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The Natural Water Garden

Chapter 2: Wetland Ecology for Gardeners

by Leslie Yetka and Susan Galatowitsch

Typically, we experience wetlands from a distance—as casual spectators from a boardwalk, boat or a ridge—because being in a wetland means sinking into ooze, precariously hopping from one clump of tangled roots to another, slogging through dense vegetation and, of course, fending off pesky mosquitoes. However, careful observation of wetlands reveals a distinctive combination of water, soil and plants that harbors a great variety of species unable to exist in any other type of habitat. Familiarizing yourself with the ecology of these habitats, learning how the water, plants and topography of native wetlands interact, is the first step in restoring or creating a wetland garden.

The Movement of Water

To truly understand wetlands and wetland plants, you first need to know a bit about hydrology, or the movement of water. Water creates wetlands, and you usually will find wetlands where water accumulates at a rate faster than it drains away. As you visit different wetlands, you will notice that some are inundated year-round while others are not, that some are shallow and others are deep. For example, the otherwise dry lands of California's mountain valleys are occasionally punctuated by shallow depressions that hold water for brief periods in the spring. When these vernal pools fill after heavy rains or snowmelt, they erupt with aquatic plant and animal life. Eventually, the water evaporates into the atmosphere or percolates down through the soil, recharging the groundwater. Any seeds that have been dropped by the plants become dormant until the next rainfall fills up the pool again. At lower elevations, this groundwater may again reach the surface as springs or seeps, providing a steady supply of water to wetlands year-round.

Other wetlands are created by the regular ebb and flow of rivers, lakes and oceans. Whereas coastal wetlands are affected by the twice daily tides, littoral or lake-edge wetlands also experience a regular rise and fall of water levels. Floodplains, the low-lying lands along rivers, often flood after heavy storms. Even after most of the water has receded to the main waterways, some may remain trapped in the lower depressions of these floodplains. Thus, water can remain close to the surface year-round, supporting extensive forests along with pockets of emergent wetland vegetation such as cattails and arrowhead. Shrubs, including buttonbush and willows, grow as a fringe along the river channel. Periodic flooding along the Mississippi River supports floodplain wetlands from temperate Minnesota all the way down to subtropical Louisiana.

Wetland Plants

Plants of the emergent zone such as pickerel rush grow partially in water. Most of the time their roots are submerged.

Plants of the emergent zone such as pickerel rush grow partially in water. Most of the time their roots are submerged.

As wetlands flood, soil microbes quickly deplete oxygen in the soil, using it, as we do, to respire. In flooded soils, upland plants cannot get the oxygen they need. However, wetland plants are able to cope with this by producing large, interconnected cells called aerenchyma that extend from the shoots to the roots. Acting like straws, these chains of cells flush oxygen down to the roots of the plant, creating an oxygenated environment for them. By splitting lengthwise the leaves of thick-stemmed plants like cattails and bulrushes, you can easily see these air chambers.

Even with their special adaptations to flooding, individual species of wetland plants have differing limits as to the depth of water they can tolerate. A plant's ability to tolerate a certain water depth is what determines its zone. Wet meadow zone species, including Carex lanuginosa and swamp milkweed (Asclepias incarnata), thrive best in soils that undergo alternating wet and dry periods. Sedges and other small perennials, including marsh marigolds, tend to be located at the water's edge where they remain wet throughout the growing season. Emergent zone plants such as cattails and bulrushes are best suited to persistent flooding; just enough of the leaves need to remain above the water to photosynthesize sugars for the entire plant. The leaves of floating and submersed aquatics such as water-lilies and pondweeds photosynthesize in water, enabling them to live in deep water, beyond the reach of even the tallest emergents. These species are so suited to growing in deep water that their stems lack the strength to hold the plant upright without the support of the surrounding water. Submersed aquatics can grow in shallower water if emergent aquatics and suspended particles have not diminished light penetrating the water.

Water Chemistry and Plant Life

Wetlands differ not only in water quantity but also in chemistry. Wetland waters range from alkaline to acid, depending on groundwater and organic substrate. Because bogs receive water directly from rainfall rather than from surface runoff or groundwater, they are mineral-poor and acidic. Acid-loving plants such as the common pitcher plant (Sarracenia purpurea) and sphagnum moss (Sphagnum spp.) thrive in these acidic conditions.

As water flows through the ground, soil minerals and nutrients are dissolved and transported into wetlands, resulting in a high mineral content or alkaline condition. Called fens, these wetlands harbor a distinct group of species known as calciphiles. For example, grass-of-Parnassus (Parnassia glauca) and sterile sedge (Carex sterilis) are abundant in fens but rare elsewhere. Alkaline wetlands are also common in arid regions such as the Great Plains and the Desert Southwest, where water from surface runoff (rather than groundwater) evaporates, leaving behind dissolved minerals. Only a few salt-loving plants, such as saltgrass (Distichlis spicata), are able to withstand these conditions. They either prevent salt from entering their roots, or are able to excrete it through nodules. Known as halophytes, these plants tend to have thick stems and cuticles, wax-like substances on their leaves that prevent excessive water loss.

Vital Disturbances

Disturbances like hurricanes and floods can be catastrophic for cities and farms. However, many wetland ecosystems are not only resilient to periodic disturbances but dependent on them. For example, the floodplains of rivers in the western United States used to support stands of cottonwood trees, which are now declining. The cottonwoods rely on spring flooding for the germination of their seeds. However, once irrigation reservoirs were built to capture winter snowmelt, the floods ceased and so did cottonwood regeneration.

Severe droughts can also rejuvenate some wetlands. Emergent plants of prairie wetlands germinate on wet, not saturated soils. Only during severe droughts, which occur every ten to fifteen years, does water in large, deep marshes draw down to expose saturated mudflats. Seeds stored in the soil seedbank germinate on the mudflats. These plants grow and expand when precipitation returns. As their populations increase, they attract muskrats, which graze on the emergent plants faster than the plants can make new shoots. The marsh then becomes lake-like until the next drought.

Some types of wetlands are more resilient to disturbances—both natural and human—than others. Wetlands fed by precipitation tend to be less sensitive to perturbation because they are accustomed to fluctuating water levels throughout their growing season. On the other hand, bogs and fens—wetlands fed by groundwater—are more sensitive to disturbances. These wetlands harbor plant communities that have stabilized over a significant period of time. In some northern wetlands, the accumulation of undecomposed organic matter, known as peat, can be over 10 meters thick and 10,000 years old. They often support sensitive and slow-growing rare plant species with low reproductive rates, including orchids, lilies and irises. Even minor changes in hydrology can have a severe impact. If these wetlands are artificially drained, the peat rapidly decomposes and the land can actually drop significantly, cracking with fissures up to 100 feet deep.

In urban areas, stormwater flows to man-made wetlands may be similar in timing to natural wetlands but dramatically different in magnitude. For instance, an undisturbed wetland without stormwater drainage may experience about a foot of change in water depth a year. An urban wetland, by comparison, may suddenly rise three or four feet due to runoff from streets, roofs, concrete and other impermeable surfaces. This dramatic rise in water level may be more than some plants can tolerate.

Fertilizer is another human-introduced disturbance that can harm wetlands. Fertilizers applied to lawns are often lost to surface runoff, and eventually end up in wetlands. Certain wetland plants, such as cattails, thrive on this fertilization and grow at the expense of others. The result is a wetland with very few species.

Although garden wetlands typically are not as sensitive to periodic climatic disturbances as natural wetlands are, wetland gardeners should mimic natural disturbances as much as possible. For example, it is not a good idea to add water at a time in the natural cycle when the water should be down because germination may be adversely affected.

The Right Wetlands for Your Region

Various combinations of geology, precipitation and temperature result in a rich diversity of wetlands. For example, sedge meadows are native only to the temperate north, while riparian desert forests with cottonwoods (Populus spp.) and saltbush (Artiplex spp.) are uniquely found along rivers in arid regions. When preparing to restore or create a wetland, be sure to visit healthy wetlands in parks, wildlife refuges or preserves in your area. Look at sites with conditions similar to those on your property and plan to mimic both species composition and vegetation zones. Notice the arrangement of plants. In many wetlands, the organization of vegetation indicates the flooding patterns. Over the years, the bands or zones of emergent, floating and submersed aquatics may migrate up and down a shoreline, depending on water-level fluctuations. Extensive shallow wetlands with little topographical change, such as sedge meadows, have fairly uniform mixtures of plants.

Ask the site manager or a local naturalist about the hydrology of any wetland you are planning to use as a model. You need to know whether your wetland will rely on surface runoff or groundwater flow. Then be sure that these water sources that were historically important have not been diverted or obstructed by roads, ditches or filling. Consult your county soil conservationist to help you determine how similar the soil on your property is to the the soil in the wetland you are using as your model. Similar soils bode well for the success of your wetland project because you can be reasonably certain that the plants you have chosen will find the nutrients, texture and water conditions suitable.

The patterns of vegetation, or lack thereof, at any wetland site greatly determine the types of wildlife that visit it. Waterfowl and shorebirds tend to forage in shallow water and nest in tall vegetation found along the water's edge. Mammals such as muskrats and beavers find materials for dens and food reserves in forested wetlands. Wetland vegetation provides habitat for insects and amphibians, which in turn provide food for other wildlife.

Keeping Out Invaders

Unfortunately, wetlands support more than their fair share of invasive species. Purple loosestrife (Lythrum salicaria), reed canary grass (Phalaris arundinacea), hybrid cattail (Typha x glauca), tamarisks (Tamarix chinensis, T. parviflora and T. ramosissima), glossy buckthorn (Rhamnus frangula)and water hyacinth (Eichhornia crassipes) are just a few examples of the rapacious plants that can overrun a wetland if not kept in check. Some species, like purple loosestrife, were deliberately introduced for their horticultural value. Some, like hybrid cattail, have become invasive because they can tolerate pollution such as road salt and fertilizer.

Invasive species can spread rapidly and eventually replace all the native vegetation in a habitat. Large stands of any single plant reduce the species diversity, as well as alter ecosystem processes such as the frequency of wildfires, the availability of water or nutrients and the rate of soil erosion—ultimately resulting in habitat loss. While the federal Clean Water Act has slowed the draining and filling of wetlands, it does not regulate habitat loss due to invasive species. Therefore, thousands of acres of American wetlands are lost each year due to plant invaders.

Newly replanted wetlands are particularly vulnerable to invasives because these aggressive species are typically good colonizers of open sites. The best way to keep them out of your wetland is to pull out the plants by hand, including the roots, as they colonize. Never let invasives seed. Large populations of invasives often can be controlled only by chemical herbicides.

Knowing which species are native to wetlands in your area is an essential part of reducing habitat loss from invasive species. Some invasive exotics are available commercially. Don't buy them—even if growers claim they are so-called infertile strains; studies have shown that purple loosestrife cultivars claimed to be infertile, for example, are not. It's generally wise to avoid non-native species in your wetland garden. A gardening experiment with an exotic aquatic could be the cause of our next serious wetland invader.

Wetland Zones

A natural water garden should mimic the zones and plant species found in nature. The wet meadow zone, where the wetland grades into upland, typically is dominated by a few grasses or sedges and wildflowers.

A natural water garden should mimic the zones and plant species found in nature. The wet meadow zone, where the wetland grades into upland, typically is dominated by a few grasses or sedges and wildflowers.

Wetland plants are typically arranged in a series of concentric bands or zones determined by water depth. Plants that are able to tolerate similar depths of water grow in similar zones.

The wet meadow zone is found where the wetland grades into upland. It regularly undergoes periods of wet and dry. Depending on where in the country a wetland is located, the wet meadow zone may be dominated by emergent plants or shrubs and trees.

Adjacent to the wet meadow zone lies the emergent zone, dominated by soft-stemmed, herbaceous plants that grow partially in water. Most of the time, their roots are submerged and their leaves and stems are exposed to light and air. Emergent species include cattails, pickerel weed, arrowheads, bulrush, grasses and sedges.

Submersed aquatics and floating plants like water-lilies, water hyacinths and pondweeds are found in deep water. Their leaves are capable of photosynthesizing under water.

Wetland Types

Marsh: Supports soft-stemmed herbaceous plants including grasses, sedges and reeds.

Swamp: A closed-canopy wetland dominated by woody plants: flood-tolerant trees and shrubs.

Bog: Characterized by high acidity, poor drainage, sphagnum mosses and peat.

Susan Galatowitsch is assistant professor of landscape ecology at the University of Minnesota in the departments of Horticultural Science and Landscape Architecture. She is co-author of Restoring Prairie Wetlands: An Ecological Approach (Iowa State University Press).

Leslie Yetka of the University of Minnesota Department of Horticultural Science, holds an M.S. in Horticultural Science with a minor in water resources. For the past seven years, she has conducted research on revegetating sedge meadows in created and restored wetlands, and on the plant ecology of peatlands.