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www.arborage.com ARBOR AGE n SPRING 2015 21 INDUSTRY INSIGHTS 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) fi rst 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 fi ndings. "I had been researching which landscape plants triggered the most allergies for fi ve years when I realized just how important the dioecious species (those with male and female fl owers on different individuals) were," said Ogren. "I tried to get photos of both the male and female fl owers; I discovered that male trees were everywhere, but I could rarely fi nd the females to photograph." This refl ected the industry's bias toward using male cultivars to avoid the "messy" fruit litter of female trees. At fi rst 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 fl owers 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 Dr. Tom Ranney's lab at NC State is working on developing a lacebark elm (Ulmus parvifl ora) that won't develop fruit or appreciable pollen, which will be good news for allergy sufferers. PHOTO BY MICHELLE SUTTON Oak trees tend to have highly allergenic pollen. PHOTO BY MICHELLE SUTTON