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www.urban-forestry.com 13 gum (Liquidambar styraciflua)—but has had mostly failures with those species. Hence, these are some of the few spe- cies they avoid planting in the fall. Upper Arlington, Ohio Superintendent of Parks and Forestry Steve Cothrel has learned to give credence to his local nursery's advice in light of when his fall planting program takes place. Cothrel says that his fall B&B trees are typically dug and shipped in October. "Digging stock of certain species later in the fall— when plants are more fully dormant— might help with the 'fall hazards' from what I've heard," he says, "but that would not work with our installation schedule. It's all we can do as it is to get trees planted before serious winter arrives." Based on this installation schedule and his many years of obser vation, Cothrel avoids fall planting of oaks (except swamp white oak), magnolias (Magnolia spp.) (including tulip tree), river birch, black gum, beeches (Fagus spp.), hawthorns (Crataegus spp.), horsechestnut (Aesculus spp.), and baldcypress (Taxodium distichum). However, he says that he has obser ved no readily apparent sur vival dif- ferential between fall- and spring-planted 1.5-inch/3.8-cm caliper trees that some consider touchy in the fall, including lindens (Tilia spp.), zelkova (Zelkova serrata), goldenrain- tree, callery pear (Pyrus caller yana), and elms (Ulmus spp). Five Branches of Transplanting Success In her work in her home city of Ithaca, NY and with dozens of other municipalities, Nina Bassuk has found that for bare root trees under 2-inch (5 cm) caliper, it has been easy to success- fully fall-transplant many of the trees frequently appearing on fall hazard lists—including London planetrees, goldenraintrees, crabapples (Malus spp.), zelkovas, cherry trees, pear trees, oaks, and elms. This tells us that season + species isn't the only key interaction in the fall hazard picture; production method comes into play as well. Bassuk says that the five major interacting factors influencing transplanting success are season, species, production method, size, and aftercare. Season is best understood in its interac- tions with the other factors, while the essential nature of good aftercare (watering, weeding, mulching, avoiding damage to bark, etc.) is obvious. Species: Bassuk and colleagues are currently investigating why some tree species are more difficult to transplant than others. It has to do with the ability of a given species to maintain hydraulic conductivity by avoiding cavitation (obstructing bub- bles in the xylem) after its roots are severed. Why is bur oak (Q. macrocarpa) so difficult to transplant while its close relative, swamp white oak, is relatively easy? Bassuk says, "Our early results show that bur oak isn't able to take up water from its severed root system after transplanting. There's no water uptake until it makes new roots, so if it doesn't get adequate water at planting and thereafter, that exacerbates the problem. Meanwhile, swamp white oak can take up water from its own root system after transplanting; it has better hydraulic conductivity than bur oak." Production method: Bare-root digging takes exponentially more roots along than does B&B digging. For many species, if the bare root trees are handled properly, (see www.hor t. cornell.edu/uhi), bare root can translate to higher surviv- al in spring or fall because of this larger root system. For notoriously difficult-to-transplant species, Bassuk says that container-grown trees are actually the safest bet, because the entire root system is brought along/there's no root sever- ing to trigger cavitation. However, Bassuk acknowledges that container-grown trees of the species and size city foresters desire are not always readily available. Size: Bassuk says, "In terms of transplanting, for both B&B and bare root, the larger the caliper of tree, the poorer the results (bare root trees above 2-inch/5-cm caliper are especially at risk). For instance, even though spring planting works better for bur oak, with larger caliper there will be high rates of failure, even in spring and in the best conditions." This is because the larger the tree caliper at transplanting, the greater the percentage of its roots are lost in the move; it takes a longer time for a root system to grow to support that larger canopy than it does to support a smaller canopy. "In my expe- As an alternative to B&B transplanting, bare root trees handled with the UHI method can be purchased, shipped, and planted much more affordably. Photo Courtesy Onondaga County Soil and Water Conservation District