Friends of the Arboretum Newsletter
Spring 2009 – Volume 13, Number 1
An Introduction to Sex Expression in Plants
By Denny Werner, Ph.D., Professor, Department of Horticultural Science, NC State University
Some time ago, while giving a tour of the JCRA to a visiting group, we happened upon the beautiful Asian fringe tree (Chionanthus retusus) adjacent to Elm Circle. As I pointed out the attributes of this fine plant, I also called attention to the significant fruit set on the tree last year. During the discussion, I proposed some possible explanations for the significant fruit set, which was somewhat out of character for this specific tree that generally has demonstrated only light fruit set or an absence of fruit production in prior years. Ultimately, the subject of sex expression in plants arose in my discussion, and at that time, the interest level of the group blossomed. Hence, I decided to discuss this topic in my writing for this newsletter. The subject of sex expression is expansive, and this piece is meant to provide the reader with only some general and practical information in this interesting area of plant biology.
Higher plants exhibit a myriad of sex expression phenotypes. Sex expression refers to the sexual diversification that occurs in the flowers of higher plants at the species or population level. Most flowering plants produce flowers that contain both female (pistils) and male (stamens) sexual organs. Such flowers are called perfect or hermaphroditic flowers. Perfect flowers are usually associated with high levels of self-pollination. However, genetic mechanisms that control the spatial separation of the stamens and pistils, or differences in the relative timing of pollen release and female receptivity within an individual flower or between flowers on the same plant (dichogamy), can promote considerable outcrossing even in perfect flowered taxa. For example, many species in the walnut family (Juglandaceae), such as pecan (Carya illinoiensis) demonstrate dichogamy. In populations of these taxa, some individuals release pollen before pistils on the same tree are receptive (protandry), while in other individuals, pistils are receptive prior to pollen dispersal (protogyny). Both mechanisms tend to encourage, but not enforce, outcrossing. Another mechanism that promotes and often enforces outcrossing is called self-incompatibilty. Self-incompatibility prevents self-pollination (pollen transfer within a cultivar or clone), even though functional male and female gametes are produced by the plant. This mechanism is mediated by the lack of pollen germination or pollen tube growth through the style after self-pollination. Many of our popular fruit trees, such as apple and pear, exhibit this phenomenon. Mechanisms that promote outcrossing in plants are important in plant evolution, as outcrossing increases genetic diversity and potential fitness in a population, while self-pollination results in inbreeding, leading to a decrease in genetic diversity and a potential reduction in population fitness due to inbreeding depression.
Some flowering plant species (about 10%) produce unisexual flowers containing only male or female sexual organs. Gender variation, in which a species or a population of a species produces plants with both male and female flowers on the same plant, is called monoecy. Many members of the gourd family (Cucurbitaceae), such as melon species in the genus Cucumis, commonly demonstrate this sex expression condition. Conversely, dioecy describes a condition in which a species or population of that species contains separate female and male plants, such as demonstrated in Ginkgo biloba and many species of Ilex. Although these two general sex expression categories are widely recognized, rarely are circumstances that simple in biology. Many variations on these two general categories have been recognized and described. In monoecious taxa, the relative number of female and male flowers varies considerably between individual plants in a population, with some plants functioning primarily as males, and others functioning primarily as females. Variation in the strict dioecious condition is also common. A sex expression condition called polygamodioecy refers to the phenomenon in which hermaphroditic flowers are produced in conjunction on plants that produce primarily male flowers (androdioecy) or female flowers (gynodioecy). Both the gynodioecious and the androdioecious condition exists in Chionanthus, meaning that plants that primarily function as males or females can be recognized, but in both cases hermaphroditic flowers may be produced in variable numbers on each type, depending on the genetics of each plant.
Also, environmental conditions during the time of flower bud initiation almost certainly plays a significant role in influencing the extent of hermaphroditic flower formation in any year, but this area of research has been largely unexplored due to the difficulty in conducting controlled experiments examining such in woody plants. Recall that in most woody taxa, flower buds are initiated the summer or fall prior to the actual time of flowering (anthesis) in the subsequent year, so it is the environmental conditions in the summer prior to flowering that most greatly influence flower bud set and sex expression of the subsequent flowers. It is interesting to ponder the basis for the significant fruit set in 2008 on our Chionanthus retusus that historically has set low amount of fruit. One can speculate that the highly stressful conditions of extreme heat and drought in summer of 2007 may have promoted the initiation of a greater number of hermaphroditic flowers in year 2007 on our typically male tree, as compared to a normal summer with less heat and drought stress, leading to increased fruit production in 2008. You may ask if any experimental evidence exists to support this contention. Again, the literature is limited in this area, but some interesting studies have been conducted in redvein maple, Acer rufinerve. A recent study by Japanese scientists (reference 2) examined sex changes in A. rufinerve as related to plant health and environmental conditions. Like Chionanthus, A. rufinerve is polygamodioecious. Interestingly, these scientists reported that a dramatic shift in sex expression from male to female or bisexual occurred in the spring following a year of reduced precipitation and drought stress. Perhaps a similar phenomenon is occurring in other dioecious woody species also. This is an interesting area of biology, one that is ripe for more research, particularly in woody taxa.
Members of the arum family (Araceae) represent a particularly interesting group of plants from the perspective of sex expression. Members of this family are characterized by an inflorescence called a spadix that contains many individual flowers. The spadix is partially enclosed in a vegetative hood-like structure called a spathe. One well-studied arum, our native Jack-in-the-pulpit (Arisaema triphyllum), often demonstrates an interesting biannual "sex change," where an individual plant can produce an inflorescence that contains only male (staminate) flowers in one year, a vegetative shoot containing no flowers the next year, and still the following year produce an inflorescence that contains only female (carpellate) flowers, or both staminate and carpellate flowers on the same plant (the monoecious state). This ability to undergo "sex change" from year to year is referred to as "gender lability." For an excellent review of the topic of gender lability, see the manuscript by Helena Korpelainen, Ph.D., (reference 1).
In Ariseama triphyllum, the ultimate sex expression of an individual in any particular year is a function of the genetics of that individual plant, plant size, population density, sexual expression history, and environmental conditions such as temperature and moisture. Young, immature (i.e., small) plants typically produce mostly male flowers; female expression occurs later in life as the plant ages and increases in size. Population density can have an effect on sex expression, both directly through biochemical interaction between neighboring plants and indirectly through the impact of population density and competition on plant size. Plants tend toward male expression when population density is high, and female expression is favored at lower population densities, such as at the periphery of a population. Plants that exhibit largely female expression in one year, and hence set a high number of fruit, often produce no flowers or exhibit only male expression the following year. This can be explained by the fact that a net drain on stored reserve (photosynthate) occurs after significant fruit production, and plant survival is jeopardized if the plant continues to produce female flowers and fruit in subsequent years. To ensure survival, it must reduce its reproductive effort by changing its gender to male or reverting to the vegetative state for a few years after fruiting. Hence, gender lability is an adaptive mechanism that has evolved in some plant species to ensure survival.
I would encourage you to include some Arisaema triphyllum, or other members of the Araceae in your garden, and observe these interesting dynamics in your own back yard. Observing and appreciating these subtle changes in the plants in our gardens from year to year increases our knowledge of the biological mechanisms taking place in our own back yards, and adds another dimension to our gardening pleasure.
1. Korpelainen, H. 1998. Labile sex expression in plants. Biological Review. 73:157-180.
2. Nanami, S., H. Kawaguchi and T. Yamakura. 2004. Sex change toward female in dying Acer rufinerve trees. Annals of Botany. 93:33-740.
No Lace Here!
By Tim Alderton, Research Technician
Ferns have been around for over three hundred million years, giving them just a few years to diversify into all forms and fill every environmental niche possible. Delicate, soft, lacy, deep green, glossy foliage growing in cool, moist shade: that is what we all think of the quintessential fern. Broad, undivided, thick, plastic-feeling, blue, or hairy fronds growing in baking full sun: that is the polar opposite of the quintessential fern. Many ferns, though, fit this second scenario! Oddballs, right? These un-fernlike ones fascinate me most.
I will talk about a few of these strange plants in the Arboretum as well as some we don't have, discussing ferns for shade to part shade. The first group is some ferns from the Polypodiaceae. This includes our native resurrection fern (Pleopeltis polypodioides) and rock polypody (Polypodium virginianum). Neither of these species is in the gardens because they are not easy to establish, but several of their relatives intrigue me and are easy to grow in your own garden.
The genus Pyrrosia is probably the most un-fernlike that we have. It is a genus that includes anywhere from 65–74 evergreen species distributed in Africa, southern and eastern Asia, and into the south Pacific islands. In the wild, they typically grow as epiphytes and lithophytes. Fine, star-like hairs are present on both the upper and lower surfaces of the typically strap-shaped fronds when they are young, and shed from the upper surface when mature. Only a few species have proven hardy in our climate, but there are others yet to be tested. Of the tried and true, there are three species that are becoming more widely available and should be used. The most common of these is Pyrrosia lingua. This species in native to China, south into Indochina, and is found on the islands of Taiwan and Japan. The fronds of the typical P. lingua are undivided, strap-like, and held upright on a ridged petiole along a spreading rhizome. Many selections of P. lingua have been made in Japan and China, emphasizing crested, wavy-edged, lobed, or variegated fronds. The next two species, P. hastata and P. polydactyla, look very different from other Pyrrosia because they have lobed fronds. P. hastata hails from South Korea and Japan. Its leaves are hastately lobed with typically three, but sometimes, five lobes that give it the appearance of a bird's foot. The side lobes are individually about 3" long and the central lobe is about 5" long. All the lobes are about 0.5" to 0.75" wide. The total leaf width is about 5". The underside of fertile fronds is covered in orderly rows of cinnamon-colored sori (clusters of structures producing and containing spores). P. polydactyla is native to Taiwan. It is similar in appearance to P. hastata, but has six to eight lobes on its palmately divided frond, fitting its name perfectly since polydactyla roughly translates as "many fingers." The individual lobes may be longer than that of P. hastata. Both P. hastate and P. polydactyla have short rhizomes that form a tight clump, topped in foliage that grows 6" to 10" tall. All the Pyrrosia like well drained soils and are unfazed by drought once established.
The genus Phlebodium consists of only four species found in tropical and subtropical America. It is very closely allied with Polypodium and is often included in that genus. There is one species, Phlebodium pseudoaureum, which we have been growing successfully at the Arboretum. P. pseudoaureum is native to much of tropical America and ranges into southern Florida. Where it is native, it grows as an epiphytic evergreen; but in all but the most sheltered of locations here, it will be terrestrial and deciduous. The fronds grow 12" to 15" tall and are a glaucus blue-green, pinnately lobed, and held upright on a rigid petiole. Double rows of golden sori line the lobes of fertile fronds. Fronds arise from a spreading blue-green rhizome covered in golden hairs that match that of the sori.
Another family of ferns, the Dryopteridaceae, includes many of the most common ferns that we see in gardens, including the shield ferns (Polystichum), wood ferns (Dryopteris), and the florists' favorite, leatherleaf ferns (Rumorha). These are some of the most quintessential of all ferns, but one in the Lath House stands out. Dryopteris sieboldii just does not fit in with the bunch; it is an oddball for this family. It is one of 250–350 species of Dryopteris found throughout the Northern Hemisphere. When I first saw this fern while working at Plant Delights Nursery, I almost thought there was a labeling issue since it did not look like any other Dryopteris that I had seen. The fronds are only once pinnately divided and have undivided pinnae (a division of a compound frond equivalent to that of a leaflet of a compound leaf). The edges of the pinnae irregularly undulate, trying to form pinnules like its fernier cousins, but it just can't do it, as they are all webbed together. The evergreen fronds' pinnae are arranged in three to five pairs up the rachis, with a terminal pinna at the tip that is 6" to 12" long. The texture of the foliage, when handled, is thick and leathery, adding to its un-ferny quality. A native of China, Japan, and Taiwan, it is perfectly at home here in the eastern United States, taking the heat and drought better than many of its lacy relatives.
Yet another family of ferns with some oddball kin is the Blechnaceae. This family includes our native netted chainfern (Woodwardia areolata) and Virginia chainfern (Woodwardia virginica), both deciduous species that have spreading rhizomes. Of the 12–20 species of Woodwardia, the cool one of the bunch is Woodwardia orientalis. This fern, as its name implies, comes from Japan and China. An evergreen to semi-evergreen with fronds that originate from a non-spreading rhizome, it has the look of a tree fern without the trunk. The fronds can grow 3' to 6' long, arching out from the crown. Emerging fronds are glossy and often pigmented bronze to red, which age to a deep glossy green. This is a somewhat ferny fern with fronds that are pinnate with pinnatafid pinnae. The un-fernlike character of this plant is its reproduction method. Most ferns have dust-like spores that float through the air and the lucky few land in a favorable location and grow. W. orientalis is a bit different. It produces spores in the same fashion as most ferns, but it also does more. It grows a nursery of small plantlets right on its fronds. In late summer and fall, small plantlets emerge from the upper surface of the mature fertile fronds, which then fall to the ground, and, if conditions are right, grow into new plants.
These are just a few of the shady characters that can be grown in our gardens. Many more are worth trying, like Coniogramme japonica, Asplenium scolopendrium, Lepisorus bicolor, and Diplazium subsinuatum, just to name a few. These all ignore that stereotype of lacy and delicate ferns. Lacy or not, another advantage is that deer find ferns to be inferior roughage. Try some of these un-ferns in your garden and confuse your neighbors.
By Tim Alderton, Research Technician
To go along with my strange ferns, you might try some hardy begonias. I have been trying some different begonia species and hybrids in my own garden, as well as admiring them for years at Juniper Level Botanic Gardens. This past year, I planted one rhizomatous hybrid called Begonia 'Lime Swirl'. It did rather well in the neglect of my personal garden in well-drained soil in part to full shade. I dug a division last fall and placed it in the greenhouse, but left a portion in the ground to test. I have been pleasantly surprised when checking it throughout the winter months, finding that the rhizome was still firm and green with vegetative buds waiting to break when the warmth of spring returns. I also planted a few Begonia boliviensis 'Bonfire'. This is a tuberous species from, you got it, Bolivia, as well as Argentina. It has beautiful bright orange flowers. This is also proving hardy in the Raleigh area.
At JLBG, I have been eyeing Begonia heracleifolia var. nigricans for a few summers now. This rhizomatous species has large dark green edged leaves with a paler central streak down each of the angular palmate lobes. While Mark Weathington was in Taiwan, he collected a few species as well, some rhizomatous as well as other, more cane-like, begonias. These will be interesting to try in the landscape. I love trying these supposedly tender begonias that would not be expected to survive the winter. I am then excited to find them return. Have fun experimenting! I hope to try some of these out in the gardens in the future, so keep an eye out.
Pollination: The Major Contribution of the Honey Bee
By Charles Heatherly, Volunteer Beekeeper
Poets have a special talent for giving stature to such a simple mystery as to why a honey bee gently lands on a flower, extracts its sweet nectar, and leaves with a grain of pollen attached to its hind legs. It is an exercise of survival for the bee who manufactures the nectar into honey, essential food for the colony, and the pollen provides vital protein for the baby bees.
We first think of honey bees in terms of the honey they produce, which last year in the United States totaled some 150 million pounds, its wholesale value to the economy estimated at $150 million. That is dwarfed, however, by the value of the pollination work honey bees do for our crops of fruits and vegetables, worth some $19 billion, according to a recent National Research Council study.
Honey Bee Value to Agriculture
Extensive studies have quantified the benefit of honey bee pollination to various food crops. For example, we know that almonds, cucumbers, apples, avocados, melons, cranberries, asparagus, broccoli, carrots, cauliflower, and celery are entirely dependent upon honey bee pollination for their seeds to set and yield fruit. Blueberries and blackberries are 80 percent more productive with the aid of honey bees; grapefruits, and apricots, 90 percent. Seven of the nine crops that provide half of the vitamin C available to the human diet depend almost entirely upon external pollination for the production of fruits or sees (oranges, cabbages, green peppers, tomatoes, melons, tangerines, and watermelon).
California almond growers must rent a million honey bee colonies to pollinate their billion dollar crop, for which they pay prices sufficient to entice East Coast beekeepers to truck thousands of colonies across the country and back each year.
Pollination is the process by which pollen is transferred from the anther (male) to a receptive stigma (female) flower part. In its quest for sweet nectar, which is usually secreted deep within the flower, the honey bee coincidentally comes in contact with the pollen-laden anther. As the honey bee maneuvers through the flower, it brushes the pollen grains across the stigma; hence, fertilization and seed formation occurs. The rear legs of the honey bee are miraculously equipped with Velcro-like grips that bind pollen grains securely for the return flight to the hive.
One Third of the Food We Eat …
In North Carolina, some 10,000 beekeepers, mostly hobbyists with 10 colonies or less, produce $15 million worth of honey each year, but the value of honey bee pollination to farm crops is estimated at nearly $200 million.
One-third of the food we eat is dependent upon honey bee pollination. We know that because agricultural scientists, in their effort to boost production of essential food crops, have studied the honey bee for eons. Not so well known, however, is the impact of honey bee pollination to ornamentals, though we suspect it is significant.
Alfalfa, clover, and sunflowers are heavily dependent upon the honey bee. We know that because their plants and seeds are important commodities.
Honey Bees in the JC Raulston Arboretum
I can find no research that defines the benefit of honey bee pollination to ornamentals. However, I am convinced it is substantial. Take the Nellie R. Stevens holly, for example. In September of 2007, when I first established four honey bee colonies at the southeastern corner of the JC Raulston Arboretum, there were few berries on the holly trees. Last year, with honey bees in residence for a full year, the holly berries were prolific. It should be noted that a late and extended freeze over the Easter weekend in 2007 severely damaged most plants and trees flowering throughout the state.
I noticed one thing about the prolific crop of holly berries last year that may be an indication of a visit by the honey bees: the berries are especially thick along a band from about 8'–20' high. I have also observed bees in the Arboretum visiting plants throughout the year, even during mid-winter when there are no flowers elsewhere.
There is another side to the pollination scenario—the perspective of the honey bee who benefits immensely from the nectar and pollen available from the non-food crops, such as the tulip poplar tree, the most abundant source of nectar throughout much of the growing season in North Carolina. The tulip poplar is found throughout the state from the coastal plain, through the Piedmont and even into the western Appalachia region. While honey bees have to visit many small flowers to gather a full load for delivering to the hive, the large, yellow, bell-shaped tulip poplar flower is rich in nectar and sufficient to supply several bees with their quotas.
Another important source of nectar and pollen is the maple tree, which flowers in mid-winter, usually late January. Most people, except for beekeepers, are unaware of the maple flowers because its flowers are tiny and have no petals. Maples are very important to honey bees because their early flowers incite the colony foragers to bring in nectar and pollen, which inspires the queen to begin laying eggs, as many as 2,000 a day, for the quick build-up.
Aside from their economic value to agriculture, pollinated plants contribute a variety of ecosystem services to mankind, among them: water filtration, carbon sequestration, and flood and erosion control. Nor should we overlook the food value to birds and other animals of the fruit and seeds that are a direct result of honey bee pollination.
Honey Bees in Decline
There are fewer bee colonies in the United States today than at any time in the last 50 years. The number of commercial U.S. bee colonies declined from 5.9 million in the late 1940s to 4.3 million in 1985, and 2.4 million today. The decline of honey bee colony health has escalated since the 1980s with the arrival of new pathogens and pests. The spread into the United States of varroa and tracheal mites, and more recently the small hive beetle, have created major new stresses on honey bees.
The value of bee pollination to ornamental plants would be almost impossible to measure," said Stephen Bambara, Extension entomologist at NC State University. "Any plant with ornamental fruit or berries and any annual plant propagated by seed, requires pollination," he added.
The honey bee is the workhorse of pollination. When you see a bee on a flower in the garden, be assured she is fulfilling an important role, not only in producing the sweet honey we enjoy, but also in growing the food we eat.
2008 New Plantings
By Mark Weathington, Assistant Director and Curator of Collections
The past year has been a great one at the JCRA in terms of getting plants in the ground. Regular moisture for most of the growing season meant we could keep planting from early spring right through until fall. Over 900 different accessions from sources around the globe were planted during 2008 throughout the Arboretum.
A major planting area last year was the newly created Xeric Garden, installed where the Southwest Garden previously stood and utilizing the mature specimens from that collection. Forty-one agave were planted last summer with 37 of them in the Xeric Garden alone, while 16 of the 17 yucca taxa planted last year were also added to this area. The Xeric Garden became home to more than just woody lilies though. Almost one quarter of the new plantings in 2008 went to this display of drought tolerant, water-wise plants. New taxa include rare conifers like Widdringtonia nodiflora (African mountain cedar), Juniperus seravschanica, and Wollemia nobilis (Wollemi pine) as well as southwestern and Mexican oaks, Quercus wislizenii, Q. berberidifolia, and Q. durata. Broadleaf evergreens, such as Pittosporum tenuifolium 'Argentea Nana' (dwarf silver-leaf New Zealand pittosporum), Callistemon brachyandrus (prickly bottlebrush), and Calluna vulgaris 'Golden Carpet' (gold heather), work with the agaves and cacti to make this a truly four-season garden, while deciduous flowering shrubs add color bursts throughout the season. Highlights include the rare native Styrax platanifolius subsp. texanus (Texas snowbells) with white, bell-shaped flowers, several Punica granatum (pomegranate) cultivars, and the large-flowered Chilopsis linearis 'Bubba' (desert willow). Bulbs and herbaceous perennials complement the woody plants, providing color and texture in both summer and winter.
New selections of popular plants are always exciting to evaluate. Different flower and foliage color, vigor, or growth habit can add a lot to the garden. Some of these selections include the ground covering Lagerstroemia indica 'Rosey Carpet' (crepe myrtle), the variegated Cyrilla racemiflora 'Spring Cloud', the respectively dwarf and fastigiate Ginkgo biloba 'Troll' and 'Blagon', and the cutleaf Hydrangea arborescens 'Emerald Lace'. Some of these plants will certainly add quite a bit to the gardener's palette while others may prove to be a mere flash in the pan.
This year is shaping up to be a great year also. Watch the master plan continue to transform the JC Raulston Arboretum as we plant out the newly redesigned and expanded Asian Valley. It will feature new selections of Asian plants as well as wild collected species from "the mother of gardens." Other areas to watch for new plantings are the Lath House, Geophyte Border, and Conifer Collection.
At the JCRA, we're always looking for new species that we haven't grown previously. NC State University's Jenny Xiang, Ph.D., supplied us with seed for a couple of new plants from her treks in China, and we have grown them out and planted them on the grounds. Cornus eydeana is a newly described evergreen species of dogwood that we are hoping will be hardy for us here in Raleigh. We'll pass on more information as we learn about this exciting plant. Another new plant for us is the horsechestnut, Aesculus wangii (possibly the same as A. assamica). Either way, it has already shown some potential by retaining its leaves until frost. Other new species that we've added to the grounds include Phoebe formosana, a broadleaf evergreen in the laurel family; the beautyberry Callicarpa kochiana; and Hydrangea hirta, one of the five species, along with over a dozen cultivars, of hydrangea planted last year.
We hope you take the opportunity to come by the Arboretum often to view our constantly evolving and changing collections. Let us know about plants that impress you or about new plants that you think we should evaluate.
Musings on "Great Garden Plants"
By J. C. Raulston, Ph.D., North Carolina State University Arboretum Founder
(Editor's note: The following article was discovered in 2007 in one of the boxes that remained in Kilgore Hall after we moved out in 2002. J. C. originally prepared it for the December 16, 1993, Christmas Plantsman Luncheon hosted by William H. Frederick, Jr. The article brought back many rich memories to the few who read it so we thought we'd include it in this newsletter for everyone to enjoy.)
In an almost weekly event, I am asked to lecture here and there around the country on a wide variety of subjects—many of which carry the theme of good or recommended plants for various purposes—"native plants," "flowering vines," "conifers," "most profitable/marketable," "winter interest," "screening," etc., with endless variety. In general, during these events, I have the luxury of 40-60 minutes to pontificate and an 80-slide reel of photographs to give a range of the plants I feel fit the specific categories of the moment from the "thousands of plants one can't possibly do without."
I've recently been put to a new and possibly ultimate challenge which has occupied much of my daydreaming and thinking time over the last month. In two weeks, I will make a 14-hour round-trip drive to do a "slide lecture" to a select group of plantsmen peers—with a major problem inherent in the mission. The requested theme is "hardy landscape plants of the HIGHEST GARDEN MERIT"—not "curiosities," not "esoterica," not "profitable." A tough enough assignment in its own right—but strengthened by the additional requirement of a limit of only five plants, and only five slides total in the presentation. AAAUUUUGGGHHH!!
The list has churned and churned endlessly (and will likely continue to do so until departure)—just what is "high garden merit"?—and how to divide them out? As necessary whittling mechanisms, I've decided they need to: be basically tough and not need pampering, with relatively low maintenance (little or no special environments, modified soils, watering, pruning, spraying, etc.); have interest or appeal over a fair length of time; that they have high ornamental value at least at some point (showy or interesting flowers, fruit, bark, foliage, texture, etc.); be "available" to the public (no use of my normal "I've got the only one in the United States" tricks); and be represented in my slide files with an excellent photograph (an ironic, but realistically very important part of "marketing and promotion of plants" today).
And to force myself to variety, there will be one each in the categories of: an herbaceous perennial (a major concession here, but necessary); a broad-leaved evergreen; a deciduous shrub; a tree (we needed both deciduous and evergreen categories); and a conifer. Which left me without other "essential" categories of bulbs, vines, ground covers, etc. Only five plants? How painful!
The Herbaceous Perennial
In one sense, perhaps the easiest, as everyone knows I know nothing about these "things that die to the ground every year." But I do have a variety of favorites, enough to make even this decision difficult. Amsonia hubrichtii was finally chosen as it has a six to seven month period of continual high ornamental interest (rare in perennials), is new enough that it is not yet in most perennial references (an American plant the English haven't grown yet to convince Americans of its value) but available in recent years through a goodly number of native plant and speciality perennial growers. There are 25 species of blue stars (Apocynaceae) native to the United States and Japan—but only A. tabernaemontana is common in the ornamentals industry.
A. hubrichtii is native to the American Midwest. It has beautiful, pale blue, star-like flowers in terminal clusters in early summer; very handsome, thin, linear, graceful, green foliage through the summer on plants reaching 2'–3' in height and 3'–4' wide; and, rare for perennials, a long, spectacular fall foliage color display of brilliant ginkgo-yellow fading to golden brown. It is easy to propagate by seed, rooting of softwood cuttings, or division of clumps—and is exceptionally stress tolerant with no pest problems. Probably useful in USDA Zones 5–9 (4?) and best in sun. (My five runners-up include: Asclepias tuberosa, Baptisia pendula, Calylophus drummondii, Iris siberica 'Caesar's Brother', and Muhlenbergia dumosa.)
The Broad-leaved Evergreen
As the standard background of the entire Southern landscape, with thousands of choices—a real toughie. But my choice of Mahonia 'Arthur Menzies' is an easy and comfortable one with its many merits. In the mid-South, leatherleaf mahonia, M. bealei, is the standard representative of this genus—a fine plant in its own right. But in this large genus of over 70 species in the Berberidaceae family, one of the superstars is M. lomarifolia from Myanmar which can reach 40' in height, with long leaves up to 2' in length, containing dozens of leaflets, and huge terminal candelabras of yellow, fragrant flowers in midwinter—and sadly a USDA Zone 8-9 non-hardy species (at least long-term in Raleigh; we grow it as a short-lived woody perennial).
In 1961, a group of M. lomarifolia seedlings were sent from the Strybing Arboretum in San Francisco (where it grows magnificently) to the University of Washington Arboretum in Seattle for trial. In 1964, a severe freeze reduced this group of plants to "brown pulp" except for one seedling—which was subsequently identified as an accidental hybrid of M. lomarifolia and M. bealei that occurred by natural crossing of adjacent parent plants at Strybing.
'Arthur Menzies' (named in honor of "the most knowledgeable horticulturist in California") is intermediate in characters between the two parents. The leaves are 1'+ in length—longer than M. bealei and shorter than M. lomarifolia; it will probably be 12'–18' in height at "maturity" in most areas; and it generally flowers in Raleigh from early December through mid-January with showy and wonderfully fragrant yellow inflorescences to 1' in length. (Similar "competitors" in the Mahonia world are the several M. ×media cultivars from M. lomarifolia × M. japonica crosses in England—but these tend to flower later in the winter here.)
Sadly, it has remained a relatively unavailable plant except for local propagation and use in the Pacific Northwest where it originated. Although propagation is fairly easy by terminal or single-node stem cuttings under mist, this requires a parent stock plant for availability of wood, and plants provide low annual yields for slow buildup. It is available in small quantities from speciality nurseries with hunting, and, happily, several large wholesale nurseries in North Carolina are now in the buildup stages, using the half-dozen older plants in the NCSU Arboretum for propagation so it can begin to enter our local market in the near future. Useful in USDA Zones 6–8 and best with winter shade to prevent foliage scorch in periods of bright sun coupled with low temperatures. (My five runners-up include: Abelia ×grandiflora 'Confettii', Magnolia grandiflora 'Little Gem', Nandina domestica 'Moonbay', Osmanthus fragrans var. aurantiacus, and Viburnum tinus.)
The Deciduous Shrub
The musing on this category made me aware of how difficult the "long season of quality ornamental interest" is to achieve here. The truly long-flowering things like Buddlejas and dwarf spireas are of somewhat "fuzzy" ornamental interest. But a truly outstanding plant finally emerged from this sorting process—Hydrangea quercifolia 'Flemygea' (Snow Queen™)—oakleaf hydrangea (Hydrangeaceae).
This southeastern U.S. native plant has large, showy, white inflorescences in summer which dry and remain attractive through winter, interesting cut-leaf "oak-like" foliage which has good fall color, and wonderful winter bark on older plants. It is tough with good stress tolerance once established and can be grown in sun or shade, has no insect or disease pests, is easy to propagate and is rapid growing with an ultimate size of 5'–8', and is long-lived. (My five runners-up include: Enkianthus perulatus, Hamamelis ×intermedia 'Sunburst', Hydrangea anomala ssp. petiolaris (staked and eventually grown as a shrub), Poncirus trifoliata 'Flying Dragon', and Viburnum plicatum var. tomentosum 'Summer Snowflakes'.)
A real dilemma here—how could I possibly bypass my much beloved winter-flowering Prunus mume, Japanese flowering cherry, that I've promoted for so many years? In a category with many outstanding candidates, it finally falls to the "long season of merit" criteria with less than wonderful foliage in summer. So, by "default," Cercis canadensis ssp. texensis 'Oklahoma' (Fabaceae) becomes the automatic choice. This superior seedling variant of the Texas redbud was found in the Arbuckle Mountains of Oklahoma and was introduced by Warren and Son Nursery of Oklahoma City in 1964. A small tree to 15'–25', with great heat and drought stress tolerance, with the specific landscape merits of a rich flower color of the darkest magenta of all redbud species and cultivars, and large, thick, leathery, glossy foliage so handsomely appealing one would grow the tree for ornamental merit even if it never flowered. It is useful in USDA Zones 6–9 and does best in full sun, though it will tolerate light shade.
The only thing keeping this exceptional plant from being one of the most widely grown in the United States is the relative difficulty of its propagation—requiring budding, and one of the trickiest of the woody plants to bud successfully due to its very thin cambium. So suppliers can never meet demand though it is grown more and more each year—and not that difficult to find now with a bit of searching. Recently, its similar "sister" plant, the white-flowering form of the same species called 'Texas White' has yielded to tissue culture techniques—so although propagation specialists have not yet been successful with 'Oklahoma' in tissue culture, it does appear possible to break this barrier to mass production in the future. (My five runners-up include: Acer palmatum 'Beni Kawa', Lagerstroemia fauriei 'Fantasy', Prunus mume, Salix babylonica, and Styrax japonicum 'Emerald Pagoda'.)
A truly impossible choice with the diverse variety in sizes, forms, and "looks" available—and additionally tricky with my coniferaholic partner Kim Tripp breathing down my neck to check my conifer "political correctness." "We've" finally settled on Cephalotaxus harringtonia 'Prostrata' for some unusual conifer properties. This slow-growing, mid-height (to 2'–3'), ground cover shrub native to Japan can be grown in either full sun or heavy shade, has a very different "feathery" texture than the ubiquitous junipers normally grown for ground cover use, will tolerate poorly drained, hot soils of the southeast which do in many conifers, and, of increasing importance in the eastern suburban gardens, it is totally ignored by deer browsing on other garden delicacies.
It is easy (though slow) in propagation from hardwood cuttings in winter and the only negative characteristic keeping it from the mass market is the slow growth which precludes fast, cheap production for consumers. With the awarding of the Pennsylvania Horticultural Society Gold Medal in 1993, production and marketing of this outstanding plant should certainly increase. Useful in USDA Zones 5–9 in sun or shade. (My five runners-up include: Cedrus atlantica 'Glauca Pendula', Chamaecyparis nootkatensis 'Pendula', Cryptomeria japonica 'Elegans', Juniperus deppeana 'McFetters', and Taxodium ascendens.)
Only five plants? Absolutely ridiculous!
The NAPCC Certified Cercis Collection at the JC Raulston Arboretum
By Mark Weathington, Assistant Director and Curator of Collections
The North American Plant Collections Consortium (NAPCC) is a program of the American Public Gardens Association in cooperation with the USDA's Agricultural Research Service and the U.S. National Arboretum. In this program, institutions commit to holding and developing a collection of documented living plants according to professional standards of collections management. They share collections information with other public gardens to compare holdings and to identify duplications and gaps. This approach makes efficient use of available resources, strengthening collections through combined collaborative activities. This network of botanical gardens and arboreta work to coordinate a continent-wide approach to plant germplasm preservation, and to promote high standards of plant collections management.
Participating institutions undergo a thorough evaluation to determine their suitability for the program. Gardens must demonstrate exceptional curatorial and management practices, meticulous record keeping, and a commitment to the long term care of a specific taxonomic group.
The nature of the JC Raulston Arboretum's mission, combined with its small size, means that the collections are constantly evolving as new plants are added and older specimens removed. Committing to the long term curation of a single taxonomic group was only undertaken after careful consideration of the limits this would impose on the dynamic nature of the collections. Because the aims of the NAPCC to make germplasm available for selection and breeding, taxonomic studies, evaluation, utilization, and other research purposes ties in closely with the goals of the JC Raulston Arboretum, we decided that this would be a mutually beneficial collaboration.
Cercis (redbud) has been an important part of the collections of the Arboretum from the start. The first accession dates to 1978, shortly after the Arboretum was established. While most public gardens have their share of redbuds, few have significant collections of the often hard-to-find species and cultivars. Depending on your taxonomist of choice, there are 6-11 species of Cercis and several other sub-specific forms. The JCRA currently holds 7 species and 40 distinct taxa with a concentration in the forms of Cercis canadensis. While there is very little replication of taxa in the JC Raulston Arboretum, the history of freely sharing propagules with other gardens and nursery professionals has ensured that replicates of our collections can be found all over the world. The stated goal is to collect all known species and subspecies for comparison as well as collecting as many of the various named forms of redbud as possible.
Considerable confusion abounds in the literature—and consequently the marketplace—surrounding the Asian redbuds. The JCRA NAPCC collection will help to clear up the taxonomic confusion surrounding this group of landscape-worthy trees and shrubs and provide propagules to other researchers and the green industry. Side-by-side comparisons of living plant material will enable us to develop keys for identifying plants. The collection will also help the JCRA's education mission, both to the public and to NC State students, as we impress on them the importance of coordinated and collaborative efforts to preserve germplasm ex situ.
By the mid 1990s, two selections of Cercis had been selected and named by J. C. Raulston, Ph.D. The first, Cercis canadensis subsp. mexicana 'Bonita', was grown for its exceptionally glossy leaves with regularly undulate margins. The other, C. glabra 'Celestial Plum' (originally named and still sometimes seen as C. yunnanensis 'Celestial Plum') was selected for its blue-green foliage and loads of light plum purple flowers in early spring. Once thought to be lost from cultivation, this outstanding selection is making a resurgence. In more recent years,the JCRA Cercis collection has been used as source material in the breeding efforts of Denny Werner, Ph.D., plant breeder in the Department of Horticultural Science at NC State University. Over the next several years, he will be releasing new plants in conjunction with the JCRA. Plants coming out of his program in the near future include weeping forms with both variegated and purple leaves as well as the purple-leafed Cercis canadensis subsp. texensis 'Merlot'. We anticipate even more exciting forms of our native redbud to emerge from this innovative breeding program. The newest introduction from our existing collection is a Cercis chinensis that we have been watching for years. It is one of the very first redbuds in our extensive collection to show color, it absolutely covers itself in lavender-pink flowers, and is one of the last to finish flowering. The exuberant flower display starting at the very beginning of March and continuing through the end of April gives us the hope of returning spring and ushers in the "March Madness" of our garden's floral display. To honor the memory of Coach Kay Yow, we have named this exceptional plant after this remarkable women—C. chinensis 'Kay's Early Hope'.
Formatted into HTML by Christopher
Programs and Education Coordinator
JC Raulston Arboretum
Department of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7522
© The JC Raulston Arboretum, March 2009