Story by NYSUFC Editor Michelle Sutton
See also our 2015 post on this topic
This post gives a research update from Nina Bassuk and accounts of bare root transplanting success with specific species from Green-Wood Cemetery in Brooklyn and Tree Watertown.
In her research, Cornell Urban Horticulture Institute Director Nina Bassuk explores tree transplanting in all its facets: species selection, site preparation, production method (bare root, container, B&B), root desiccation and root circling considerations, caliper size, and aftercare. More recently, she added hydraulic conductivity (water uptake) to the conversation as she sought answers as to why certain species tend to develop cavitation (also known as embolism or air bubbles) in their xylem when they are exposed to a drought stress event.
How does studying water uptake add to the conversation around fall planting? “There’s this pervasive idea that certain species are inherently fall transplanting hazards,” Bassuk says. “Nurseries post lists of “fall hazards,” but these lists don’t take into account variables like tree production method, handling of fall-harvested trees, tree caliper size, and care of the trees after planting. We can also ask, how many of the species on ‘Fall Hazard’ lists fail because they are prone to cavitation and drought stress regardless of production method, and what can we do to overcome that? In our quest for tree biodiversity, we don’t want to rule out a long list of species that could, with due consideration, be successfully planted in fall.”
Note: At the end of this post are three helpful lists: tree species that, with proper consideration, Bassuk has found easy, moderate, or difficult to transplant bare root. These lists should be considered in tandem with the findings of Dr. Bassuk’s latest research, as summarized in this post, and the observations of other contributors to this post. Consider, too, your own planting site conditions and ability to achieve the bare root best practices as laid out in Creating the Urban Forest: The Bare Root Method.
In a June 2021 Tree Fund webinar, Why Do Some Trees Transplant Better Than Others? A Look at Some New Research on Water Uptake, Bassuk first lays out the advantages and disadvantages of the three main production methods, then presents on several extended research studies that have implications for transplanting success and for the notion of “fall hazards.” The studies measure shoot extension, leaf area, and hydraulic conductivity—three important indicators of successful establishment of trees. The latter is measured by a hydraulic conductance flow meter.
In the early 2010s, Bassuk and team transplanted containerized 2.5 inch caliper swamp white oak (Quercus bicolor, a famously easy to transplant oak) and bur oak (Q. macrocarpa, a notably difficult to transplant oak) in spring and fall. They measured hydraulic conductance of the fine roots before, and six months after, transplanting to see how well water was moving through the xylem. Both spring and fall transplanted Q. bicolor showed robust hydraulic conductance, but Q. macrocarpa’s water uptake declined significantly after six months, regardless of season. These findings correspond with field observations by Bassuk and urban foresters of the extreme ease (Q. bicolor) or difficulty (Q. macrocarpa) of transplanting these two species.
In a subsequent study, Bassuk and team experimented with small (3 gallon) and large (10 gallon) containerized Q. bicolor and Q. macrocarpa trees. Because pot-bound circling roots can be a disadvantage of the container growing method, the team introduced root pruning to compare with non-root-pruned controls. They found that, with root pruning, the 3-gallon trees showed a significant increase in water uptake, while the 10-gallon trees did not. This finding speaks to the observed ability of smaller trees to regenerate roots more readily than larger trees, take up water more readily, and therefore better establish after transplanting. It also speaks to the benefit of root pruning (please see Dr. Bassuk’s Tree Fund lecture for details about the specific root pruning technique used in this study.)
In a third study, Bassuk and team transplanted small (1.5 inch-caliper) and large (4 inch-caliper) bare root trees of six species in Ithaca in both spring and fall. They left a control group of the same trees at Schichtel’s Nursery, in place/undug. The six species were Q. bicolor and Q. macrocarpa, ironwood (Carpinus caroliniana), Kentucky coffeetree (Gymnocladus dioicus), tupelo (Nyssa sylvatica), and scarlet oak (Q. coccinea). The measurements taken beginning the growing season after transplanting were hydraulic conductance, shoot extension, and rate of photosynthesis.
For the smaller caliper (1.5 inch), spring-transplanted trees, hydraulic conductance was significantly better for the smaller-caliper trees than for the larger caliper (4-inch) trees; this was true across species. For fall-transplanted trees, the results were more mixed: smaller caliper trees still had better hydraulic conductance than larger caliper trees, but not to the same degree as with the spring-transplanted trees. Further, fall-planted trees showed more variation by species in hydraulic conductance.
Shoot extension was measured starting in the year after transplanting. Spring-transplanted, smaller caliper trees had greater shoot extension across species than larger caliper trees planted in spring. For fall-transplanted trees, the shoot extension results were more mixed, varying by species.
Spring-transplanted smaller caliper trees showed better rates of photosynthesis than spring-transplanted larger caliper trees. However, for fall-planted trees, the rate of photosynthesis was roughly equal between smaller and larger caliper trees.
In a related study, Bassuk and team transplanted three species (Q. bicolor, Q. coccinea, and Q. macrocarpa), balled-and-burlapped (B&B), at three different caliper sizes (1.5 inch, 2.5 inch, and 4 inch). They harvested and replanted the B&B trees at Schichtel’s Nursery to minimize the effects of soil conditions. Trees of the three caliper sizes were dug, balled and burlapped, and then replanted in the same nursery plot. This replant-on-site approach was to provide maximum control for site conditions, with the knowledge that the nursery soil and the nursery’s maintenance practices were ideal for tree growth.
The study used 54 trees altogether, six for each permutation of species x caliper. Some plants established and thrived, while others lived but showed significant dieback. The larger the tree, the more dieback was observed, and all six of the 4 inch-caliper Q. macrocarpa died. All six died, despite impeccable planting and maintenance, and ideal soil, at Schichtel’s Nursery.
The failure of all six 4 inch-caliper bur oak trees is no surprise; it fits with Bassuk’s decades of field observation. “Larger is not better,” Bassuk says. “If the tree’s too big, problems arise with water uptake—and even if the tree survives, you get dieback … not the thriving tree you want,” she says.
Bassuk emphasizes that regardless of production method, transplanting aftercare remains essential, especially in year one. “So even if you planted in spring and watered throughout the summer, we really recommend watering in the fall as well, because that’s when you get the most intimate contact between soil and roots,” she says. “Watering is the most important component of aftercare. How much water is needed varies on climate and year; in central New York where I live, we water newly planted trees at least twice a week when it’s dry.”
Seven Takeaways from Dr. Bassuk’s Research
- Size matters most: small plants are more easily transplanted successfully: roots take up more water, shoot growth and leaf area are increased.
- Container production is safest with difficult-to-transplant species because you have all the roots in the ball (except for root-bound plants—but root pruning increases water uptake).
- “Fall Hazards” have more to do with size than species. Planting at small caliper improves fall transplanting for most species and production methods.
- Poor site conditions exacerbate transplanting difficulties but good site conditions do not eliminate transplanting difficulties (witness the example of the large caliber bur oaks in perfect growing conditions that all died).
- Watering after transplanting (spring or fall) is critical for success. Proper mulching helps with this.
- The ability of roots to take up water is more important than the volume of roots, although greater volume of roots also contributes significantly to transplanting success.
- There are still things we don’t understand about species differences.
Tree Watertown was an early adopter of the Cornell bare root planting method. Watertown Planning and Community Development Director Mike Lumbis heard UHI Director Nina Bassuk speak about bare root at the 1996 ReLeaf conference in New Paltz. “The first year we tried bare root was 1997,” Lumbis says. “We could immediately see how much more affordable it would be for the City and how much easier it would be for the volunteers who come to the spring Rotary tree planting events and the fall Tree Watertown tree planting events.” Watertown gets its bare root trees from Schichtel’s Nursery and Chestnut Ridge Nursery, both of whom have hydrogel root dipping setups in place per the UHI bare root method protocol.
Lumbis works closely with City Planner and Urban Forestry Coordinator (and Council Board Member) Mike DeMarco, who was hired by the City in 2016. We spoke with DeMarco about recent bare root trends in Tree Watertown’s planting. In the fall of 2021, he selected 17 different taxa for the 50 trees that volunteers planted, and in the fall prior, he selected 20 different taxa for the 50 trees. All of the trees planted by volunteers in fall are bare root.
In terms of oaks, DeMarco has been eschewing ubiquitous red oaks in favor of other oaks with good to excellent tolerance of the stresses of the urban environment, like swamp white oak (Q. bicolor), bur oak (Q. macrocarpa) and ‘Urban Pinnacle’ bur oak, Crimson Spire oak (a hybrid of Q. robur x Q. alba), chinkapin oak (Q. muehlenbergii), Regal Prince oak (a hybrid of Q. robur f. fastigiata and Q. bicolor), and shingle oak (Quercus imbricaria). All are planted bare root at 1.25 or 1.5-inch caliper, and many of these are suitable for fall planting. DeMarco has found that chinkapin oaks really must have a drier site. “In wetter areas, they don’t withstand transplanting shock, we have found,” he says.
One of the selections DeMarco has been particularly excited about is the fruitless osage orange (Maclura pomifera) cultivar ‘White Shield’, which tolerates dry, windy, hot sites; high soil pH; occasional wet conditions; and salt spray. It has no serious pests or diseases. “It does well bare root and in fall,” he says. “I find it fascinating that a tree native to Oklahoma like osage orange is doing so well here in the most challenging sites,” DeMarco says. “It’s also very vigorous—for us, up to 36 inches a year of shoot growth— which means you do need to have a young tree pruning program or plan in place.”
Other trees that Tree Watertown plants successfully bare root and in fall include baldcypress (Taxodium distichum), dawn redwood (Metasequoia glyptostroboides), paperbark maple (Acer griseum), eastern redbud (Cercis canadensis), and katsura tree (Cercidiphyllum japonicum).
Green-Wood Cemetery’s Bare Root Successes
Green-Wood Director of Horticulture and Curator (and Council Board Member) Joseph Charap started planting bare root trees in the iconic Green-Wood landscape in 2017. He says that he was inspired by Nina Bassuk’s advocacy for the utility of the bare root method approach; in recent years, Green-Wood has focused on using bare root trees to rapidly and effectively reestablish canopy for a variety of reasons, including replacing trees lost to storms.
Charap says, “Bare root has proven to be an immensely successful, rewarding approach for us, with much, much less mortality. This year, from a successful application with the DEC’s Urban and Community Forestry Grant program, we are primarily using bare-root material as the dominant approach to plant throughout our landscape.” For evergreens, Green-Wood still relies on B&B material, because they are never in a state of dormancy and as such, are more prone to desiccation.
Thus far, Green-Wood has planted 174 bare root trees in their effort to do succession/storm recovery planting. They’ve stayed in the 1 to 2 inch-caliper range, with a rare 2.75 inch-caliper selection, and they plant in both spring and fall. “With bare root trees, we can plant near mature, senescing trees without unduly disrupting the big tree root systems,” Charap says. “Our carbon footprint is also much lower when we plant bare root. We don’t need any machinery to plant bare root, and the shipping of bare root trees is less costly because they weigh so much less than B&B trees.”
In 2017, J. Frank Schmidt & Son Co. Nursery donated bare root trees of their own introductions to Green-Wood: Emerald City tulip tree, Royal Raindrops crabapple, First Blush cherry, Pink Flair flowering cherry, and American Dream swamp white oak. They all established well and are performing beautifully. Charap also connected with Brotzman’s Nursery and eventually Schichtel’s Nursery and is now ordering bare root almost exclusively.
The few failures he’s seen with bare root were with very small caliper yews (Taxus sp.), but when one of the four survived and flourished, the payoff was still there because they were so affordable ($35/each) and easy to plant. He also lost one of two bare root baldcypress (Taxodium distichum)—but the remaining one is doing great. “We’ve used Nina Bassuk’s Bare Root Method guide to help with our selections, but we occasionally plant species that aren’t recommended for bare root, because we can afford to trial and evaluate,” Charap says.
Some of the more unusual trees Charap has tried out successfully bare root include a Liriodendron tulipifera x chinensis and a Tilia oliveri. He shared with us his bare root order for 2022, which includes more hybrid tulip trees, yellow buckeye (Aesculus flava), Mandarin Jewel Kousa dogwood (Cornus kousa), Farges filbert (Corylus fargesii), hawthorn (Crataegus phaenopyrum ‘Ohio Pioneer’), sweetgum (Liquidambar styraciflua ‘Worplesdon’), Chinese cork oak (Quercus variabilis), striped maple (Acer pennsylvanicum ‘Joe Witt’), sugar maple (A. saccharum ‘Powder Keg’), horsechestnut (Aesculus x arnoldiana ‘Autumn Splendor’), yellow birch (Betula alleghaniensis), bitternut hickory (Carya cordiformis), yellowwood (Cladrastis kentukea), Kentucky coffeetree (Gymnocladus dioicus ‘Prairie Titan’), Carolina silverbell (Halesia carolina), butternut (Juglans cinerea), osage orange (Maclura pomifera ‘Witchita’), Cucumber magnolia (Magnolia acuminata), swamp white oak (Quercus bicolor), American linden (Tilia americana), and elm (Ulmus ‘St. Croix’). In a future update, we can learn how these selections fared. 🌳
Trees Easy to Plant Bare Root
Acer campestre Hedge Maple
Acer x freemanii Freeman Maple
Acer miyabei Trident Maple
Acer platanoides Norway Maple
Acer pseudoplatanus Sycamore Maple
Acer rubrum Red Maple
Acer saccharum Sugar Maple
Acer truncatum Shantung Maple
Catalpa speciosa Northern Catalpa
Cercidiphyllum japonicum Katsura Tree
Cladrastis kentukea Yellowwood
Cornus mas Cornelian Cherry Dogwood
Cornus foemina Gray Dogwood
Fraxinus spp. Ash
Gleditsia triacanthos Honeylocust
Gymnocladus dioicus Kentucky Coffee Tree
Koelreuteria paniculata Goldenrain Tree
Maackia amurensis Amur Maackia
Malus spp. Crabapple
Parrotia persica Persian Parrotia
Platanus x acerifolia London Plane Tree
Prunus ‘Accolade’ Accolade Flowering Cherry
Prunus sargentii Sargent Cherry
Prunus serrulata Japanese Flowering Cherry
Prunus virginiana ‘Canada Red’ Chokecherry
Pyrus calleryana Callery Pear
Pyrus ussuriensis Ussurian Pear
Quercus bicolor Swamp White Oak
Quercus palustris Pin Oak
Quercus rubra Northern Red Oak
Robinia pseudoacacia cultivars: Black Locust ‘Purple Robe’, ‘Pyramidalis’,
Globosum’, ‘Bessoniana’, ‘Twisty Baby’
Sorbus alnifolia Korean Mountain Ash
Sorbus intermedia / aria Whitebeam Mountain Ash
Syringa reticulata Japanese Tree Lilac
Tilia americana Basswood
Tilia cordata Littleleaf Linden
Tilia euchlora Crimean Linden
Ulmus americana and elm American Elm hybrids
hybrids except ‘Frontier’
Trees Moderately Difficult to Transplant Bare Root
Alnus glutinosa European Alder
Amelanchier spp. Serviceberry
Betula spp. Birch
Celtis occidentalis Hackberry
Cercis canadensis Redbud
Corylus colurna Turkish Filbert
Crataegus crus-galli inermis Thornless Cockspur Hawthorn
Crataegus viridis ‘Winter King’ Winter King Hawthorn
Eucommia ulmoides Hardy Rubber Tree
Prunus subhirtella var. autumnalis Higan Cherry
Quercus robur English Oak
Tilia tomentosa Silver Linden
Zelkova serrata Japanese Zelkova
Trees Difficult to Transplant Bare Root
Carpinus spp. Hornbeam
Crataegus phaenopyrum Washington Hawthorn
Ginkgo biloba Ginkgo
Liriodendron tulipifera Tulip Tree
Nyssa sylvatica Tupelo
Ostrya virginiana American Hophornbeam
Quercus alba White Oak
Quercus coccinea Scarlet Oak
Quercus imbricaria Shingle Oak
Quercus macrocarpa Bur Oak
Quercus muehlenbergii Chinkapin Oak
Quercus prinus Chestnut Oak
Taxodium distichum Baldcypress
Ulmus ‘Frontier’ Frontier Elm
Ulmus parvifolia Lacebark Elm