Hellebores: A Comprehensive Guide - Hardcover

Burrell, C. Colston; Tyler, Judith Knott

Hardcover

ISBN 10: 0881927651 ISBN 13: 9780881927658

Verlag: Timber Press, 2006

Inhaltsangabe

A mere 10 years ago, hellebores were considered connoisseurs' plants - subdued in coloration, hard to find, and the subject of much snobbery. Today, however, they are among the hottest perennials, thanks to their early bloom, long-lasting flowers, shade tolerance, handsome foliage, and the profusion of new colors and forms that have recently become available. Authors Cole Burrell and Judith Tyler have produced what is arguably the definitive book on this genus, packed with up-to-the-minute, comprehensive information on growing, maintenance, design, hybridization and selection, and trouble-shooting. This lavishly illustrated volume will interest hellebore lovers at every level of interest.

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Über die Autorin bzw. den Autor

C. Colston Burrell is a garden designer, award-winning author, photographer, naturalist, and teacher. He grows hellebore species and hybrids alongside natives and other treasures in his garden in the Blue Ridge Mountains near Charlottesville, Virginia. Colston holds two master's degrees: one in horticulture and one in landscape architecture from the University of Minnesota. He is a lecturer in the College of Architecture and Landscape Architecture at the University of Virginia and principal of Native Landscape Design and Restoration. He is the author of many garden books and has twice won the American Horticulture Society book awarda"in 2007 for Hellebores: A Comprehensive Guide and in 1998 for A Gardener's Encyclopedia of Wildflowers. Colston is a contributing editor for Horticulture and frequently writes for Landscape Architecture and American Gardener. He lectures internationally on design, plants, and ecology. Judith Knott Tyler and her husband, Richard Tyler, have owned and operated Pine Knot Farms since 1982 on land that has been in Judith's family for six generations. Pine Knot is a wholesale and retail nursery specializing in herbaceous perennials, with an emphasis on hellebores. An artist by training, Judith channels her creativity into making gardens, layering the forms and textures of plants into living sculptures that change with the seasons. Judith brings a magnitude of life experience to her nursery work including time spent landscaping, producing vegetables for a communal farm in Canada, and even tutoring underprivileged children. Hellebores fascinate Judith because as she says, "Any plant brave enough to reach its peak flowering time during the dull days of late winter, deserves a place of pride in most gardens. If we consider the wide range of conditions many of the species will tolerate, there is truly a hellebore for almost any garden."

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Hellebore Morphology

Who could fail to be smitten with hellebores? Anyone with an appreciation of beauty is quickly seduced. Hellebores seem to cast a spell from which you never awaken. Is it the flowers? They lack cloying fragrance to draw you in. Many are small and subtly colored. Even the most colorful fall short of riveting or festive. What can it be? Their precocious nature is part of their allure, but what makes them so much more than alluring? For many, hellebores are an obsession. What other flower, save for cousin Adonis, punches through the ground with flowers open while frost and snow surround them? Like most early bloomers, Adonis is fleeting. Not hellebores; they bloom for two months or more — longer where spring arrives slowly and nights stay cool. Their rich green foliage provides summer and winter structure. For us, they are the whole package — fantastic foliage, precocious bloom, and beautiful flowers. In short, elegant simplicity.

Most hellebores are native to mountainous regions of Europe, in open oak and beech woodlands, scrub, grassy meadows, and on rocky slopes. These areas are characterized by limestone bedrock and calcareous, humus-rich soils. Western Europe, including the British Isles, is home to three species. The odd plant out is Helleborus thibetanus, which is native to China, well outside of the epicenter of hellebore distribution. The rest of the species fall in between, with the bulk of them centered in the Balkan region of the former Yugoslavia. In all aspects, the plants are both charming and beautiful. In their native lands, few species but the Christmas rose (H. niger) are grown in gardens. This wildling is often found in churchyards due its association with the birth of Christ. Today, we value the wild species for their delicate beauty in woodland and informal settings. (Refer to Chapter 3 for more information on species and their distributions.) The hybrids are by far the most popular garden hellebores worldwide, cherished for their early bloom, showy flowers, and decorative evergreen foliage.

Flowers

Hellebore flowers lack conspicuous petals. Five petal-like sepals create the show, a characteristic shared by most members of the buttercup family (Ranunculaceae). Relatives such as buttercups (Ranunculus), windflowers (Anemone), and winter aconite (Eranthis) share the open, cup-shaped flower. Inside the five showy sepals hides a ring of up to 32 subtle petals, modified into tubular or funnel-form nectaries. The center of the flower sports two to ten pistils surrounded by multiple rings of up to 125 stamens. The ovaries, or carpels, of the flower are an essential characteristic for distinguishing certain species. Some carpels are fused at the base, while others are free. After pollination, the sepals of most species fade to rose or green and often persist in an attractive state until the seed is ripe.

Occasionally, double flowers occur in the wild as a result of spontaneous mutation. Doubling is known to occur in H. dumetorum, H. niger, H. orientalis, and H. torquatus. Doubling occurs when the nectaries, which are actually modified petals, become petaloid, or petal-like. In the place of the tubular nectaries, flattened petals are produced inside the ring of fi ve sepals. The effect can be quite attractive.

Sexually speaking, hellebore flowers are protogynus, which means the stigmas are receptive before the stamens in the same flower shed their pollen. This adaptation generally keeps a flower from fertilizing itself and encourages cross-fertilization. Honey bees (Apis mellifera), bumble bees (Bombus spp.), and other small bees are the primary pollinators of hellebores. They visit the flowers quite freely and move pollen throughout the garden. The flowers can be self-fertile, however, and the pollen from a single flower can fertilize the same flower if the stigma is still receptive. More commonly in nature, pollen from one flower fertilizes another flower on the same plant. This is, in fact, a common occurrence in our gardens, though cross-fertilization from one plant to the next also occurs freely.

Flowers are borne clustered at the tips of persistent, leafy aerial stems or on ephemeral flowering scapes with leaflike bracts. The flowers are carried singly (occasionally doubled), as is common in H. niger, or in clusters of three or more. The flowers emerge in winter or early spring, before the new leaves in most species. True leaves are borne on long petioles. After the seeds ripen, the flowering stems do not bloom again.

Fruits and Seeds

The fruit of a hellebore consists of a cluster of dry follicles surrounded by persistent sepals. Follicles split along one inward-facing seam to release the dark brown or black seeds, which are ovoid and sometimes flattened on two sides so they stack neatly in their vessels. The seeds of some species possess fleshy arils called elaiosomes, which encourage ants to distribute them. This dispersal mechanism is called myrmecochory. Sex pheromone mimics are exuded by the eliasomes to lure ants to the ripe seeds. The plants win because the seeds are taken away from the parent, and with less competition they have a better chance of survival. The ant, for its part, gets some fats and starch from the elaiosome — not a bad deal for all involved.

Hellebore seeds experience a combination of endogenous dormancies. This means that when the seed is released from the follicles, the embryo must undergo morphological and physiological changes before it can germinate. The embryo within the seed is immature or not fully developed at the time of dispersal in early summer. The underdeveloped embryo matures as the seed ripens. A chemical inhibitor further suppresses germination. This kind of dormancy is called physiological inhibition. Once the embryo is fully developed and the hormones that inhibit germination are turned off by chilling, a process called cold stratification, the seed is ready to germinate. Most fresh hellebore seed germinates after a warm ripening period followed by modest chilling for 60 to 120 days.

First to emerge as the seed germinates is the radicle (seed root), which penetrates surprisingly deep in a short time. Two untoothed cotyledons (seed leaves) emerge next, and soon after them the first true leaf, which is divided in most species. Of course, there are exceptions. Helleborus vesicarius exhibits a more complex dormancy and forms only cotyledons the first season. After a second chilling, the shoot and first true leaves emerge. Helleborus thibetanus is also a maverick. The cotyledons never emerge from the seed. Instead, a single three-parted leaf emerges straight from the germinating seed. Most hellebores flower within two to four years of germination. (See Chapter 7 for more details.)

Stems and Leaves

The prevailing convention is to divide hellebores into two groups based on whether or not the plants produce persistent aboveground stems. The terms used for describing the two different growth forms are acaulescent, or stemless, and caulescent, or stemmed. The majority of species are considered acaulescent, the stem being entirely underground and represented by a rhizome. Caulescent species bear persistent aboveground stems with distinct leaves borne up the stem. The flowers of caulescent species are borne at the apex of year-old stems. After flowering, the old stems die, and new stems emerge from the crown that bear flowers the following year. The succulent nature of the stems makes them susceptible to winter damage. As a result, they are far less hardy than their acaulescent relatives.

In reviewing the taxonomy and anatomy of the genus Helleborus, Will McLewin (of Phedar Research and Experimental Nursery, Stockport, U.K.) and Brian Mathew (who recently retired from Kew Gardens, Richmond, U.K.) propose additional classes of plants that are intermediate between the two conventional types. The main characteristic used to separate caulescent from acaulescent is the persistent, aerial stem. Though not aerial-stemmed, acaulescent species do indeed have short, usually subterranean stems with very short internodes. Though McLewin and Mathew currently maintain the status quo, H. niger, H. vesicarius, and H. thibetanus do not fi t neatly into the traditional classifi cation. These hellebores bear short stems that differ structurally from either established group as well as from each other. In time, the terms used to describe hellebore stems will undoubtedly change.

Hellebore foliage is beautiful and perplexing. It helps if you have a penchant for geometry and the ability to count above 20. Most hellebore leaves are variations on a theme. The thick petiole joins together three leaflets at the same point. The leaflets are generally oval to lance shaped and are often, though not always, toothed. Those with three undivided leaflets are deemed ternately divided. When the two outer leaflets are divided, the term applied is palmate. When the outer divisions are fi nely divided and seem to circle the petiole, the leaves are called pedate. The outer leaflets of most species are usually divided or lobed, and some species display leaves cut into as many as 100 pencil-thin segments.

Most species are deciduous in the wild, though in gardens the abundance of moisture and nutrition can keep them green and growing. Some, like H. thibetanus, are summer dormant, while others fall quiescent in autumn and winter. Helleborus orientalis and hybrids, as well as H. niger, are fully evergreen, as are all the caulescent species. Helleborus vesicarius is unique in displaying winter foliage after complete summer dormancy.

Rootstocks

Hellebores grow from thickened, slow-creeping rhizomes with thick, fleshy, sparsely branching roots. These thick rootstocks account for the longevity of acaulescent hellebores. The rhizomes grow horizontally and produce new buds at the bases of the current year's stems. Leaves and flowering scapes arise directly from the rhizome in most species. The ephemeral bloom stalks of stemless species leave behind rounded, flat scars.

Older rhizomes develop a spiraling or corrugation (ridging), with each band representing a node where a leaf was attached. In time, the multistemmed crowns become congested and woody, with dozens of eyes. Old roots seem to serve mostly for anchoring the plant, while new roots, which are produced in autumn and spring as the rhizome elongates, provide maximum water and nutrient uptake. Root growth appears to be temperature dependent, with cool soil promoting rapid growth and warm soil slowing or stopping root growth altogether. Older roots branch when damaged, but the bulk of activity is associated with fresh growth.

Details zur Ausgabe

• Verlag: Timber Press
• Erscheinungsdatum: 2006
• ISBN 10: 0881927651
• ISBN 13: 9780881927658
• Einband: Tapa dura
• Sprache: Englisch
• Anzahl der Seiten: 296
• Kontakt zum Hersteller: Nicht verfügbar