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14 FALL 2015 ARBOR AGE www.arborage.com INDUSTRY INSIGHTS 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.hort.cornell. edu/uhi), bare root can translate to higher survival in spring or fall because of this larger root system. For notoriously difficult- to-transplant species, Bassuk said that container-grown trees are actually the safest best, because the entire root system is brought along/there's no root severing to trigger cavitation. However, Bassuk acknowledged that container-grown trees of the species and size arborists desire are not always readily available. Size: Said Bassuk, "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 caliper are especially at risk). For example, 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 experience, size trumps season and species and production method," said Bassuk. For this reason, she recommends that arborists, whether in spring or fall, consider planting B&B and bare root trees at 2-inch caliper or less; research shows they have higher establishment rates and catch up with or even outgrow their larger-caliper, more stressed counterparts. MITIGATING FALL HAZARD TRAITS Based on his review of the literature and his experience in NYC, coauthor and Director of Street Tree Planting for NYC Parks, Matthew Stephens created a table (seen here) that breaks down the four main traits for species presumed to be fall hazards and then gives advice to counteract these traits. His first piece of advice is to Harvest at the Right Time. During the fall, trees reach dormancy at different rates/on different schedules; depending on weather patterns, some trees go dormant very late in the fall. One reason that some tree species are thought to be fall dig hazards is that they have been harvested too early. An example of this can be seen with the genus Quercus, which goes dormant extremely late in the fall. A possible solution is to strategically procure trees from slightly more northern climates. For example, if Quercus were on a planting plan in the fall and timelines were tight, it would be wise to obtain trees from a reasonable northern source where there is an earlier digging season. This earlier digging could allow two to three weeks of additional time for planting ahead of locally grown trees. Regardless, Stephens believes it is absolutely critical — no exceptions — to ensure that trees are harvested in a dormant state. The reality is that an arborist's work timeline can drive when trees get dug in the nursery industry, and nursery people understandably want to secure sales. Sometimes nurseries would rather sell the tree, attaching to it a "fall hazard" caveat, than lose the sale if they wait to dig the tree during optimal conditions when the trees are fully dormant. If they want to do more fall planting, arborists and cities should consider adjusting installation schedules to the needs of the species they are planting — e.g., planting oaks late in the fall. The second mitigation strategy is to Root-Prune Coarse- Rooted Trees. Some trees that have been labeled fall hazards have coarse root systems that regenerate more slowly. The genus Nyssa has a tap-root-like root architecture when compared to Metasequoia, which has a much more fibrous root system. Research indicates that the coarse roots of scarlet oak (Q. coccinea) require approximately three to four times longer to regenerate than the more fibrous roots of pin oak (Q. palustris). Also, if a tree with coarse roots is harvested in the fall and a harsh winter comes along, freezing the soil early and consistently, the tree becomes more easily desiccated due to an inability to absorb and replenish water. Therefore, any coarsely rooted tree should be prepared for digging in the fall by root pruning the previous spring. Careful root pruning in the beginning of the growing season forces a more fibrous root system by fall harvest time. For example, a 2.0- to 2.25-inch-caliper tree could be prepared in spring by spading two or three sides of the root system as if it Tree Species Presumed to be Fall Hazards COMMON FALL HAZARD TRAITS Minimize Moisture Loss Thin Barked Trees Harvest Time Trees with Coarse Roots Acer rubrum X X Betula sp. X X X Carpinus sp. X X X X Celtis sp. X X Cercis sp. X X X Cercidiphyllum japonicum X X Cornus sp. X X Crataegus sp. X X Fagus sp. X X Halesia sp. X X Liquidambar styraciflua X X Liriodendron tulipifera X X Malus sp. X X X Nyssa sylvatica X X Ostrya virginiana X X X X Platanus sp. X X Pyrus sp. X X Quercus sp. X X X X Taxodium distichum X X Tilia tomentosa X X Zelkova serrata X X TABLE BY MATTHEW STEPHENS