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TREES, PLEASE: Think Trees Are Making You Sneeze?

Allergies are on the rise. Although grassy plants and food sensitivities account for much of this ascent, there is startling evidence that ever-increasing pollen is the result of an imprudent notion that litter-free trees make the best urban choices. In a nutshell, the goal to reduce fruit drop has resulted in a preponderance of single-sexed male trees producing copious pollen in a never-ending pursuit of elusive females.

So, what happens if there are few, if any, females to be found? Prodigious, light-weight pollen grains go airborne seeking to reproduce, but instead, float aimlessly, wafting around our urban spaces looking for people to irritate. Smog may bind with pollen, creating lingering doses long after a tree releases a typical morning dispersal. Commercial growers sell predominantly male plants of dioecious species like ash, ginkgo, pepper, bay and mulberry and monecious trees get grafted from scion-wood to produce male-clonal specimens. Nurseries now sell male-sexed versions of juniper, Italian cypress, xylosma, cottonwood and others, and some olive species claim no fruit or pollen production. Is that really a virtue? They’re not all equally allergenic, but it’s a slippery slope.

It is also true that spring rhinitis arises from California’s native plants. In San Diego County, ashes, cypress, oaks, willows and walnuts, produce troublesome airborne allergies, as do sagebrush, saltbush and coyotebrush. Oak tree pollen is lightweight and can travel for miles. Are you a native plant devotee who unwittingly incorporated native species into your garden, but later discovered sensitivity to pollen? Was this explained at purchase?

Alder Pollen.  Attribution: Toni Vakiparta [CC BY-SA 4.0]

Remember, pollination is the process of transferring pollen grains from the male anther of a flower to the female stigma. The details get complicated. Some trees have conspicuous flowers that contain both male and female parts, and pollen doesn’t have far to travel. “Perfect” flowers don’t rely on wind or insects, but can self-pollinate, although some have mechanisms to encourage cross-pollination. Non-native acacia pollen may appear ominous and plentiful, but it is also sticky and heavy, and drops close to home, so it’s typically considered a low-allergen genus. Conifers like pines have male and female cones on the same tree, which typically rely on wind to get the pollen from the base of the tree where the male cones dominate, to the top where receptive female cones linger. Beware of windy days if you are sensitive. Although there are differing examples of tree species that may flare up seasonal allergies, it’s typically male-sexed trees that are causing cumulative urban aggravation.

Whether we plant natives, grafted clones, sterile hybrids or imported tree species, there are many reasons to justify tree diversity. Everything we do has consequences. Additionally, almost all plants seem to be having longer pollen-producing seasons due to higher levels of atmospheric CO2. The good news, is that dioecious female trees are totally pollen free. If you’re seeking a hypoallergenic species, consider a female holly, persimmon, carob, mulberry, Chinese pistache, pepper, smoke tree, or mulberry. If this all sounds confusing; well it is…

Don’t rush to assume that trees are your allergy culprits, because the bane of urban gardens “Bermuda grass,” might actually be your nemesis. We need to plant more, not less trees. It is also likely that increased allergies stem from human immune systems gone awry from too much hygiene. So, go ahead, and play in the dirt!

Additional information can be found at the links below:

Member Robin Rivet is an ISA Certified Arborist & City of La Mesa Environmental Commissioner – contact her:

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