Dogwood Lane
The Quarterly Journal of the Mary May Binney Wakefield Arboretum ___________________________________
Volume Three, Issue 1 - Fall 2020
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Cultivating a passion for trees |
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As a certified arboretum, naturally we are interested in all things related to trees: how they grow, where they grow, what hampers their growth, how big will they grow and for how long. Because arboreta are educational institutions, we are also committed to impart to our visitors the importance of trees, their varied roles in our ecosystem, and as a vital tool to address climate change. The benefits of trees are many: they produce oxygen and store carbon dioxide- in fact, roughly half a tree's weight is stored carbon absorbed from the atmosphere. In addition, they provide critical habitat for a wide assortment of birds, animals and insects, and are a renewable source of building material (lumber for our homes). They cool our communities with shade, promote water quality, and reduce storm water run-off and associated costs - the list is long and impressive!
There are countless reasons to love trees. This issue of Dogwood Lane explores this passion for woody plants and several different ways botanists, tree lovers, and collectors have satiated their hunger for the unusual, the spectacular, and the rare specimens that beguile the botanical world.
Awakening the Passion through Discovery
Whether it is through studying horticulture or arboriculture, or by a chance encounter on a casual walk through a park or arboretum, some people, simply put, fall in love with trees - and when that happens, often their desire to learn about and possess rare and unusual varieties can become an insatiable hunger.
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Sometimes the encounter can be with one particular tree or a collection of trees — and often tree lovers can easily recount that moment. Whether it was the Arnold Arboretum's awe-inspiring and well-loved cork tree, whose branches reached out and supported countless children for decades before dying in 1995, or the Olmsted Elm at Fairsted, most of us can remember a tree that made a special impression on us, igniting a desire to know more about them. https://arboretum.harvard.edu/stories/requiem-for-a-cork-tree. |
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The Sensational Story of a Horticultural Treasure |
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One tree that provides that type of jaw-dropping, "stop dead in your tracks" impact is the enchanting Dove Tree, Davidia involucratra. An encounter with the tree in full bloom may have been enough to motivate Polly to start her own arboretum. Discovered by botanists during one of many legendary "plant hunting" expeditions to China, this rare tree "blooms" in a manner similar to the Chinese Dogwood, with a pair of gigantic and unequal sized bracts that gradually turn from green to creamy white while embracing and protecting a small and unspectacular button-like mass of stamens. In bloom, its appearance is breath-taking and its appeal irresistible - as Michael Dirr who wrote the "bible" on trees remarked "...when observed in flower there is an insatiable urge to secure a plant for one's garden." The only plant in its own genus Davidia, the species is one of 22 in the Nyssaceae or Tupelo family.
The story behind the "discovery" of the Dove Tree is as intriguing as the tree itself. The tree was first noted by a French missionary Armand David in 1869, who began collecting plant and animal specimens for his work teaching science to children. His work was of such great interest to the Natural History Museum in Paris that they encouraged the mission to free him to focus solely on collecting specimens in China.
On his second collection expedition along the Yangtze River and into the mountains, David discovered numerous woody species including rhododendrons, maples, and a number of herbaceous plants including primulas, gentians and lilies. This was also the expedition where he discovered the dove tree, later named Davidia involucrata in his honor. This exciting horticultural find was overshadowed by an animal he found on the same expedition, the Giant Panda, which he introduced to the outside world as the "black and white bear."
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Described and named after its discoverer in 1871, the Davidia was subsequently sighted on another exhibition by Augustine Henry, an English physician with a great passion for botany then stationed in China. In 1893, he wrote enthusiastically, "Davidia is worth any amount of money. I saw only one tree of it, but doubtless there are others in the district ...Davidia is wonderful."
Discovery of the dove tree did excite nurserymen who sought to secure the plant. The Veitch nursery hired a young plant collector to seek it out. The 22 year-old was E. H. Wilson, who would later become one of the most famous plant hunters, renowned for this find among hundreds more. Ironically, he did find a dove tree at the location he was given, but it was now only a stump and a new house built from its logs. In the weeks that followed, he discovered other trees in bloom and shipped seed back to England in 1901.
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This find proved to become one of the most admired and sought after hardy exotic trees. When in bloom, its graceful white bracts bedeck the branches, appearing like a flock of doves or abundance of handkerchiefs.
The Dove tree here at the Mary May Binney Wakefield Arboretum is located on the lowest terrace of the formal garden near the metasequoias and the Kalmia Garden and typically blooms in May.
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A poignant legend is associated with the Dove tree. During the Han dynasty, the beautiful Zhaojun Wang, the best known of China's "political brides" was married off by the emperor to the king of a distant northern tribe. It is said that Zhaojun, missing her family, sent a letter home attached to a dove that flew nonstop for a thousand miles, finally landing and dying on a dove tree branch outside her family’s home.
The Dove tree (Davidia involucrata) was renamed to honor the dedicated messenger.
Learn more about the Dove Tree's fascinating story by reading the 2005 Arnoldia article,
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Fabulous Fruit and Falling in love with Propagation |
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We like to remind our visitors that the word "arboretum" essentially refers to a "museum" of "trees." As such, our arboretum has a number of different "collections" that often boasts a variety of "treasures" to display. In fall, one of these is the dazzling fall foliage that bedecks our wide variety of trees, but another is their fabulous fruit now on display.
This time of year, we encourage visiting children to gather some of the treasures that have fallen to the ground as a scavenger hunt. They provide another path to a deeper interest in different tree species, and often, a deeper affection.
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Chinese Chestnut
(Castaneda mollissima)
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"Beautyberry"
(Callicarpa americana)
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Snowbell
(Styrax japonicus)
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Different seeds have different needs, and often it is not enough to just take a seed home and plant it in a pot. The seeds in an apple, if planted and grown, will not produce that same yummy fruit we may have just eaten. The reason for this is that when the flowers are pollinated enabling the pollinated flower part to become the fruit we enjoy, the seeds produced are not "true" to that tree - they were likely pollinated by pollen brought from one or more other trees. Our apple producing trees are all grown as trees fusing two separate plants: a scion or young plant shoot of a particular apple variety grafted onto one of a handful of carefully developed varieties of rootstock - the base and root portion of the tree. |
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This is also true of most varieties of flowering trees because they are monecious, meaning that male and female flowers are found on a single individual plant. To produce fruit, dioecious plants such as kiwi and some varieties of holly, require both a male and a female plant located close enough in order for pollination to occur. During the growing season with thousands of flowers in bloom simultaneously, pollinators are carrying pollen from tree to tree blending the genetic material from various trees. Only if the flowers are bagged and hand-pollinated can cross-pollination be prevented to ensure the seeds produced are genetically identical to the parent plant.
Other seed varieties require periods of stratification (a cold, moist period that breaks seed dormancy) as they would experience in their native environment. In some cases, for example oaks, the seed (acorn) that doesn't get eaten by an animal and gets buried or covered with a bit of dirt is likely to germinate after a natural period of cold or winter. Plant propagators will often replicate the species natural stratification through artificial means such as refrigeration or applied heat to simulate natural conditions that the seeds must experience before germination can occur.
Most of the trees and shrubs at the Mary May Binney Wakefield Arboretum can be grown from collected seed after being cleaned and properly stratified and or scarified (providing a means for the seed to breech an impenetrable seed coat that would naturally break down through repeated freezing and thawing). Many of our plants were grown by Polly Wakefield after being germinated from seeds propagated using these methods or from rooted cuttings.
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With such a wide variety of tree species represented in the Wakefield Arboretum's collection, some really stand out as "showstoppers" because of their spectacular fruit. A few of these include the sapphire berry, callicarpa, winterberry, black walnut, silver bell, snowbell, and Japanese maple. |
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Japanese maple
(Acer palmatum)
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Seven-son flower (Heptacodium miconioides)
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Sapphire berry
(Symplocos paniculata)
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Cultivating and patenting Kousa Dogwoods
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Polly Wakefield's magnificent arboretum literally "grew" out of an interest in and affection for propagation. She took the same propagation class at the Arnold Arboretum for many years and collected seed from the same three Cornus kousa dogwoods there. She ultimately planted more than 600 kousa dogwoods here at her estate. Like apple trees, kousa dogwoods are cross-pollinated by a variety of insects. When cross-pollination occurs, the resulting seed will be a natural hybrid and may have some characteristics of each parent or may look totally different than either. Polly was fascinated with the huge variation that occurred as she expanded her collection over many years. She took notes and photographs observing the characteristics of each individual kousa dogwood. Some kousas had bracts, or false leaves, that were tinged with yellow or were long and teardrop shaped while others were almost completely round and overlapping. Once she discovered certain trees in her collection that exhibited special and or unusual features she decided that they were worthy of introducing into the trade by applying for a plant patent for individual cultivars of kousas or a plant variety that she produced by selective breeding.
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Self-sown, naturally occurring varieties of unnamed kousa dogwood cultivars. |
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“In 1930, the United States began granting patents for plants. By 1931, the very first plant patent was issued to Henry Bosenberg for his climbing, ever-blooming rose. Under patent law, the inventor of a plant is the person who first appreciates its distinctive qualities and reproduces it asexually. In other words, a plant can be created (such as by breeding or grafting) or it can be 'discovered' as by a 'plant hunter.' In order to acquire a plant patent, the inventor must have actually asexually reproduced the plant. Asexual reproduction means that the plant is reproduced by means other than seeds, usually accomplished by cutting or grafting of the plant. Asexual reproduction is the cornerstone of plant patents because that is what proves that the inventor (or discoverer) can duplicate the plant. The patented plant also must be novel and distinctive.” https://www.nolo.com/legal-encyclopedia/plant-patents.html
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More self-sown and naturally occurring varieties of kousa dogwood cultivars. |
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Once Polly selected the plants for patenting, she submitted detailed applications for each plant. Applications included descriptions and images of all details of the plant: fruit, flower, seed, leaves and bark. The application might also highlight any disease, drought resistance or other special characteristics such as unusual hardiness. She ultimately received 7 patents for Cornus kousa cultivars: Fanfare, Silverstar, Moonbeam, Triple Crown, Twinkle, Greensleeves and Milky Way.
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When Polly had selected the plants and they received the patents, it was then important to propagate them. The most effective method of propagation for Cornus kousas is from softwood cuttings.
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For this, cuttings are taken in early spring from each tree, stripped of all of the leaves with the exception of 2 then they are ‘stuck’ in growing medium. These delicate cuttings require a heavy daily dose of mist to keep moist while developing delicate roots. Polly accomplished this process by two methods. She converted her small chicken house into a mist house and she sent many cuttings to the Arnold Arboretum to propagate in their mist house. Both methods proved successful. It is now possible to find several of Polly Wakefield’s selections in the nursery trade. Polly considered herself an amateur propagator but these techniques take a commitment of time energy and understanding proper propagation techniques. |
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"Getting to know you" - "Keying" Trees
How learning to identify trees deepens our relationship with trees
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Often when a person finds themselves falling in love with trees and beginning to recognize different trees by their leaves, they sometimes are anxious to learn how to ID (identify) the trees they encounter. Recently, scores of "apps" for your cellphone have been developed that take the work out of plant identification. At arboretums like the Mary May Binney Wakefield Arboretum, many trees have identification tags or signs. Nevertheless, it is educational and empowering to learn how to identify various trees through careful observation of their branching, bark, leaves, twigs, and fruit. An ID provided by an app may be speedy and convenient, but may not be accurate and most importantly, strips the inquiry of any interactive learning or true understanding about the plant's genus or species.
Apps that speedily provide identification of trees are often based on utilization of a dichotomous key, a tool that uses a series of statements consisting of a pair of choices describing the characteristics of the tree, such as is it evergreen or deciduous (drops its leaves in fall). When the user chooses the answer, they are directed to choose between another pair of statements, each answer narrowing down ultimately to the identity of the tree. When the app requires a person to make progressive observations rather than simply providing the answer by image recognition, it encourages the observer to begin to consider the characteristics of the tree and enables a richer exploration of it as a species.
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All in the family- or rather "genus" - the Maple or Acer Genus
When we are novices at making tree IDs, we can be thrown off by an unusual species within a genus whose leaf looks nothing like the better-known species. In this case, the fruit or seed can provide a critical clue. While leaves and bark of a genus can vary greatly, often the fruit will not "lie" about its relation to the species. The Maple or Acer genus provides a useful example. While most can identify what we think of as the maple leaf from the Canadian flag and emblems, there are dozens of different shaped maple leaves. The Acer genus is comprised of over 120 species of trees, over 20 of which can be viewed at the Mary May Binney Wakefield Arboretum. The leaves on these trees vary greatly in shape from palm-shaped or "palmate" to a leaf of three parts, resembling poison ivy. What does not vary greatly is the seeds produced by all these different species. While varied in size, the seeds of nearly all acer species are double samaras, a type of dry fruit/seed covered by a casing that extends into a wing-like shape - think the clothes-pin shaped or helicopter-like seeds children often hang on their nose. The "wing" enables the seed to "fly" out from the canopy and away from the mother plant where its chance to grow is increased due to there being more space, more access to water, and more sunlight. It is thought that because many maple species have large crowns, winged seeds eventually evolved enabling them to be transported to more hospitable places to grow, increasing the tree's ability to perpetuate the species. While the seeds of some other species have wings, such as the tulip and ash tree, the double samara is characteristic of the acers and provides a clue even the beginner tree ID-er can bank on.
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Profile of one of our special native maples -the versatile Red Maple
Latin name: Acer rubrum
The US has 13 native species of maple (Latin: Acer) trees, but there are over 120 species world-wide, most in Asia. Among maple trees locally, Red Maple (Acer rubrum) is the most abundant native tree today in eastern North America. It also has the greatest north-south range of any tree in the eastern forests - found in Newfoundland to Florida. With its wide geographical distribution and ability to thrive in very different climates and growing conditions, it is believed to be the most adaptable tree in eastern North America. Known for its brilliant scarlet color in autumn, it is easy to identify and distinguish from other maples. Acer rubrum generally live to be 100 years old and 100 feet tall. The greatest Acer rubrum - known as a “Champion Tree” - is located in the Great Smokey Mountains National Park: it is 141 feet tall and seven feet in diameter at four and half feet above ground. We have several massive Acer rubrum in our woods - likely well over 100 years old. The next time you come for a visit, ask us where they can be found - well worth the view!
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Improving on a Classic: "Thornless" Honey Locust
Plants have evolved for millions of years to the trees, flowers, seeds, and phenological relationships we know today. Scientists believe that evolutionary changes in plants occur initially as mistakes or mutations, but mistakes that in fact increase the viability and success of the species. In modern times, geneticist and horticulture professionals have often seized upon these mutations or variants to cultivate and commercialize a variety that "improves" a beloved plant that has some not so loved characteristics. Such is the fascinating story of the "thornless" honey locust.
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Honey Locusts at Mary May Binney Wakefield Arboretum
Polly Wakefield planted several dozen honey locust, Gleditsia triacanthos on her property that are now mature trees; some 50 to 60 feet tall. Currently no records have been found as to why she planted so many of these trees that are an important part of this cultural landscape. Her notes and records suggest many were planted where stately elms once stood and they are prized as grade stabilizers for steeply sloped landscapes like this one. It may also be because they are imposing trees that grow quite large and have massive double compound (bipinnately compound) leaves made up of dozens of tiny leaflets that provide dappled shade and are a cinch to rake up in the fall. Polly may have appreciated that they provide an important source of food for wildlife- their long pods are consumed by many different species of animals. Whatever the reason for the inclusion in our collection, they are a great source of interest for many of our visitors who are often more familiar with the popular street tree that is a thornless variant of the same plant. During their visits here, children often marvel at the size of the thorns poking out on all sides of these trees. According to scientist, these thorns are thought to have evolved to protect the trees from browsing Pleistocene megafauna, which may also have been involved in seed dispersal (when they would eat the pods), but the size and spacing of them is also useful in defending against smaller herbivores such as our local deer population.1
In the 18th century, plant enthusiasts observed that some of the trees in the wild did not have thorns yet had all the other characteristics of honey locust. Scientists and plant breeders found that while this honey locust was distinct because it was thornless, it is not otherwise different enough to be classified as a separate species. In fact, offspring from the thornless trees will sometimes have thorns and must be cloned. This twist of genetics has led botanists to classify thornless honeylocusts as a form of honeylocust, or “a seedling variation where a percentage of seedlings exhibit a distinctive characteristic.”2
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It's all in a name, or is it? |
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Plants that have been commonly used in gardens and neighborhoods for centuries often have several different "common" names. In regions in the South, honey locusts were sometimes called Confederate pin trees because the thorns were used to pin uniforms together during the Civil War. In addition to the name honey locust, the species was referred to as "honeyshucks." The "honey" part of the name refers to the sweet flesh inside fresh seedpods. It's said that this pulpy material was once chewed by farm boys and is still chewed by farm animals when they can find it. These animals somehow know not to consume the slightly toxic seeds of the plant. When raked up and left in the compost, a sweet fermenting smell is emitted by the decomposing pods. The “locust” part of the name apparently comes from the rattling sound made when the pods are
shaking. This sound was thought to sound like a swarm of locusts.
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As students of arboriculture and tree huggers get more serious about learning about plants, they realize that using the scientific or botanical latin names is much clearer. For example, there are countless varieties of beautyberry including two we have here also known as kolkwitzia and the other callicarpa. As a species, honey locust was first described in the mid 1700's by the Swedish botanist, Karl von Linné. He gave it the genus name Gleditsia in honor of J. G. Gleditsch, a German botanist at the time. The species name, triacanthos, is from the Greek for “three” and “spine,” referring to the thorns of honey locust that often are three-branched. The Latin designation for the thornless form of the tree is appropriately, inermis, meaning, “unarmed,” or when applied to plants, “without prickles or thorns.”
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Mutations that are like finding a jackpot - one in a million
The Hunt for Witches' Brooms
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If you have visited the Mary May Binney Wakefield Arboretum in the past several years you will have had the opportunity to see the new dwarf conifer garden behind the Isaac Davenport mansion. Many of these dwarf conifers were propagated from cuttings of “witches' brooms.” Polly Wakefield had an interest in dwarf conifers and collected several specimens. It is unclear as to whether or not she ever propagated any, but we have added dwarf conifers to her collection over the past three years that now is a Reference Garden for the American Conifer Society. |
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Broom Hunters
Today there are "Broom Hunters" that enjoy collecting witches' brooms from trees and propagating them. A small group of broom hunters travel the world, traipsing through forests to collect witches' brooms. More common in higher elevations, each witches' broom is the only one of its kind in the world. It is thought that higher altitudes, with higher levels of solar radiation, trigger more mutations and brooms. It is not uncommon when driving the higher elevation roads of the mountains of Colorado to see brooms often. Because witches' brooms often occur in the upper reaches of trees that can be 50 to 90 feet and too difficult to climb, "hunters” will shoot the broom out of a tree with a shotgun, and the others rush to pick up the pieces that fall to the ground, hence the name witches' broom hunters! Many of the hunters own nurseries, and may eventually sell some plants created from these brooms. But often collectors do it for fun and for the camaraderie. Brooms hunters often donate some of their haul to the American Conifer Society, where gardeners and other plant aficionados will bid to own one at the organization's annual auction. One of the desired “prey" of witches' broom collectors today is to find the slowest growing witches' broom to produce the smallest plant possible. The limit seems to have been reached by the discovery of more than one broom consisting of simply a tight cluster of buds with no shoots.3 Witches' brooms are found all over the world on many different conifers and other plants.
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Abies koreana 'Kohouts Icebreaker' is a relatively new dwarf cultivar of Korean fir from a broom of 'Hortmann’s Silberlocke’. It was discovered by Jörg Kohoutof Elstra, Germany. This plant was voted most outstanding by plant of 2014 by American Conifer Society.
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A broom found on a spruce tree here at the Mary May Binney Wakefield Arboretum. |
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What exactly is a "Witches' Broom?" |
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The English term witches' broom comes from the German word hexenbesom. Both parts of the compound word are found in English hex meaning to bewitch and besom meaning a bundle of twigs.4 There is much folklore that surrounds witches' brooms. It was believed that witches used these formations to rest during their travels and were also used by elves and goblins. Witches brooms can be found on many species of plants and are caused by fungus, bacteria, viruses and mites. Normally in plants, especially evident in trees, the leading shoot will produce an auxin, a plant hormone, which will slow the growth of the secondary and tertiary shoots to prevent them from overgrowing it. Interference in this mechanism by mutations or cytokinins (a phytohormone) induced by fungi, insects, nematodes, phytoplasmas, viruses or other outside agencies can cause plant apices to develop into witches' brooms.5 Witches' broom are not always caused by a pest or disease. Sometimes they form because the tree is stressed from a broken branch or because pruning was not done properly. It can also be caused by a genetic mutation or environmental conditions that led to the death of the terminal buds of shoots. If the witches' broom is caused by a genetic mutation, there will likely only be one cluster of twigs in the tree. Conifer trees, such as pine, fir, spruce and juniper, might be affected by a genetic mutation that causes witches' broom.
3 Derek Spicer., American Conifer Society
4 Al Fordham, Arnoldia, Dwarf conifers from witches'-brooms, June 1967
5 Derek Spicer., American Conifer Society
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Articles written and edited by Debbie Merriam, Mark Smith and Erica Max.
Layout by Erica Max.
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For a printable copy of this or any prior issue of Dogwood Lane, click here or visit the news tab on our website.
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Mary May Binney Wakefield Arboretum | 617-333-0924
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