusively planting pollen-producing trees.

“Some cities are starting to plant more fruit trees so you have urban foraging opportunities, and that is a very new idea,” Bratt said. “I think those kinds of ideas will grow a lot in the next 50 years. 

The reason for this lack of fruit-bearing trees is deceptively simple: they cost more. That extra cost comes from one main source, their higher maintenance needs.

“Fruit-bearing trees require a lot more water. So, the costs of that are different depending on where you are,” Lade said. “In California, the costs of switching to something that would require way more water is tremendously expensive.”

While some states like Minnesota that are rarely in drought might be able to handle the extra water use, drought states like California, Nevada, and New Mexico could not.

Furthermore, certain types of trees produce more pollen than others, and cities have been largely ignoring this facet of tree planting.

“Some trees produce a lot of pollen that can be problematic from a human health standpoint and others produce less and we should think about that when deciding what trees to plant in public spaces,” Bratt said.

For example, a common urban tree is the pin oak, which is attractive to cities because they are smaller than other oak trees. However, pin oak trees and oak trees in general produce a lot more pollen than other tree varieties.

 The solution to this problem however is not to plant fewer trees in cities. The benefits of urban trees are numerous and planting fewer of them would have adverse effects on communities.

 “In terms of the benefits [of trees] they’re mostly in providing shade, trying to prevent urban heat traps, and they’re usually planted in greenways and can help slow down water to help with flood mitigation strategies,” Lade said.

 Expanding on urban heat traps, Lade explained how the communities often affected by them are minority communities and poorer areas of cities.

 “If you take the average temperature in [high minority and impoverished] blocks of cities, they can be way higher, 10, 20 degrees higher on the same day [than in other areas],” Lade said.

 The cause of this disparity can be traced back to historical disparities in neighborhood investment, which affects how many trees get planted in certain areas.

 “[Poorer neighborhoods and minority neighborhoods] have less tree canopy cover, just fewer trees, than more wealthy neighborhoods,” Bratt said. “For example, Macalester-Groveland which is a very wealthy neighborhood relative to the rest of the city, there’s a lot of trees here compared to a place like Rondo which was redlined and has a lot less tree canopy cover.”

 Ensuring that cities address these disparities by planting more trees in affected neighborhoods can have a big impact on eliminating urban heat traps and ensuring that across cities, people have a cool place to stay.

 However, this still leaves the problem of pollen production unsolved, because the way that cities plant trees has been both increasing pollen production and is unsustainable for the trees themselves.

 “For a long time, monocultures of trees were planted around [the Twin Cities],” Bratt said. “Having all of one species in a city makes the city very prone to a pest or something coming through and destroying everything which is what happened with Dutch elm disease.”

 Cities can diversify the types of trees they plant, whether they produce fruit or not, or the species of trees. This is important not only for pollen production but also for ensuring the health of the trees themselves.

 There are three prominent facets of tree planting that cities can focus on: shade cover, pollen production, and species diversity. For example, many cities plant palm trees which provide very little shade. Planting trees that have more shade cover can help alleviate urban heat traps. Oak trees produce massive amounts of pollen, and planting trees that produce less pollen would help alleviate allergy issues. Finally, avoiding monocultures of trees would ensure they aren’t prone to diseases and ensures the long-term sustainability of a city’s urban tree population.

 Changing the way cities plant trees would impact public health and local communities. Planting more fruit trees to reduce pollen production might be too expensive in some areas, but some states like Minnesota could do it. However, one thing that cities around the country can do is be more aware of the types of trees they plant, ensuring they aren’t creating monocultures, are equitably planting trees and are providing shade for their citizens on hot summer days.  

[email protected] 

Sources: 

https://www.nature.com/articles/s41467-022-28764-0#Sec2

https://www.pnas.org/doi/10.1073/pnas.2013284118#sec-1

https://www.nifa.usda.gov/about-nifa/impacts/yes-allergy-seasons-are-getting-worse-blame-climate-change