Past Tree Mobs™
The Snow Queen of Summer
Come July, many ornamental trees and shrubs have flowered and are putting their energy into seed development. Not so for oak leaf hydrangeas. The heat hits and they send forth showy panicles of creamy white contrasted against textured, deep green leaves. Hydrangea quercifolia ‘Snow Queen’ ranks high on Supervisor of Horticulture Andrew Gapinski’s list of favorite shrubs. The group met at Acc. # 318-94*A.
It’s chilly out there…for the Mountain Hemlock
Mountain hemlock (Tsuga mertensiana) is native to mountains of the western US, where it can be covered in snow for up to 10 months of the year. It’s hard to imagine now that summer is finally here in New England, but much of mountain hemlock’s native habitat won’t be free of snow until mid-July or later! Putnam Fellow Ailene Ettinger spoke about this species’ adaptations to cold environments, how it may be affected by climate change in the future, and how research like hers, conducted in natural habitats and among the collections at the Arnold Arboretum can help us understand how this and other tree species might adapt to climate in flux. The group met at Acc. # 693-77*A.
Clematis: What a plant will do for love
The beauty of clematis may trick you into believing that these spectacular and bizarre flowers were bred to ornament your lampposts and trellises, but in fact, they evolved to attract bees. There are over 300 species of clematis that display incredible natural diversity of flower shape, color, and smell. This explosion of diversity attracts bee pollinators from the tropics to the sub-arctic. The seemingly endless malleability of form and color has enabled the creation of hundreds of cultivated varieties that now accent yards around the world. The Arnold Arboretum holds an exemplary collection of both natural species and cultivated forms of clematis. The group met at the heart of this collection in the Leventritt Shrub and Vine Garden at Acc. # 372-2003, Clematis viticella ‘Betty Corning’, and heard from Harvard Assistant Professor of Organismic and Evolutionary Biology Robin Hopkins about form, color, and the science of attraction.
An American All-star: Yellowwood
As one of the seldom-planted but more beautiful flowering trees native to North America, yellowwood, Cladrastis kentukea, deserves attention. Taking up to 20 years to first flower and only producing good shows every two-to-three years, it is a tree for the patient and committed gardener. Curatorial Fellow Jordan Wood spoke about its discovery by André Michaux in 1796 and the Carolina expeditions of Michaux and his son François André. The group met at Acc. # 13055*B, a broadly spreading specimen accessioned into the Arboretum’s collections in 1881.
You say Catalpa, I say Catawba!
The Arnold Arboretum’s catalpa collection holds 39 accessions, representing two species native to North America (Catalpa bignonioides and C. speciosa), three species that hail from China (C. ovata, C. bungei, and C. fargesii), and some cultivars. Often referred to as bean trees for their 6- to 16-inch slender fruits, they reveal their glory in June with clusters of sweetly scented flowers. Arboretum Director William (Ned) Friedman spoke about the catalpa collection and intercontinental (eastern Asia/eastern North America) disjunct species, so characteristic of many of the fine collections at the Arnold Arboretum. The group met at Acc. # 17664*A, Catalpa fargesii, a rarity in its native habitat and in horticulture.
Each year the many species of Amelanchier bring dramatic floral displays to New England in May and June. Amelanchier nantucketensis, the Nantucket shadbush, serves as a fascinating study in the interplay of natural hybridization, conservation, plant reproduction, and land use in New England. Arboretum scientist Dave Des Marais discussed the role that the Arboretum plays in preserving rare species, and how Amelanchier nantucketensis is serving as a valuable tool to understand the genetics and ecology of natural populations. The group met at the centerpiece of our amelanchier collection, Acc. # 934-89*A.
Malicious men may die, but [Malus] never.
I’ve toyed with Moliere’s quote about malice from Tartuffe in this Tree Mob’s title, however there may be some truth to my version. Apples (Malus), thought to have originated in central Asia, have been around for upwards of 7 million years. Visiting researcher and recipient of the Jewett Prize, Ling Guo, is a horticulturist, professor, and curator in the Department of Plant Introductions at the Beijing Botanical Garden. Ling has spent several months studying the Arnold Arboretum’s crabapple collection on Peters Hill and talked about her research. The group met at Acc. # 1645-52*A, Malus x arnoldiana.
Distinctively Deciduous Conifers
Deciduous gymnosperms that used to dominate vast areas of the earth in ancient times went through a dramatic reduction in species richness and ecological dominance due to climate change and competition from angiosperms. Now, extant conifers are overwhelmingly evergreen. Metasequoia glyptostroboides is one of the few representatives of the extant conifers with deciduous leaf habit. For this tree mob, Putnam Research Fellow Guang-You Hao talked about the ecology of deciduous conifers, which may provide clues to their extinction and habitat shrinkage as well as their persistence under certain conditions. The group met at Acc. # 524-48*Z.
It’s a Tree. It’s a Shrub. It’s Alnus!
The flower catkins paired with the pine-like pseudocones of the genus Alnus are a charm in the late winter and early spring months in Boston. The Arnold Arboretum’s diverse collection of Alnus showcases the variability of species from East Asia to Europe, and locally within Massachusetts. In this Tree/Shrub Mob, Curatorial Fellow Joyce Chery highlighted the world distribution, ecology, and lifecycle of this fascinating group—cousins to the birches—as well as some associated taxonomic horrors. The group met along Willow Path, where most of the Arboretum’s Alnus collection is situated, at Acc. # 125-2005*B, Alnus incana ssp. rugosa.
Conjuring spring with Hamamelis vernalis
In New England, it is a bit magical to see a plant flowering through the dead of winter and on into spring. Such is the beauty of Hamamelis vernalis, typically in bloom from January to March, allowing us to say, “There’s always something in bloom at the Arboretum.” Visitor Educator Maggie Redfern spoke about the horticultural history of the type specimen of Ozark witch hazel. The group met beside Rehder Pond at Acc. # 6099*D.
Sargent’s Weeping Hemlock
Contradictions, lost or unrecorded facts, and misstatements attributed to Sargent’s Weeping Hemlock (Tsuga canadensis f. pendula) shroud the plant’s origins in mystery. Senior Research Scientist Peter Del Tredici illuminated what has been discovered about this plant by digging into historical records, speculating about locations, and in some cases, taking core samples to count rings. The group met at Acc. # 1514-2*A, a sculptural beauty, on Hemlock Hill Road.
Techno Mob: Archaeological Mapping
Professors and students of Boston University’s Department of Archaeology are spending a few days this fall semester at the Arnold Arboretum to study the remains of historic archaeological sites. The team is using a variety of surveying equipment and geophysical instruments to map remains both on and below the ground surface. Professor Chris Roosevelt and his students met at the site of the former Bussey Mansion to learn about techniques used today to map and interpret what remains on the land. The group met on Beech Path at Acc. # 655-93*A, sugar maple (Acer saccharum).
Seduction and Dispersal: Euonymus Species
Arnoldia editor Nancy Rose spoke about seed dispersal in general and specifically about the fruiting structure of euonymus species. Nancy showcased some of the species in the Arboretum’s collection and compared native and non-native characteristics and discussed issues of invasiveness. The group met on Bussey Hill Road near its intersection with Valley Road at Acc. # 1518-83*A, Euonymus europaeus.
The Bitter and the Sweet of Vines
Research conducted by Putnam Fellow Stacey Young in the collections is flagged by blue tape that you may have noticed as you wander the Leventritt Shrub and Vine Garden. There, and elsewhere in the landscape, Stacey is examining biological, morphological, and ecological traits of both North American and Asian temperate lianas (woody vines). Come learn about the ecology of invasiveness and decline within the bittersweet genus, and in general, consider how vines climb. The group met in the Shrub and Vine Garden at Acc. # 269-95*A, anglestem bittersweet (Celastrus angulatus), a non-invasive (we believe) Asian relative of the highly invasive oriental bittersweet (Celastrus orbiculatus).
Actinidia polygama: The Next Superfood?
Rachel Brinkman, Horticulture Apprentice, has been keeping an eye on the various hardy kiwi vines (Actinidia sp.) in the Leventritt Garden, which displays seven different species. She will speak about Actinidia polygama, silver vine, whose fruit might be ripe for sampling. The group met in the Leventritt Garden at Acc. # 460-83*B and learned about this demure but vitamin-rich relative of the kiwifruit commonly found in grocery stores.
Landscape with Edibles: Prunus maritima
Kristina Jones, Director of the Botanic Gardens and faculty member in Biology and Environmental Studies at Wellesley College, spoke about beach plum, Prunus maritima. Interested in ecological approaches to growing food such as edible forest gardens, and partnering with permaculture experts to analyze productivity and sustainability of designed edible ecosystems, she spoke about the food value and horticultural uses of this stone fruit native to the eastern seaboard from New Brunswick to Virginia. The group met at Prunus maritima forma flava, Acc. # 262-92*C on Peters Hill.
A Landscape Transformed
In recent years we have assigned a group project to our Hunnewell Horticultural Interns as culmination of their experience in the Arboretum’s landscape. This year’s interns concentrated on an area just below the Explorers Garden, along Oak Path. Intern Joe Leonard spoke about the group’s design and process for transforming a once unwelcoming and challenging-to-manage area of the Arboretum. The group met on Oak Path near its intersection with Beech Path.
Cattail leaves stand nearly vertical and can be up to 6 feet tall. How do they do it? Professor Lorna Gibson from the MIT Cellular Solids Group spoke about the sandwich structures within these reeds that provide stiffness and strength and explained how such structures work in both plants and engineering. The group met along Meadow Road at the edge of the meadow.
The Seaside Alder
Alnus maritima, the seaside alder, is a bit of rarity, and our specimen of this plant is truly unspectacular. However, its story is intriguing. Endemic to the United States, it is found in three populations, remnants from a time when the plant was perhaps more widespread. Matt Jones, a 2013 horticultural intern at the Arboretum with an International Master’s in Horticultural Science and a Master of Science in Botany from the University of Oklahoma, spoke about this member of the Betulaceae. The group met at Acc. # 1217-4*A.
Get to know the oft forgotten pawpaw, Asimina triloba, with the largest edible fruit native to North America. It is the only species of the Annonaceae (a tropical family that includes cherimoya, guanabana, and soursop) that is hardy in New England. Research associate Iñaki Hormaza, visiting from Spain, spoke about this humidity-loving small tree. Meet at Acc. # 205-91*A along Peters Hill Road.
Dragonflies had wingspans of 60 centimeters 300 million years ago. Today, they are some of nature’s most accomplished fliers, spinning and flipping as they catch mosquitoes, flies, spiders, and more. This Insect Mob focused on dragonflies, their life cycle, flight patterns, and vital ecological importance. Led by Mary Salcedo, Harvard University Graduate Student in Organismic and Evolutionary Biology, the group met at Rosa virginiana, Acc. # 106-2006*A.
Eastern Cicada Killers
Straying from our woody plant theme this week, we learned about the Arnold Arboretum’s resident population of Eastern cicada killers, Sphecius speciosus. Birder, photographer, and volunteer Bob Mayer spoke about the life cycle and habits of this native insect on Forest Hills Road near the weeping cherries, east of Dawson Pond, at Acc. #: 290-87*A, Prunus subhirtella ‘Pendula’. [pdf]
Clethras: Sweet Scents of Summer
The group met in the Leventritt Shrub and Vine Collection for a comparison of Clethra species, both native and exotic, and their cultivars. Propagator Jack Alexander spoke about habitat, floral characteristics, and what he would look for in the quest to find the ideal summersweet shrub in the wild. The group met at Acc. #: 287-96*A. [pdf]
Lime or Linden: Tilia cordata
Michael Dosmann, Curator of Living Collections, told the story of a cultivar of Tilia cordata, collected by dendrologist and longtime Arboretum staff member Alfred Rehder, whose birthday was 150 years ago this year. The group met at the summit of Peters Hill at Acc. #: 12112*A. [pdf]
Conifer aficionado Dennis Collins, Horticultural Curator at Mount Auburn Cemetery, talked about some of the reasons why Taxus, a genus of long-lived conifers, have captivated humans for religious, military, medicinal and horticultural reasons over the past few thousand years. The group met at Acc. #: 169-99*B, Taxus cuspidata. [pdf]
Plants and Climate Change
Boston University professor Richard Primack used Pinus aristata, bristlecone pine, to illustrate the timing of leaf-out as related to climate change. Dr. Primack has been using Thoreau’s notebooks, Arnold Arboretum herbarium specimens, and other historical records to compare leafing and flowering times through the past century to the present. The group met in the dwarf conifer collection at Acc. #: 721-61*A. [pdf]
Chionanthus Here and There
Nancy Rose spoke about disjunct species Chionanthus retusus and C. virginicus. These fringe trees are found on widely separated continents, one in Asia and the other in North America. She compared their similarities and differences, and considered how they have evolved and adapted in their disparate homes. The plants are located in the Explorers Garden on Bussey Hill at Acc. # 13051*A. [pdf]
Japanese Azaleas: Rhododendron molle ssp. japonicum
Join longtime Arboretum volunteer and rhododendron aficionado George Hibben to learn about the qualities of Japanese azaleas that endear them to hybridizers, with focus on the Arboretum’s Rhododendron molle ssp. japonicum. Meet at Acc. #: 28-98*A. [pdf]
Mutants in Our Midst
Ever wonder how new horticultural forms and varieties come to be? For example, how does a green-leafed shrub morph into a gold-leafed variety, or a white-flowered plant develop from a species with magenta blooms? Join Ned Friedman, Director of the Arnold Arboretum, for the first of three mobs he will lead this spring exploring naturally occurring plant mutations that are the source of most of our horticultural wonders. Meet at Acc. #: 10-68*B, Cercis canadensis, commonly known as redbud. [pdf]
Pyrus pyrifolia, Sand Pear
Senior Research Scientist Peter Del Tredici spoke about one of our older specimens, Pyrus pyrifolia, Acc. #: 7272*C. Native to China, Japan, and Korea, this plant was grown from seed that was collected in Hubei Province, China by E.H. Wilson in 1907. A standout in the Arboretum’s landscape, this plant has provided multi-season beauty on Bussey Hill for more than a century. [pdf]
Lilacs: Syringa oblata and Syringa x hyacinthiflora Hybrids
Lilac season has begun with the earliest of the lilacs coming into full bloom. Plant propagator and lilac expert Jack Alexander introduced us to Syringa oblata and told the story of the creation of the first cultivar of Syringa × hyacinthiflora. Met in the lilac collection on the slope of Bussey Hill at Syringa oblata, Acc. #: 381-2001*A. [pdf]
Dwarf Sweet Box, Sarcococca humilis var. digyna
Sue Pfeiffer, horticultural technologist, explained Sarcococca humilis var. digyna, Accession #: 667-2003*MASS a diminutive, sweet-scented evergreen groundcover, native to China. Sue will also discuss other members of the Buxaceae on display in the Leventritt Shrub and Vine Garden. [pdf]
Jasminum nudiflorum, Winter Jasmine
Brian Leib, manager of the Arboretum’s research greenhouses at the Weld Hill Building, spoke about this species from northern China in the Oleaceae. Grown as a ground-covering shrub or trained as a vine, winter jasmine’s yellow flowers signal the coming of spring. See a specimen in the Leventritt Shrub and Vine Garden at Accession #: 654-2003*A. [pdf]
Chimonanthus praecox, Winter Sweet
Chimonanthus praecox, a member of the Calycanthaceae, is a treasure introduced from Asia before this country was a nation. Michael Dosmann, Curator of Living Collections, spoke about the plant’s history, its virtues, and the challenges it presents to northern gardeners. Find our one and only specimen at the entrance to the Explorers Garden, just off Bussey Hill Road at Accession #: 236-98*A. [pdf]
Bornean Trees in Boston
Cam Webb, an Arboretum researcher based in Indonesian Borneo, discussed tree genera that occur both in Southeast Asia and in New England, and what they tell us about the history of tropical rain forests and adaptation to the cold. He highlighted Accession #: 801-87*A, Diospyros virginiana, the common persimmon, south of Meadow Road and opposite Faxon Pond. [pdf]
A California Native Thrives in New England
The bright green foliage of California incense cedar can enliven the dullest winter day. Calocedrus gets its name from the Greek kallos, meaning “beautiful,” and cedrus, originally from Latin meaning “evergreen conifer.” Unlike most other conifers, male cones of incense cedar shed pollen on winter’s winds. Visitor Education Assistant Maggie Redfern presented a pair of trees laden with pollen cones at the base of Peters Hill along South Street, including Accession #: 450-86*G, Calocedrus decurrens. [pdf]
Most people who can identify trees have come to recognize them by their characteristic leaves, flowers, or fruit, present during the growing season. But with deciduous trees, one can’t rely on these features during the winter months. Fortunately, each species of tree has a unique signature which can be found by studying its twigs. Arborist Kyle Stephens discussed twigs and the information they hold about their identity and their growth habits, featuring Accession #: 695-80*B, Ailanthus altissima forma erythrocarpa, opposite the cork trees along Meadow Road.
Sleeping Beauty: A Flower Bud Fairy Tale
Deciduous trees appear bare, with no of leaves and flowers during the winter. However, the flowers and leaves that will appear in spring are already present, though hidden. These tiny flowers and leaves begin to grow at the end of summer and remain protected inside the bud in a state of dormancy through the winter. Then the warm kiss of spring wakes them from slumber. Visiting researcher Erica Fadon showed what is happening inside the buds of three different trees in the Bradley Rosaceous Collection, at Accession #: 931-51*A, Prunus sargentii ‘Columnaris’. [pdf]
Fagus sylvatica ‘Tortuosa’
The enigmatic cultivar of the European beech, Fagus sylvatica ‘Tortuosa’, is a rarity in nature. Although now fairly common in cultivation, the tortuous habit of these trees still continues to puzzle botanists. Julien Bachelier shared the many mysteries yet to be solved at Accession #: 2420*A. [pdf]
Katsuras: Hearts of Gold
Curator of Living Collections Michael Dosmann spotlighted the katsura collection as a prelude to autumn. Some of the Arboretum’s oldest accessioned plants (including Accession #: 882*B), katsuras display amazing fall color, and senescing leaves emit the aroma of caramel or burnt brown sugar. [pdf].
The Mighty Oaks
Guest presenter Dave Barnett, President and CEO of Mount Auburn Cemetery, spoke about the oaks (Quercus spp.) that dominate the landscapes of both the Arnold Arboretum and Mount Auburn Cemetery. The talk took place in the Arboretum’s oak collection, north of Valley Road at the northern end of Oak Path. [pdf].
Locust, Legumes, and Nitrogen Fixation
Arnoldia Editor Nancy Rose discussed Robinia pseudoacacia, accession 321-48*A, located near the juncture of Meadow and Bussey Hill Roads. Known commonly as black locust, R. pseudocacia hosts nitrogen-fixing bacteria in its root system, a characteristic shared by many other members of the pea family. [pdf].
Stewartias: Natives, Exotics, and Hybrids
Senior Research Scientist Peter Del Tredici met visitors at the entrance to the Explorers Garden off Bussey Hill Road for a comparison between the many stewartias in the Arboretum collection. Members of the tea family of plants, stewartias are one of six species collected by the Arboretum in collaboration with the USDA and the North American Plant Collections Consortium.[pdf].
Keeping Track of Trees
Visitors to this Tree Mob™ at the summit of Bussey Hill were introduced to the Arboretum’s plant inventory program. Kyle Port, manager of plant records, and Stephanie Stuber, curatorial fellow, explained how and why our living collections are tracked and recorded using Pinus parviflora, accession 1539-71*D, as an object lesson. [pdf]
Senior Research Scientist Peter Del Tredici explained the reproductive biology of Ginkgo biloga at the time of day when the process is visible. Visitors met at Ginkgo biloba ‘Hayanari’, accession 824-83*A, just beyond the Hunnewell Building Visitor Center, and observed the phenomenon with hand lenses. [pdf]
Arboretum Apprentice Miles Sax Schwartz introduced visitors to Malus sargentii, accession 20408*D, on Peters Hill. Miles used this plant to illustrate the Arboretum’s long interest in cultivating crabapples, including his recent efforts to reassess and reinvigorate these plants at the Arboretum. [pdf]