Swamp milkweed, cattails and marsh marigold have names that give away their status as wetland plants. Others, such as alpine violet, iris and mountain holly are not as familiar. To qualify as a wetland cultivar, they must grow in areas that are wet for part of the year. Most wetlands are parts of larger watershed areas; wetland plants may grow in water, in floodplain lowlands or in uplands that may flood only occasionally.
The U.S. Department of Agriculture's Natural Resources Conservation Service, state natural resource agencies and native plant societies classify how wetland plants grow by sorting them into "obligate" or "facultative" groups. Obligate (OBL) wetland plants such as water lilies, American lotus and bald cypress actually grow with roots underwater. Facultative wetland plants are broken into several categories. Two major groups are wetland plants such as Canadian anemone and marsh valerian that appear mostly in wetlands (FACW) and wetland-tolerant upland area plants such as dandelions and Appalachian violets that appear less than a third of the time in wet land (FACU).
Obligate plants provide food for water-dwelling animals, including migrating waterfowl that use watersheds as "flyways." Insects that congregate in facultative wetland plants such as sedges and cattails provide food for frogs and fish that feed on them. Plum, huckleberry, deerberry and a variety of willows provide food for forest dwellers like rabbits, deer and squirrels. Wetland flora provides food for all and shelter from predators. Frogs and turtles use rushes and sedges to hide their young in spring and summer; birds use bald cypress, Atlantic white cedars and Everglades palms to hold their nests safely above tidal waters or marshes.
Wetland plants also provide environmental benefits. Shallow-rooted willows and rushes hold wetland soils in place, a step in natural flood control. Obligate ferns growing along banks hold soil in place and provide protective shelter for ground-dwelling animals. Plants clean water that runs through them, taking nitrates and returning oxygen, consuming pollution and providing clean water. Wild rice is an obligate plant and various reeds and sedges make nutritious winter food for farm animals.
Wetland Plants in the Landscape
Any gardener who keeps a koi pond or other water feature knows that a colony of water lilies, cardinal flowers or water arum provide brilliant blooms, oxygenate the water and give fish a place to hide. Rushes provide winter interest and movement when trees are bare, and sedges provide color when dormant plants are brown or gray. Louisiana irises belie their name by flourishing as far north as Canada. Willows, red maples and wetland shrubs fill damp spots, removing the need to fill and change drainage patterns in landscapes.
Some wetland plants can pose problems or become invasive. Cattail colonies that grow too large clog and fill waterways with dead rushes, which then decompose to form muck, making waterways shallower and slowing current. Plants such as duckweed, when not kept under control by avian or amphibian populations, shades the bottom of ponds. The shade cuts off oxygen production upon which fish and other water-dwellers depend, as well as facilitating troublesome algae "blooms."