By Michelle Sutton
In 2012 I attended an urban forestry conference in Sacramento. I found one presentation highly provocative because it hinted at a potential downside of the urban forest. What could possibly be negative about living in close proximity to tree canopy?
One of the many benefits of urban tree canopy is said to be reduced air pollution, which could be expected to lead to a healthier respiratory environment. But Assistant Professor in Epidemiology at Columbia University Mailman School of Public Health, Dr. Gina Lovasi, and her 11 coauthors found a surprising result for members of a group of 549 New York City children.
A statistically significant subset of those children — those whose mothers lived near urban tree canopy while pregnant — developed allergic sensitization to tree pollen by age seven. In their analysis of their findings, Lovasi et al identified pollen as a potential factor, but they didn’t have enough information to firmly lay the blame there.
However, other studies are more specific to pollen. For example, “The Association of Tree Pollen Concentration Peaks and Allergy Medication Sales in New York City: 2003–2008” by Sheffield et al found that maple, oak, and birch tree pollen concentration peaks were followed by large, statistically significant increases in over-the-counter allergy medication sales.
According to the American Academy of Allergy, Asthma and Immunology, more than 35 million Americans suffer from seasonal allergies, including to tree pollen. The pollen question is likely to become more visible as laws prohibiting the planting of the most allergenic trees are being enacted by municipalities, as has been done in Phoenix, Ariz.; Tucson, Ariz.; Albuquerque, N.M.; and Las Vegas.
Legislators and lawyers are also turning their attention toward the schoolyards of North America and how the tree plantings there, generally heavy in male cultivars, are negatively affecting children. Arborists should get in the habit of considering allergenic potential in trees and educating their clientele about the pollen hazard. Luckily, there are good resources available.
On the case
California-based horticulturist and writer Thomas Ogren (safegardening.org) first got interested in pollen because of the misery he saw his wife endure with asthma, which would be compounded by peak pollen times. Ogren is the author of two previous books on the topic (Allergy-Free Gardening and Safe Sex in the Garden) and his new book, The Allergy Fighting Garden, merges those earlier books and adds new findings.
“I had been researching which landscape plants triggered the most allergies for five years when I realized just how important the dioecious species (those with male and female flowers on different individuals) were,” said Ogren. “I tried to get photos of both the male and female flowers; I discovered that male trees were everywhere, but I could rarely find the females to photograph.” This reflected the industry’s bias toward using male cultivars to avoid the “messy” fruit litter of female trees.
At first Ogren thought this male tree prevalence was a local phenomenon, but he’s since seen it all over the world. “The thirst for litter-free trees would appear to go back very far,” said Ogren. “In Christchurch, New Zealand I found a hedge of native yew (Podocarpus totara) that was a quarter of a mile long and more than a hundred years old, and every single plant in the hedge was male.”
In Sacramento, Calif. right in front of the State Capitol building, Ogren found a row of a dozen giant Deodar cedar (Cedrus deodara) trees that were planted in 1913. “Every one of them was male… this didn’t happen by accident. And there was dense pollen all around them,” he said.
Related to this, one may wonder, “Why does it matter what I plant? My clients and I will still be exposed to pollen traveling in from elsewhere.” But in fact, most pollen settles near the originating tree; allergists even have a term for the phenomenon of “proximity pollinosis,” meaning that the big allergic response comes from the tree just outside your window or the one you just walked by.
Communities now hire Ogren to assess and redress their excessive use of allergenic male cultivars of dioecious taxa like those of box elder (Acer negundo), yew (Taxus spp.), juniper (Juniperus spp.), and mulberry (Morus spp.). He also advises on monoecious (having female and male flowers on the same tree) taxa like Norway maple (Acer platanoides) and London plane trees (Platanus x acerifolia) that produce copious allergenic pollen. (To be fair, many tree species and cultivars are not allergenic, but the ones that are can create disproportionate misery.)
Ogren developed OPALS (Ogren Plant Allergy Scale), which he has applied to measuring the allergenic potential of thousands of trees and other landscape plants, and is being used by the American Lung Association and USDA Urban Foresters. A pollen-free tree like ‘Autumn Glory’ red maple ranks at 1 (least allergenic) on the scale, while male green ash trees rate 9 (out of 10, which is most allergenic). He rates more than 3,000 trees and other plants in his new book. Ogren is working with the nursery industry to get tags made that include the OPALS rating so that arborists and horticulturists can be better informed.
Enter the polyploids
The work of Dr. Thomas Ranney at NC State University could be part of the solution. In his Mountain Crop Improvement Lab, he and his students are developing seedless cultivars of woody and herbaceous plants that otherwise are invasive. They select for and create triploids — plants that have three sets of chromosomes, and are thus unable to divide evenly during meiosis (formation of reproductive cells), which reduces fertility and prevents seed formation. This is the process used to make seedless fruits like watermelon and bananas.
According to the lab’s website, “Triploids can occur naturally or can be bred by hybridizing a tetraploid (4X) with a diploid (2X) to create seedless triploids (3X).” Fortunately, polyploidy (more than the standard two sets of chromosomes) is common in nature, yielding useful triploids and tetraploids. In addition to preventing spread of invasive species, the benefits of triploids are “enhanced flowering and re-blooming, reduced fruit litter, and reduced pollen allergens.”
“Although our primary aim is to eliminate fruit or seeds, the triploid plants generally don’t produce pollen,” said Ranney. “We have released a number of seedless cultivars including My Fair Maiden Miscanthus (Miscanthus sinensis), ‘Chastity’ trumpet vine (Campsis x tagliabuana) and we are close to releasing some seedless pears and barberries. Most of the plants we are working on at present are not big allergy causers, but the same approaches should work.” Two of the trees his lab is seeking triploids of — lacebark elm (Ulmus parvifolia) and Norway maple (Acer platanoides) — are allergenic, so pollen reduction would be welcome in those cases.
Prune away the pollen
While Ranney is working on the genetic level on a technique that could be used more widely in the future to prevent pollen production, Ogren has been thinking a lot about what to do with allergenic trees already in the landscape, apart from take them down. He is working on species- specific pruning recommendations to control pollen shed in residential and urban environments.
Ogren said it’s tricky because not only do some trees flower on new wood (this year’s growth) while others flower on old wood (last year’s growth), some monoecious species will produce male flowers on new wood and female flowers on old wood, or vice versa. So, for example, with the monoecious (and highly allergenic) oaks, male flowers form on old wood, but female flowers are formed mostly on new wood.
Said Ogren, “This is quite interesting as it means that tip pruning oaks in winter will result in much less pollen, and in many more female flowers — perhaps messier, but good for allergies and good for wild animals that eat the acorns.” He said that the same situation is true for all of the oak relatives, like the hickories, chestnuts, and birches. Also, more female flowers is always a win, because the female flowers “trap” pollen from a variety of other trees, reducing the amount that’s free floating in the wind and up our noses.
In contrast to oaks, there are the trees that produce male flowers on new wood. “Luckily there aren’t too many of them used in landscapes,” Ogren said, “because they will always pose a problem, since there’s often just no way to prune away the next season’s pollen production. Male (fruitless) mulberry trees would be the prime example of this: the harder you prune them, the more pollen they produce.”
By contrast, silver maple pollen can be strategically managed with pruning of the many male cultivars. These form the male flower buds on branch tips late in the season in the year before they bloom. If these trees are pruned during the winter, then most of the pollen for next spring from that tree will have been removed.
“This is potentially quite important,” said Ogren, “especially for those with allergies who live on blocks that have been planted with large numbers of clonal male silver maples. It also represents a totally new opportunity for pruning work for arborists. I can see down the road where this sort of pruning might well be mandated in some cities, which would be a win-win situation for both allergy sufferers and arborists.” Ogren invites readers to send him their own observations regarding pruning and pollen production (firstname.lastname@example.org).
Michelle Sutton (michellejudysutton.com) is a horticulturist, writer, and editor.