Read an Excerpt
Plants have an ability that no animal does: they can, theoretically, live forever. An orchid living in the duff on the forest floor simply grows a complete new self each yearnew roots, new stem, new leaves, new flowers. Barring some catastrophe like a systemic disease or the chomping jaws of a burrowing rodent, a herbaceous plant like the orchid can go on this way for eons like the phoenix rising anew from the ashes each spring.
Woody plants differ from herbaceous plants in that they retain the same framework from year to year, pasting on a new layer of living tissue over the dead bones of tissues past. Unlike the orchid, a shrub or tree pastes on a veneer of new tissue atop those preceding it. An old, massive oak is simply a thin, living skin stretched over a long-dead framework of supporting wood. Woody plants were the first master builders, laying down lignin and cellulose to strengthen their stems and lift their leaves higher and higher above other plants that would otherwise shade them, or browsers bent on eating them. After the invention of photosynthesiswhereby leaves manufacture carbohydrates from carbon dioxide, water, and lightthe invention of wood was the next great leap forward that allowed trees and shrubs to dominate all the places where liquid water is abundant enough to nurture their lofty canopies.
Inevitably, this very woody framework that is their strength is also their undoing, as fungi, ants, and termites discover it and begin to slowly but relentlessly undermine it from within. Trees do not die quietly. Their bleached skeletons remain like whale bones on the beach long after their living skin has withered. Colonizing plants like box huckleberry and quaking aspen combine the strategies of the oak and the orchid, outliving even the most ancient woody tree by sending up new stems or trunks from a slowly spreading root system. DNA testing has shown that individual colonies of aspen or huckleberry, which may stretch over many acres, consist of a single individual that has gradually been expanding over the last 10,000 years. Barring climate change or human interference, it could easily go on for 10,000 more!
Woody plants have been called the “bones” of the garden, and you could say the same of their role in the wild as well. They are the structure that holds everything together, both physically and aesthetically. It takes a little getting used to, planting things that will long outlive us and that we may not see reach maturity. (Rapidly growing wildflowers are much more satisfying in this regard.) In fact, it is hard to really get to know individual trees or even large shrubs because they go about their business above our heads, out of viewmysteriously. Most of the time we can catch mere hints of a tree’s ongoing life processesa falling leaf, a discarded cone, or an intricate, spotted caterpillar that has dropped from somewhere above. So planting a tree is in some senses an act of faith: faith in my own longevity maybe, faith in the future probably, faith in the power, importance, and mystery of trees certainly.
I have met very few people interested in native plants who are not also interested in the rest of life on earth. One of the things you quickly learn about woody plants is the staggering number of birds, small mammals, insects, and fungi, not to mention people, that depend on them directly for their existence. Sara Stein, the author of the Noah’s Garden series, told me recently of her unrestrained delight upon finding pipevine swallowtail butterfly caterpillars feeding on the Dutchman’s pipe she planted in her yard. Truly, if you plant it, they will come. It is with the same sense of satisfaction that I watch migrating birds feeding ravenously on the winterberry holly I planted out back near the terrace or bees buzzing around my sweet pepperbush. So, planting a tree, or vine or shrub is also an act of faith in the marvelous complexity of lifea complexity that is impossible to really comprehend but truly awesome to behold. Go plant a tree.
What Is a Native Plant?
For the purposes of this work, “native” means plants growing in North America prior to European settlement, and the term “woody plant” means any species with at least a woody (or lignified) trunk or base. The use of native plants in landscaping has become very politicized in recent years, and this is unfortunate. Like the noted writer- philosopher Wendell Berry, I am suspicious of movements, and do not want to see the enjoyment and appreciation of our wonderful flora needlessly polemicized under the auspices of a native plant movement. Therefore, I think both the term “native” and some of the reasons foor choosing to grow these plants need further clarification.
It is natural for us humans to think of time in terms of our own life spans, and it isssss certainly understandable that we see organisms that live and die over longer intervals as having an air of permanence. A Douglas fir that has lived 1,000 years is incomprehensibly old to us, and to think, further, of the land itself as having a life span is nearly impossible. In geological time, 1,000 years is roughly equivalent to 10 minutes in the life span of a person. In this sort of relative time, it has been about two years since the age of the dinosaurs and about two hours since the last Wisconsin glaciers retreated north. If we could only view the earth with some sort of super-time-lapse photography for a few minutes, geological time would be understandable as the powerful dance of continents, mountains, ice, and water, where forests ebb and flow like waves on a beach and ancient trees are mere momentary bubbles in the changing surf. With this sort of perspective it becomes clear that what we see in the wilds at this particular moment in time is but a split-second-long freeze frame in a much larger process, and to draw too many conclusions about particular elements in this composition without putting them into a geological context is a dangerous proposition. This becomes particularly important when one is discussing rarity and extinction or even the concept of what is a native plant. In geological time, a plant is native somewhere only as an airplane’s transient vapor trail is native to a particular place in the atmosphere. Species are always in a state of flux, advancing and receding, evolving and disappearing, their presence in any one place only transitory. Even the land itself is constantly moving, shifting, and recombining. How can we then say that anything is native anywhere? It is equivalent to taking a photograph of a busy street and from then on assuming that these people caught in mid step have always resided on this bit of pavement and will always continue to, unless some great catastrophe intervenes. To argue this point would be an exercise in futility.
The important point, then, is not simply what it means to be native, but what possible consequences displacing a particular species from a particular place may have on the ecosystem as a whole. This is the key argument for what ecologists call the preservation of biodiversity. Individual species do not exist in a vacuumthe actions of one species radiate outward like ripples on a pond, affecting many others. For example, say the eggs of an insect that feeds on hemlock trees are transported halfway around the globe by some freak weather event (or with human assistance). The hemlocks in the new region have never experienced this pest, have no defenses against it, and begin to die out over much of their range. As the range of the hemlock recedes, so too do the ranges of the fungi that feed on its wood, the birds that feed on its seeds, the caterpillars that consume its needles, and so on. The gap left in the forests by the death of these trees allows the advance of other trees that could not grow in the shade of the hemlock, and as this may well have been the climax tree in the forest succession of the region, the whole life cycle of the forest itself is altered. Should the diversity of life be great enough, other organisms that depend on these advancing species will move forward as well, and a few may eventually adapt to feed on the hemlock pest itself, reversing the tide and restoring some kind of balance. In a healthy ecosystem, such disturbances can over time actually increase biodiversity, and the more species there are in a particular region, the more flexible and proactive the whole ecosystem will be. Just as it is easier to write poetry with a vocabulary of 10,000 words as opposed to 200, so too will a large number of species be more responsive to change and able to restore balance after disturbance or insult. The elimination of biodiversity dams this flow, so to speakthe water still trickles by, but the salmon cannot swim and the bears cannot feast, the dragonfly nymphs have less to feed on and it is hard to say whether even the quality of the mud doesn’t change for the worse. More than any other species on the planet, we humans have the ability to shape our environment. I firmly believe that if we all decide to make an effort to restore some of the local plants to our landscapes, we will in no small way help make our own piece of the world a richer, more diverse, and by consequence a healthier place. This is not politics; it is simple truth.
How to Use This Book
Three years ago, when I decided to write The New England Wild Flower Society Guide to Growing and Propagating Wildflowers of the United States and Canada, I did so with the conviction that there was a vital need for a comprehensive guide to the culture, personalities, ecology, and propagation of these plants. But wildflowers are only part of the story. What about trees, shrubs, and vines? There is certainly more information available on native woody plants than on their herbaceous companions, but it tends to be either regional in focus or written in outline or table format. So I present to you here the companion to my book on wildflowers: a comprehensive reference on woody plants, which I hope will be useful for the novice and expert, gardener and naturalist alike, with information presented in a style that is approachable and easy to read but still informative and accurate. The plants are arranged alphabetically by genus, each entry beginning with an overview that includes anecdotes and relevant information to serve as an introduction to the group. This is followed by notes regarding basic cultural advice, garden uses, and, if relevant, the plant’s importance for particular wildlife. Each entry concludes with a list of representative species and their particularities. General cultural information and detailed notes on propagation are in a separate section at the end of the book.
I have sought to include woody plants from all of temperate North America so that you can select species that are well suited to your individual climate, light conditions, and soil. While I encourage you to grow and appreciate the plants of your area, I also know that we all as gardeners like to seek out the challenge of something new and different, so whenever possible I have included information to aid you in growing a particular plant successfully outside its native range.
Latin Names: Family, Genus, and Species
I realize that Latin is no one’s first language, and it can be a bit ponderous to use if you are not used to it. I have liberally used Latin in the text, not to impress you with my erudition, but because it really is valuable to know the Latin names and to become comfortable using them, for two reasons. First, common names are often wonderful and poetic, but many common names, like maple, are generic, so it is difficult to know which plant someone is referring to. Even more confusing are names like ironwood or sage, which get applied to multiple genera. Second, the Latin binomial name can tell you a great deal about a plant because it is part of a system of classification based on a plant’s familial or evolutionary relationships and individual characteristics. The plant kingdom has been split into categories and subcategories on the basis of characteristics of their vasculation, seeds, reproductive structures, and other features, starting with the most global categories and working down to the most specific. This classification system works as follows: There are two divisions in the plant kingdom. The first, Pteridophyta, includes all the plants such as ferns and mosses, also known as bryophytes. These are plants that reproduce without flowers or seeds, relying instead on spores. The second division is the Spermatophyta, or seed plants, which encompasses most of the plants now on earth; they are more recently evolved than the Pteridophyta. The Spermatophyta are divided next into gymnosperms and angiosperms. Gymnosperm means “naked seed”the ovules, or eggs, are not enclosed in an ovary. In this group are all of the conifers and a few related plants. Angiosperm means “covered seed,” because these evolved coverings or fruits for their ovules or seeds. This is the biggest group of plants alive today. The angiosperms are next divided into two classes: the monocots, plants with only one cotyledon (the first, or embryonic, leaf), and dicots, which have two cotyledons. Among the monocots are grasses, lilies, orchids, irises, palms, bromeliads, sedges, and a few others, so it is clear that they are pretty successful evolutionarily. The monocots are a personal interest of mine. However, most of the plants we work with are dicots, a large and highly successful group.
Both the monocots and dicots are subdivided into a series of families. A family is a fairly manageable grouping of plants that share certain recognizable characteristics. (All family names end in “aceae.” This is a nomenclatural convention that makes it easy to tell a family name from a genus or species name.) The members of the orchid family (Orchidaceae) are monocots in which the sexual parts are united into a structure called a column; the composites (Asteraceae), are dicots with flowers aggregated into distinct daisy- type inflorescences. It is very helpful to know the family a certain plant belongs too, as it gives many clues, especially regarding its propagation and habits. Certain families, like the Ericaceae and Rosaceae, have a disproportionate number of wonderful garden plants, and knowing this will help you narrow your search for new possibilities.
Families are further subdivided into genera (genus is the singular form). A genus is a smaller grouping of plants with very similar characteristics. Oaks are in the genus Quercus, honeysuckles in the genus Lonicera, and firs in the genus Abies. Though beeches and oaks are in the same family, they have been split into separate genera because they differ anatomically and cannot interbreed. Conveniently, plants within a genus share many of the same characteristics, so I have used this as the organizing principle of the book.
Genera are further divided into species. A species is a group of genetically very similar plants or animals that are distinct from other species in the genus. Humans are a speciesHomo sapiensthat is distinct from other (now extinct) species in the genus Homo, such as Homo erectus. The Latin binomial (two-name) system consists of the genus name, capitalized, and the species name, written lower case. The genus name and species name are always italicized. The binomial system is sort of like our system of phone numbers, with area codes and exchanges, which allow for a great many possible combinations. Thus Fraxinus velutina and Quercus velutina are two very different plants with the same specific epithet. Knowing that velutina means “velvety” tells you something about the plants too. With plants, the separations between species are usually more blurred than with animals. This means that you cannot assume that two plants are of the same species if they produce fertile offspring. Though you can tell from just looking that Ilex glabra is different from I. opaca, a species is an artificial construct, and plants are always in a state of flux and evolution. (Note that a repeated genus name is often abbreviated, as in the preceding exampleI. opaca rather than Ilex opaca.) When a plant occurs over a wide range or in geographically isolated areas, often there are distinctive races, called subspecies (abbreviated ssp.), which are judged to be too similar to be separate species, but are distinct enough from each other to be recognizable. Erigeron chrysopsidis ssp. brevifolius (meaning “short-leaved”) is slightly different from E. chrysopsidis ssp. austiniae (meaning from Austin, Texas). More commonly, there are minor variations within a specieslike flower color, size, level of hairinessthat are not extreme enough to be given a subspecies designation. These are noted as varieties (abbreviated as v. or var.). Botanically speaking, a variety represents some consistent, natural, and minor variation that occurs within a given population of the species. In practice, the term “variety” is often confused and applied to all sorts of things, both horticultural and botanical. Juniperus virginiana var. glau”kame- ee-SIP-ar-iss,” people will understand, rules or no rules. With this in mind, I have given suggested pronunciations, based on horticultural convention, in parentheses for each genus, with the accented syllable in capital letters.
The Genus Entries
Each genus entry has introductory text, followed by four sections: Culture, Uses, Wildlife, and Propagation.
Culture: This section is intended to give a quick idea of how hard or easy it is to grow the plants in the genus and specific requirements (such as transplanting times and soil type), to help you decide if it is appropriate for your conditions. If individual species are quite different from the rest of the genus, I try to single them out here or in the species descriptions. I have avoided pointing out every possible pest or disease that might afflict the genus, mentioning only those serious enough to threaten the health of well-sited, otherwise vigorous individuals. See the chapter on propagation (page 261) for more comprehensive information.
Uses: Here I briefly list some of the possible situations and uses the particular genus is suited for (for example, shade tree, hedge, groundcover, soil stabilization), again to give you some ideas, but not to limit your own imagination. See also “Native Trees, Shrubs, and Vines for Various Sites and Uses” on pages 303311.
Wildlife: One of the strongest reasons for using locally native plants is their inestimable value to myriad nonhuman organisms that have evolved with, and come to depend on, these plants for food and shelter. There is not a great deal of information available about plants’ relationships with specific beetles, moths, fungi, or bacteria, but there is more on their benefits to birds, mammals, and butterflies, and so I have concentrated on the latter groups. Butterflies in particular are fascinating not only because they are beautiful and well studied and documented, as compared to moths or fungi, but also because their larvae often depend on just a few species or genera of native plants for food. Planting a pipevine and watching the pipevine swallowtails show up in your yard where before there were none fosters one of the most direct and powerful connections between your own actionplanting this native species and not a nonnative vineand the benefit to biodiversity of allowing a species to reproduce in an area where it formerly could not. I like to think of butterflies as the poster children for hands- on efforts to increase the biodiversity in your own backyard, though they represent just the tip of the iceberg as far as the total number of native species that might benefit in some way when you plant a pipe-vine, American beech, or California sycamore.
Propagation: I realize that not everyone is interested in propagationbut you should be, you really should be! Accordingly, I have organized the propagation information in a table that begins on page 277. Under the genus description in the main body of the text, I have briefly indicated the general ease or difficulty of propagating from seed or cuttings for the genus as a whole. The four categories are easy (can be propagated by a beginner with little specialized equipment), moderately easy (some experience required, but again, requiring little specialized equipment), moderately difficult (requires experience, specialized equipment, or is slow to propagate), and difficult (challenging even for an expert with specialized equipment).
The Species Entries
Each species entry provides information in the following categories: Zones, Soil, Native to, Size, and Color. This is followed by a general description of the species.
Zones: I have based winter-hardiness on the revised USDA Plant Hardiness Zone Map (pages 336337). The map breaks down North America into a series of zones, from 1 to 10, experiencing gradually colder average minimum winter low temperatures. The coldest zone is Zone 1; the warmest, Zone 10. A hardiness zone indicates the areas where the average minimum winter temperatures in a given year fall between a certain range. For example, Garden in the Woods, in Framingham, Massachusetts, lies in USDA Zone 6, where average winter minimums are 10 to 0° F, though in many years we have lows well above zero and once in a great while there are years where they fall below 10° F. For each of the featured species, I have included a hardiness range that I have arrived at on the basis of personal experience, references such as Dirr’s Manual of Woody Landscape Plants, and the natural range of the plant. I include an upper and lower zone limit for each plant, the lower limit (smaller number) being the coldest zone in which it will grow and the upper being the warmest limit it can tolerate. This warmer limit needs further explanation. When I state a plant’s range as Zones 47, I mean to indicate not only its cold tolerance, but also its unsuitability for gardeners in most of Zones 810, where summers are too long and warm for the plant to thrive. (An exception is the Pacific coast, which benefits from a cool maritime climate where both winters and summers are mild.) In some cases, I have included a zone in parenthesesfor example, Zones (4)57to indicate that a particular species may be more cold (or heat) tolerant if sited carefully.
I have found hardiness ratings to be a useful general guide when selecting plants to try, but please keep this information in perspective. There is much more to determining a plant’s hardiness in a particular situation than simply its tolerance of cold. Other climatic factors include the amount of annual solar radiation, the altitude, the average relative humidity, patterns and amounts of rainfall and snowfall, reliability of snow cover, average summer high and low temperatures, and length and severity of extreme hot and cold temperatures. Prolonged temperatures below 20° F or above 90° F are much harder for many plants to endure than a brief chill or heat wave. Furthermore, factors like the windiness of the site, the freeze- and-thaw patterns and depth of soil frost, and seasonal day length at a particular latitude all play a part in determining a plant’s hardiness. So even though Sable Island, Nova Scotia; Cape Cod, Massachusetts; Raleigh, North Carolina; Albuquerque, New Mexico; and Juneau, Alaska, are all within USDA Zone 7, they have vastly different climates and it is rare to find a plant that will thrive in all of them. Plant hardiness is a subject on which I could easily write a book, but rather than developing more sophisticated hardiness ratings, I think it is simply far less complicated to grow the plants native to your area whenever possible and, further, to seek out genotypes that originate nearby if you can. An Acer rubrum from Florida will likely grow better in Georgia than one originating in Maine. If you want to grow plants from farther afield, look first for species with ranges that are close to yours in latitude or longitude. A plant from southern New Mexico will have a better chance in Georgia than Wisconsin, and a gardener in Alberta will be more likely to overwinter plants from Saskatchewan than New Brunswick. Pay close attention to soil recommendations and remember that a healthy, established plant has the best chance of survival.
After the hardiness zone in the species entry is the preferred sun exposure. “Sun” means full sun, or at least six hours of direct sun daily in midsummer; essarily mean that the plant could not thrive in a garden in Nova Scotia or New Mexico or, conversely, that it occurs within every state within the range.
I compiled habitat and range information from a number of the sources listed in the bibliography, especially Gleason and Cronquist, Manual of the Vascular Plants of the Northeastern United States and Adjacent Canada; Hitchcock and Cronquist, Flora of the Pacific Northwest; Radford, Ahles, and Bell, Manual of the Vascular Flora of the Carolinas; Morin et al., Flora of North America, vols. 1 and 2; Cronquist et al., Intermountain Flora; Steyermark, Flora of Missouri; Martin and Hutchins, A Flora of New Mexico; Munz, A California Flora; Hortus Third; and the Great Plains Flora Association, Flora of the Great Plains.
Size: The height given is the average height under cultivation. A wide height rangesay 2060 feetusually indicates that the plant height varies depending on conditions. Some species (Abies grandis, for example) can attain a great height in the wild; in such cases I include that height in parentheses just as a matter of interest, not as a realistic reflection of what to expect in cultivation. The figure given for width is the plant’s expected width within 5 to 25 years in the garden, depending on the plant’s rate of growth and whether it is a long-lived tree or shrub. This figure should give an idea of necessary spacing. Again, I have occasionally included a third number in parentheses to indicate potential size of very large wild individuals. Most vines vary greatly in both height and width, depending on how they are supported.
Color: I have tried to be as detailed as possible about flower color. Many entries list a range, say from violet to purple or white, that you might find within the species as a whole. For conifers, whose flowers are enclosed in cones, not petals, I give the cone color at the time of flowering. Similarly, for plants that produce catkins (birches, for example), I give the color of the catkins. As for flowering season, I list the time of year rather than specific months, since the weather month by month varies greatly from region to region. If your spring lasts from early March until late May (aren’t you lucky!) then a flowering time of late spring would be mid to late May.
The New England Wild Flower Society originated in response to unrestrained plant collecting that was devastating populations of certain vulnerable ferns, club mosses, and woodland wildflowers. Although collection of plants by individuals can damage local populations, the commercial collection of wild plants for the nursery trade poses much more of a threat. Short of direct monitoring, there is never any assurance that wild collection is not depleting local populations beyond their capacity to regenerate. The New England Wild Flower Society strongly recommends purchasing propagated plants and strongly recommends against purchasing any wild-collected plants.
Just as a rise in the demand for fish has nearly exterminated all the commercial fish species of the North Atlantic, so too the increasing popularity of native plant gardening threatens the health and stability of wild populations everywhere. The vast majority of plants sold today are nursery-propagated (grown from seed or cuttings/divisions of nursery stock), but the slow-growing woodland and bulbous species are especially vulnerable and are still harvested in unconscionably high numbers. These include many of the lilies and lily relatives such as trilliums, mariposa lilies, and trout lilies, bloodroot, bluebells, hepaticas, lady-slippers, and gingers, to name a few. Some shrubs, like rosebay rhododendron, are routinely cut back, dug, and sold as “cut-offs” for a fraction of the price charged for nursery-raised plants.
Forestry practices that put maximum profit above the health and persistence of ecosystems are equally disturbing. The mindless cutting of our last stands of old-growth timber and the replacement of species-diverse wild forests with genetically engineered, industrial forest monocultures makes about as much sense to me as cutting off your foot when you can’t get your boots untied. The only way to stop this depredation is to eliminate the demand. When buying woodland plants, balled and burlaped shrubs or trees, or even lumber, ask your supplier about their source. If the supplier is unsure or if the price seems too good to be true, look somewhere else. Be skeptical of supposedly “rescued” plants (plants dug from areas about to be developed or paved over). I know from personal experience of unscrupulous collectors who market supposedly rescued plants actually dug from safe, healthy populations. While I hate to see plants being destroyed under a bulldozer, the term “rescued plants” is too open to interpretation and abuse at this point to be anything more than a loophole for disreputable collectors.
Copyright © 2002 by The New England Wild Flower Society. Reprinted by permission of Houghton Mifflin Company.