The Campaign for the Living Collections kicked off last fall with plant collecting trips to China and Idaho, which Curator of Living Collections Michael Dosmann and Manager of Plant Records Kyle Port wrote about in the last issue of Arnoldia. Once newly acquired fruits, seeds, cuttings, divisions, and plants arrive at the Arboretum, the production staff at the Dana Greenhouses takes over. In this issue, Manager of Plant Production Tiffany Enzenbacher and Plant Propagator John H. Alexander III describe the process of shepherding new accessions from the greenhouse bench to final production nurseries, the last step before plants move to a permanent location on the Arboretum grounds.
Plant propagation historically has been recognized as an integral component of the Arnold Arboretum’s mission. In fact, the Arboretum’s second employee (inaugural director Charles S. Sargent was the first) was plant propagator Jackson Dawson, hired in 1873, the year after the Arboretum was established. Since Dawson was Sargent’s only employee, he served not only as the propagator but also the superintendent (Geary and Hutchinson 1980) and remained with the Arboretum until his death in 1916. The Arboretum’s other long-term and influential propagators—William H. Judd (employed from 1913 to 1946), Alfred J. Fordham (1929 to 1977), and John H. Alexander III (1976 to 2016)—and shorter-term propagators followed suit after Dawson. Through horticultural expertise, experience, and old-fashioned trial and error, they coaxed seeds to germinate and cuttings to grow roots, successfully propagating taxa novel to New England and North America. The propagation facilities have moved five times over almost a century and a half and have seen many exciting horticultural accomplishments by Arboretum propagators and production staff.
In 1873, the Arboretum shared growing space with the Bussey Institution, then relocated in 1886 to a small land plot and 20-foot by 50-foot greenhouse on the property at 1090 Centre Street, where Dawson then resided (Geary and Hutchinson 1980). These modest accommodations were soon outgrown and a new greenhouse was constructed on Orchard Street across from the Arboretum (off of the Arborway) in 1917 (Howard 1962). With intensified Arborway traffic and road widening, production was moved back to land adjacent to the Bussey Institution in 1928. As additional space needs arose, along with the desire for a more up-to-date building, ground broke to construct the Charles Stratton Dana Greenhouses in 1961. The donation for the Greenhouses was provided by Mrs. William R. Mercer (née Martha Dana), and was named in honor of her father, Charles S. Dana (Howard 1962). This complex houses the present facilities, including specialized equipment and environments for seed, cutting, and grafting propagation, greenhouse and outdoor bench space for containers, an evaluation nursery, three longer-term nurseries, a cold storage building for overwintering containers, and the Bonsai and Penjing Pavilion.
Through facility relocations and many staff changes in the years since the Arboretum’s inception, plant propagation and production have remained center stage. The current ten-year Campaign for the Living Collections (Friedman et al. 2016), which focuses on acquiring nearly 400 wild-collected plant taxa, will assuredly keep propagation in the limelight well into the future. The Campaign’s list of desiderata features taxa selected because they increase the phylogenetic and biological breadth of Arboretum collections, belong to geographically disjunct clades, are marginally hardy or threatened in the wild, or can be used to create a “living type specimen” in genomic research.
Last September, the Dana Greenhouses staff received 100 new accessions (seeds, cuttings, plants) from expeditions related to the Campaign. Seeds from many accessions have already germinated, and others such as paperbark maple (Acer griseum) may take several cycles of warm and cold stratification to germinate uniformly. We look forward to transitioning individuals through the phases of production here at the Dana Greenhouses, with the end goal of having plants in their permanent locations in the living collections for researchers to study, children to learn from, and the public to enjoy.
Propagation Material Arrives
In autumn, as plants in the living collections are slowing in growth and their foliage begins to abscise, the “growing season” in the Dana Greenhouses is just commencing. Production staff is overwhelmed with anticipation about what seeds, fruit, cuttings, and plants we will be receiving from foreign and domestic expeditions. However, once the highly sought-after fruit or cutting has been harvested from its parent and is now at long last in the hands of an Arnold Arboretum explorer, its trip to the Arboretum’s greenhouses is nowhere near complete.
As Curator of Living Collections Michael Dosmann and Plant Records Manager Kyle Port (2016) explained in the last issue of Arnoldia, the United States Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) requires a specialized permit to import foreign seeds into the United States. This permit allows for the importation of a small quantity of seeds, pending a successful evaluation for hitchhikers—noxious weed or parasitic plant seeds, insect pests, or pathogens of concern. The Arboretum typically has seeds routed to the APHIS Plant Protection and Quarantine inspection station at John F. Kennedy International Airport in Jamaica, New York. Because the several day to weeklong inspection process is so complex and vital, foreign seeds have to be clean (removal of fruit surrounding seeds), properly labeled, and limited to only 50 seeds (or 10 grams [0.35 ounces]) per package. Should the scrutinizing agent discover any unwanted travelers on the coveted soon-to-be Arboretum seeds, the entire content of the package fails to pass the test, and the voyage for that seed lot ends there.
Because much time is spent to meticulously clean the seeds and package them correctly in the foreign country, the majority pass through inspection and are then shipped on to the Arboretum, where the true journey through the production system begins. On occasion, this step is skipped and seeds are shipped directly to the Arboretum from a foreign country. Since the inspection process is required by law and is essential in mitigating the introduction of invasive and/or threating agents to agriculture and the environment, greenhouse staff sends the material to APHIS to be inspected prior to any germination treatment.
If domestic fruits (berries, capsules, samaras, etc.), cuttings, or plants are acquired, such as materials that Kyle Port collected on his expedition to Northern Idaho last fall, they are shipped directly to the greenhouse. It should be noted that obtaining material from expeditions is not the sole means by which the greenhouse procures plants. Propagules and plants are also obtained by several other methods: through Index Seminum (seed list) exchanges offered by botanical institutions, from other gardens or arboreta, or by purchasing from nurseries (particularly when acquiring cultivars). However, upon receiving any new seed, cutting, or plant, no matter what it is or where it is from, the first step that production staff takes on is accessioning.
Similar to all museums, the Arboretum has a number classification system in place so that each plant can be treated as a specimen with a unique, recognized background. The accession number is composed of a number-year unit. For example, the number 274- 2015 signifies the 274th plant material lot received in 2015. For every accession, abbreviations such as SD (seed), CT (cutting), PT (plant) denote the form of material received.
After a seedling has rooted into its growing container, the next phase through the production system beckons. The Shade House, true to its name, is covered by woven polyethylene fabric that allows only 45% of the light to pass through. This keeps the vulnerable plants less stressed after transplant. Seedlings and small cuttings or plants are transplanted into the highly organic soil of this evaluation nursery in late spring to early summer and are well tended throughout the season. Plants are mulched in and hand watered until established. There is an overhead sprinkler system for irrigating the entire nursery when necessary. Rodents have been problematic, occasionally damaging all individuals within an accession, so caging plants that they appear to be most attracted to such as horse chestnut (Aesculus), hickory (Carya), and oak (Quercus) has become mandatory in recent years.
The Shade House also offers a first test of cold hardiness. Since the vast majority of Arboretum accessions funnel through here, and because the greenhouse area is in a recognized cooler microclimate of the Arboretum (Dosmann 2015), it provides a rudimentary assessment of hardiness. However, if a species is known to be marginally hardy, one to several individuals may be containerized instead of being planted in the Shade House. Those individuals would then subsequently be planted in a warmer microclimate of the Arboretum to increase their likelihood of survival during typical Zone 6 (average annual minimum temperatures -10 to 0°F [-23.3 to -17.8°C]) Boston winters. Along with hardiness, seedlings are also evaluated for form and vigor.
Heading to the Collections
After the individuals in an accession are large enough to transplant, shrubs get containerized and trees continue their journeys through the facility into one of three longer term nurseries.
After entering the production facility as a propagule, trees take anywhere from five to seven years in the system before they are robust enough to be transplanted into the living collections. Shrubs are at the greenhouse for three to five years on average. The voyage of an accession through the Dana Greenhouses concludes when the individuals are planted into their sited location out on the grounds. Now a new, much longer passage of life begins.
The Campaign for the Living Collections is now in its second year and it is already providing greenhouse staff with exciting and challenging opportunities to germinate seeds, root cuttings, and grow-on wild-collected species that are new to the Arboretum as well as previously attempted taxa. The Campaign has reinforced the importance of horticultural research and reasserts that propagation is very much center stage, even as we near our 2022 sesquicentennial. As autumn is fast approaching and new collecting expeditions will soon start, we are once again awaiting the propagules that will be beginning their journey through the production system. We can only imagine that this is how Dawson felt during Wilson’s 1907 to 1909 expedition to China, eager to receive the 2,262 seed collections and 1,473 collections of live plants or cuttings that resulted from the trip.
Dosmann, M. 2015. The History of Minimum Temperatures at the Arnold Arboretum: Variations in Time and Space. Arnoldia 72(4): 2–11.
Dosmann, M. and K. Port. 2016. The Art and Act of Acquisition. Arnoldia 73(4): 2–17.
Friedman, W. E., M. S. Dosmann, T. M. Boland, D. E. Boufford, M. J. Donoghue, A. Gapinski, L. Hufford, P. W. Meyer, and D. H. Pfister. 2016. Developing an Exemplary Collection: A Vision for the Next Century at the Arnold Arboretum of Harvard University. Arnoldia 73(3): 2–18.
Geary, S. C. and B. J. Hutchinson. 1980. Mr. Dawson, Plantsman. Arnoldia 40(2): 51–75.
Howard, R. 1962. The Charles Stratton Dana Greenhouses of the Arnold Arboretum. Arnoldia 22(5–6): 33–47.
Wilson, E. H. 1930. Bulletin of Popular Information. The Arnold Arboretum of Harvard University (series 3, vol 4, no 10): 37–40.
Wilson, E. H. 1906. Some New Chinese Plants. Bulletin of Miscellaneous Information. Royal Botanic Gardens, Kew. 1906(5): 147–163.
Wyman, D. 1960. Plants of Possible Merit? Arnoldia. 20(2): 9–16.
Tiffany Enzenbacher is Manager of Plant Production at the Arnold Arboretum. Plant Propagator John H. Alexander III recently retired from the Arboretum after 40 years of service.
Enzenbacher, T., and Alexander, J. H. III. 2016. A concise chronicle of propagation. Arnoldia, 74(1): 2–13.