I originally saw the question below posted in an Internet gardening forum and decided to answer it. It was a question I'd been asked many times before and I knew my answer would be useful in an article on springtime allergies caused by city trees. So I saved it to share with you here.
"Does anyone else here really suffer from allergies when the seeds of
Cottonwood are flying? I KNOW it is not the cottonwood but I am really
curious as to what is pollinating at the same time."
Diana Pederson, Ingham County, Michigan, Zone 5, United States, author of Landscaping With Bible Plants:
That's a very good question.
Around here, as the "cotton" (the seeds) of the female poplars (cottonwoods and aspens) and the willows is flying about, so is a good deal of pollen from different, unrelated species of trees. It is very common at this precise time that many people are suffering from extreme bouts of hay fever and often it is this "cotton" that gets the blame. Some city arborists refuse to plant female willows or poplars because of their firm (if mistaken) belief that this "cotton" is really some kind of pollen. But it isn't pollen; it is seed. It is NOT what is causing the allergies at that time.
By the time the seeds of the female willow and cottonwoods are flying, pollen from the males of these two species is already spent. However this flying of seed coincides with pollen release of many allergenic plants.
Out West this is the same time that the millions of urban "fruitless" male mulberry trees are shedding their highly allergenic pollen. It is also the time that the olive trees are starting to release pollen. The cypress trees and shrubs are releasing very large amounts of pollen at this time too, as are the many male Ailanthus trees. At or about the same time the walnut trees are releasing a large amount of pollen, as are many species of hickory, butternut, and pecan. Perhaps the most pervasive at this point are the oaks, many species of which are still at this time covered with staminate flowers and just loaded with pollen.
At the same time that the female willows and cottonwoods are releasing all that harmless fluff into the air, the birch trees have just finished shedding large amounts of pollen, much of which is still lying around on the ground. In southern areas the alders often bloom twice (as will many birch and junipers) and the second bloom of the alders sometimes will coincide precisely with the flying of the "cotton."
Also to take into consideration is that by the point in the year when the cottony seeds of the willows and poplars starts to float about, most of the male maples and male ash, and a large number of other trees and shrubs have already released their own pollen. Unless this pollen was washed away by strong downpours of rain, much of it is still lying about, and is still causing problems, weeks or sometimes even months after it was released.
To add to all of the above, at precisely this same time, the grasses start to release pollen. The ornamental landscape clump grasses all produce huge amounts of pollen at this time, as do most bluegrass species, bentgrasses, Bahia grasses, and especially Bermuda lawns that have not been kept closely mowed. By the way, the newer hybrid Bermuda grasses are sterile and pollen-free, but not the old common Bermuda lawns by any means.
By the time this poplar and willow "cotton" is in the air, many people with allergies are already starting to suffer from "systems overload." There is so much pollen being released and so much just previously released, that it overwhelms the immune systems of many individuals. The result of course is allergy.
The sad thing about this whole affair is that all too often these female willows or female poplars, female cottonwoods, female aspens, they get blamed for the pollen from the male trees and then people cut them down! Since female trees have flowers that are electrically charged negative - (their roots are grounded) and since pollen from male trees picks up a positive + charge as they tumble about in the air, the two are mutually attractive. Female trees are powerful air cleaners, air scrubbers. Every female tree that is chopped down makes the air in that neighborhood that much more allergenic.
We need to protect our females!