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.
Seeds from our own staff collectors, collaborators, and other gardens never arrive in those colorful packets seen on garden center display racks. Our seeds may arrive in small, reseal- able polyethlene bags, coin envelopes labelled in beautiful cursive writing, or sheets of paper neatly folded into packets. All will be carefully handled as they enter the propagation process.
The first step is examination, since occasionally those packets contain more than seeds. Fruit remnants, cones, and chaff may arrive with the seeds, plus the occasional weevil or other insect. Collections made in foreign countries are thoroughly cleaned before being shipped since they will have to pass an inspection by APHIS (see page 4). Collections made within the United States by our own staff are seldom cleaned before being shipped back to the Arboretum, so at the greenhouse we often get to unpack boxes full of polyethylene bags containing rotting and fermenting fruits. Seeds from other arboreta and botanic gardens, be it foreign or domestic, are usually neatly cleaned and packaged.
Not every seed in every packet will germinate, though. We once obtained a half kilogram (about a pound) of wild-collected Chinese sweetgum (Liquidambar acalycina) seeds, but ended up with only a tablespoon of viable seeds while the rest were undeveloped. Anyone unfamiliar with sweetgum seeds could easily make this mistake since sweetgum fruits often hold more undeveloped seeds than sound seeds. Careful visual inspection may help determine sound from unsound seeds, but not always. For example, bald cypress (Taxodium distichum) seeds are not uniform and could easily be tossed out with the cones.
Before sowing, plant propagators routinely remove all that is not seed (e.g., fruit pulp, capsules, cones) because it is likely to host fungi, attract insects or rodents, or, in the case of fruit pulp, inhibit seed germination. Cleaning may involve soaking, drying, sieving, or a combination of these and other techniques. Sometimes seeds and chaff are all so tiny, and separating the two so difficult, that it only makes sense to clean reasonably well and sow it all.
Freshly collected seeds generally germinate in higher percentages than stored seeds so we go to work quickly once seeds arrive. Before sowing the cleaned seeds we need to know the best protocol for germination for that particular species. For many plants, past experience or a search of seed propagation reference materials provides well-established protocols for ger- mination variables such as soil temperature, day length, or light/dark requirements. Seeds of most temperate zone species require cold stratification, which simulates winter conditions, and will germinate in higher percentages if they first experience 30 to 120 days at tempera- tures just above freezing. We routinely place seeds into polyethylene bags containing a moist, well-drained medium and refrigerate at 40°F for 90 days.
The seeds of some species need both warm and cold stratification periods. Examples include paperbark maple (Acer griseum) and related trifoliate maples, the dove tree (Davidia involucrata), and most viburnums (Viburnum spp.). And there are also many species whose seeds don’t strictly require cold stratification (heath family [Ericaceae] members, for example) but they germinate more uniformly and in higher percentages if first given a one month cold stratification so we often opt for that treatment.
Another obstacle for germination in some seeds is the presence of an impermeable seed coat. Plants in Fabaceae, the pea family, often have impermeable seed coats, so we typically scarify seeds of any fabaceous species, whether known or new to us, by rubbing on sandpaper or a file. Scarified seeds are then soaked in water; if they “imbibe” and swell to about twice their size, they are ready to be sown or stratified. For all seeds, imbibition is the first step in germination (and why garden seed packets always exhort gardeners to “keep soil moist after sowing”).
Keeping records is an essential part of plant propagation. To track germination percentages and successful protocols, we count seeds (or make a close estimate) before they are sown. Once the number of seeds is known and a protocol has been determined, we begin the specified treatment. With species that haven’t been grown before at the Arboretum or for which no established protocol can be found, we may experiment and try a variety of treatments if there are plenty of seeds. If there are only a few seeds, we rely on experience and best judge- ment to pick a treatment.
Once stratifications (if needed) are complete, seeds are sown in flats and placed in a warm, humid greenhouse with the option of supplemental lighting. The best time to sow seeds is in the early spring but that timing isn’t always possible, so supplemental lighting allows us to lengthen the photoperiod to simulate the longer days of spring and summer. When seedlings reach sufficient size they are potted up in individual containers, ready to continue through our production system. Modern technology has changed many greenhouse peripherals—we now use LED lights, thermostats, soil heating mats, and precise irrigation—but nature’s requirements for seed germination haven’t changed, and we accomplish that in much the same way as did the Arboretum’s earliest propagators.
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.
From “free” to “friend”…
Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.
It takes resources to gather and nurture these new voices, and we depend on the support of our member-subscribers to make it possible. But membership means more: by becoming a member of the Arnold Arboretum, you help to keep our collection vibrant and our research and educational mission active. Through the pages of Arnoldia, you can take part in the life of this free-to-all landscape whether you live next door or an ocean away.