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2. Wildflower Gardens: A Gardener's Ecology
by Henry W. Art
The growing interest in wildflower gardens is part of the age-old quest for the "right plant for the right spot." Native plant gardens that are designed to take advantage of local conditions and reflect prevailing ecological processes often are less hassle than high-maintenance, formal beds and rs, and they blend more gracefully into the landscape.
The key to successful wildflower gardening is to keep in mind a few basic ecological considerations, such as variations in local climatic conditions, the structures of plant communities, and soil characteristics. Change is also a natural process that influences wildflower gardens. You should expect that over the long haul weeds will invade, plants will grow, plants will die, and desirable native "volunteers" will find your garden a welcome habitat.
Let Nature Be Your Guide
The broad, violet flower clusters of tall ironweed (Vernonia altissima) demand attention in the summer and autumn garden. Photo © C. Colston Burrell
Naturally occurring plant "communities" provide the clues to the kinds of wildflower gardens that might be successful where you live. The natural vegetation varies enormously from region to region, reflecting differences in the climate and soils. While a meadow garden of annual wildflowers and grasses may successfully endure for many decades in southern California, it is unlikely to persist for much more than a single growing season in the East. Likewise, it would be nearly impossible to grow a garden of eastern woodland spring perennials in the Southwest deserts. Different species are adapted to different sets of environmental conditions, so choose the plants that are going to be most at home in your garden.
Regional patterns of temperature and precipitation play a major role in determining the kinds of vegetation that grow. Forests are generally found in regions with abundant precipitation distributed relatively evenly throughout the year. In forested regions where winter temperatures are cold and the growing season is relatively short, coniferous evergreen forests usually dominate. Deciduous forests with trees that lose their leaves in winter are usually found in regions with milder winters and longer growing seasons. It is obvious to anyone who has seen these two types of forest that the light conditions are dramatically different, as are the wildflowers. While the deciduous forest may cast dense shade during the summer, plenty of light strikes the forest floor in late winter and early spring, and wildflowers proliferate. Coniferous forest, however, casts perpetual shade, and wildflowers tend to grow only in the sunlit gaps where trees have fallen.
Grasslands are found in regions that receive less precipitation than forested areas, and often there are distinct wet and dry seasons, as well as periodic droughts. In North America the transition between forests and prairies begins in the Mississippi River Valley, where annual precipitation is less than about 35 inches. With decreasing amounts of rain and snow, prairie grasses tend to decrease in height, although their roots may penetrate very deeply in search of water. Fire and grazing by large mammals also have played important roles in the maintenance of prairies. Prairies are not just uniform grasses from horizon to horizon; they are extremely diverse plant communities, usually with dozens of different species of perennial (and even some annual) grasses, a great variety of wildflowers, and some shrubs.
Deserts usually receive less than 15 inches of precipitation per year. Like grasslands, they typically have dry and wet seasons, although the wet season may not come every year. In North America the major desert regions extend from the Great Basin, between the Sierra Nevada/Cascade Mountains and the Rockies, south into Mexico. The Great Basin deserts have cold winters and are dominated by shrubs like sagebrush, while the warm deserts to the south have mild winters and a greater diversity of shrubs and cacti and other succulents. Periodically, especially during El Nino years, abundant winter precipitation in the desert triggers an amazing display of annual wildflowers in the early spring.
Natural gardeners should take their cue from not only regional vegetation types but also local conditions like topography, drainage, and soil composition. For example, because air temperature decreases as elevation increases, coniferous forests often grow on the higher slopes of mountains in regions that have deciduous forests at lower elevations. Furthermore, at any particular elevation, slopes that face south are sunnier, warmer, and drier than slopes that face north. Slopes that face west are heated by the afternoon sun and therefore tend to be warmer and drier than east-facing slopes. These small differences in topography often lead to large differences in the kinds of vegetation present locally.
Topography also influences the drainage conditions at potential garden sites. The tops of hills and ridges tend to have better drainage and drier soils than the bottoms of slopes. Flat areas at the bases of long slopes or near rivers may have sufficient water year-round to support distinctive wetland vegetation. Marshes, dominated by grasses and sedges, and swamps, with shrubs and trees adapted to grow in wet soils, are vital environments that regulate flood water, control pollution, and provide wildlife habitat. While wetland conditions may create challenges for the natural gardener, they also provide opportunities to use the rich palette of species adapted to these habitats.
Sizing up Soil
An understanding of your soil is essential for successful wildflower gardening. Soil is not just dirt, but rather a living medium that consists of particles of different sizes, often varying in mineral composition, air spaces, organic matter, moisture, and creatures ranging from bacteria to earthworms. Sandy soils tend to be dry, since they comprise large particles with relatively big spaces in between—conditions that do not retain moisture very well. Soils rich in clay usually drain poorly, as water cannot pass between the tightly packed, microscopic particles. Different wildflowers are adapted to different soil types. The addition of organic matter improves both soil aeration and its ability to retain moisture, although in wetlands an abundance of organic matter can contribute to the mucky soil conditions.
One of the most important soil factors affecting the growth of wildflowers is pH, a measure of relative acidity or alkalinity. On the pH scale, from 0 (most acidic) to 14 (most alkaline), 7 is neutral. The pH of a soil not only results from the chemical composition of the soil minerals, but also is influenced by the decomposition of organic matter, the biological activity of soil organisms, and the chemistry of rain or snow falling on the site. In general, soils that form on top of limestone bedrock are more alkaline and have a higher pH than those formed from granite bedrock.
Many wildflowers survive only within a limited pH range, so you should choose species suited to your soil conditions. Keep in mind that the addition of organic matter such as mulch or compost to wildflower gardens can either raise or lower the pH of the soil, depending on the kind of material added. Pine needles, oak leaves, and peat moss tend to acidify the soil, while the leaves of maples and birches make the soil more alkaline.
The Constancy of Change
Plant communities, whether a designed wildflower garden or "natural" vegetation, are constantly changing. Some changes occur on a seasonal or daily basis with various species emerging, flowering, producing fruits, and going into dormancy at different times of the year. In practical gardening terms, this means it is possible to design a wildflower garden with a long season of interest.
Other changes occur over a longer timeline as plant communities respond to disturbances. When a forest is cleared and the land abandoned, it usually does not return immediately to its predisturbance state, but undergoes a gradual recovery through the process of "succession." Fast-growing species with highly mobile seeds are usually the first to arrive. Since many of these species, which we customarily call weeds and pull from our gardens, are short-lived and require lots of sun, they are eventually replaced by less mobile, slower-growing species that are adapted to shade. In other words, in forest regions, open land is first colonized by annual grasses and low-growing plants, then perennial wildflowers and shrubs, and eventually various trees. There may be further changes in the species of trees that dominate the recovering forest, with a shift from the aspens and pines that arrive fairly early to oaks, maples, beech, hemlock, or other species that arrive later, are better adapted to growing in the shade, and live longer. Grasslands and deserts have similar stories to tell, but theirs involve different species in the return to predisturbance conditions.
One of the lessons to be learned from succession is that change is a natural ecological process. A meadow garden in a forested region must be periodically disturbed by fire, weeding, or mowing or it will become a forest. However, a meadow in a grassland region may be more easily maintained, since it resembles the natural vegetation.
When designing your wildflower garden, let nature be your guide. The more observant you are of local environmental conditions, the more successful you will be.
Henry W. Art is a garden writer, Samuel Fessenden Clarke Professor of Biology, and the director of the Center for Environmental Studies at Williams College in Williamstown, Massachusetts. He is the author of A Garden of Wildflowers (Storey Communications, 1986) and four different regional editions of Wildflower Gardener's Guide (Garden Way Publishing, 1987, 1990, 1991). He has a doctorate in forestry from Yale University.