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22 SPRING 2015 n ARBOR AGE www.arborage.com INDUSTRY INSIGHTS 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 benefi ts of triploids are "enhanced fl owering 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-specifi c pruning recommendations to control pollen shed in residential and urban environments. Ogren said it's tricky because not only do some trees fl ower on new wood (this year's growth) while others fl ower on old wood (last year's growth), some monoecious species will produce male fl owers on new wood and female fl owers on old wood, or vice versa. So, for example, with the monoecious (and highly allergenic) oaks, male fl owers form on old wood, but female fl owers 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 fl owers — perhaps messier, but good for allergies and good for wild animals that eat the acorns." He added that the same situation is true for all of the oak relatives, like the hickories, chestnuts, and birches. Also, more female fl owers is always a win, because the female fl owers "trap" pollen from a variety of other trees, reducing the amount that's free fl oating in the wind and up our noses. In contrast to oaks, there are the trees that produce male fl owers 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 fl ower 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 (tloallergyfree@earthlink.net). Michelle Sutton (michellejudysutton.com) is a horticulturist, writer, and editor. Horticulturist and researcher Tom Ogren consults widely on the allergenic potential of urban and residential landscapes. Tom Ogren's newest book, published by Ten Speed Press, was released in February 2015. It includes allergenic rankings for more than 3,000 trees and other plants. PHOTO COURTESY OF TOM OGREN PHOTO COURTESY OF TOM OGREN