Plant Invasive Species

Brazilian elodea (Egeria densa)

Native to Brazil and Argentina, Brazilian elodea is a popular aquarium plant often sold in pet stores and available in school science kits under the name Anacharis. When it is introduced into freshwater, it forms dense beds that reduce water quality, disrupt wildlife habitat, slow the flow of water, and impede recreational activities.

This noxious weed is a submerged, freshwater perennial plant found in both still and flowing waters including lakes, ponds and quiet streams. It prefers low light and tolerates variable water quality (turbidity, pollution, etc.) and can survive under ice for short periods—but not prolonged freezing. The plant grows mostly underwater but forms dense mats along the surface that can cover hundreds of acres. Leaves grow in whorls of three to six around the stem making a cylindrical shape, and the stems are very leafy compared to the native elodea. The leaf edges appear smooth to the naked eye but the margins are minutely toothed, visible with low magnification. A distinguishing characteristic is the smooth midvein on the underside of the leaf. Small white flowers appear from June through October. They have three glossy petals that appear wrinkled, and float on or rise above the water’s surface on thread-like stems. Brazilian egeria is commonly mistaken for the native elodea (Elodea canadensis) or common waterweed, as well as the exotic hydrilla (Hydrilla verticillata).

Hydrilla (Hydrilla verticillata)

Hydrilla is native to Europe, Asia, and central Africa. It was first introduced in Florida in 1958 for use in the aquarium industry. Hydrilla is a submerged, rooted, perennial plant that forms dense colonies. Leaves are blade-like, about 1⁄8 inch and 3⁄8 inch long with small tooth margins. The underside of the leaf has a red midrib with one to four spines or conical bumps, making them feel rough. Leaves are usually four to eight in a whorl. Hydrilla produces tiny, translucent, white to reddish flowers on long stalks. Plants flower from June through October.

Hydrilla is able to establish itself in low-light waters over 20 feet deep and then grow towards the shallow banks. It branches profusely after reaching the surface and forms thick mats that hinder recreation, navigation, and water intakes. It grows quickly and outcompetes and eliminates native species. Hydrilla can grow in almost any freshwater—in variable conditions with either low or high nutrient amounts, or a wide temperature tolerance (68–86 ºF).

Hydrilla reproduces rapidly—by fragmentation, from seeds, and it also produces 1⁄4 inch turions at the leaf axils and potato-like tubers attached to the roots in the mud. Hydrilla is a List A noxious weed in Colorado, so eradication is mandatory.

Eurasian watermilfoil (Myriophyllum spicatum)

Eurasian watermilfoil, also called spike watermilfoil, is a submerged, herbaceous aquatic plant. Native to Europe, Asia and Northern Africa, it was introduced into the USA in the 1940s and is one of the most destructive aquatic plants known. This highly aggressive species colonizes a variety of habitats, including both moving and standing waters. It grows rapidly—about one foot per week—and forms extremely dense mats. The mats crowd out native species, disrupt the food chain, displace wildlife habitat and clog waterways, stopping or slowing the flow of water. Eurasian watermilfoil also seriously alters the physical and chemical characteristics of lakes and streams. Its infestations alter aquatic ecosystems by shading out native species and providing choice mosquito larvae habitat. Dense mats impair all forms of water based recreation.

Pink or olive green stems grow to the water surface, usually extending 3 to 10, but as much as 33, feet in length and frequently forming dense mats. Stems of Eurasian watermilfoil are long, slender, branching, hairless, and become leafless toward the base. New plants may emerge from each node (joint) on a stem, and root upon contact with mud. The feathery dark green leaves of Eurasian watermilfoil are finely divided and occur in whorls of 3 or 4 along the stem, with 12–20 pairs of fine, thin leaflets about 1⁄2 to 2 inches long. These leaflets give milfoil a feathery appearance that is a distinguishing feature of the genus. Eurasian watermilfoil produces small yellow, 4 parted flowers on a spike that projects 2 to 4 inches above the water surface from June to September. The fruit is a hard, segmented capsule containing four seeds.

Native watermilfoil, Northern watermilfoil, also called Shortspike watermilfoil, (Myriophyllym sibiricum) and Parrotfeather (Myriophyllum aquaticum) are very difficult to distinguish from Eurasian watermilfoil. Eurasian watermilfoil is a List B noxious weed.

Parrotfeather (Myriophyllum aquaticum)

Parrotfeather is native to the Amazon River in South America, but is now found worldwide. This plant was probably introduced to North America around the 1800s as an ornamental species. It prefers a warmer climate and is chiefly found in the southern parts of the United States. Parrotfeather is a freshwater plant which prefers shallow waters less than 5 feet; it can be found in lakes, ponds, and streams. Parrotfeather seriously alters the physical and chemical characteristics of lakes and streams. Its infestations alter aquatic ecosystems by shading out algae and providing choice mosquito larvae habitat. Dense infestations also cause flooding and drainage problems, and its mats restrict recreational activities.

Parrotfeather gets its name from its feather-like leaves which are arranged around the stem in whorls of four to six. Submersed leaves are 0.6 to 1.4 inches long and have 20 to 30 divisions per leaf. The emergent leaves are 0.8 to 2 inches long and have 6 to 18 divisions per leaf. The bright green emergent leaves can be very stiff and a darker green than the submersed leaves. The emergent stems and leaves can grow up to a foot above the water surface and look almost like small fir trees. Parrotfeather has both submersed and emergent leaves, with the submersed form being easily mistaken for Eurasian waterfilfoil (Myriophyllum spicatum).

Giant Salvinia (Salvinia molesta)

Giant salvinia is native to Southeastern Brazil. It is a small freefloating fern that grows in clusters and develops into dense floating mats or colonies in quiet water, undisturbed by wave action. It can clog irrigations canals and drinking water lines, and foul hydroelectric plants. This species impairs all forms of water-based recreation and has disastrous effects on the natural communities. Giant salvinia can completely cover waterways preventing the passage of sunlight and oxygen that native plants, fish, insects, and other species require, as well as covering open water that migrating waterfowl need to survive.

The floating leaves of giant salvinia are oblong (1⁄2 to 11⁄2 inches long) with a distinct midrib along which the leaf may fold forming a compressed chain-like appearance. Leaves occur in whorls of three with two floating leaves and one submerged leaf. Young plants have leaves that lie flat on the surface, whereas older plant leaves become thick and curled at edges forming upright chains that become dense mats of floating plants. Giant salvinia has rows of leaf hairs that have a single stalk that divides into four branches that reconnect at the tip, giving the hair a cage-like or egg-beater appearance when magnified. Underwater, the leaves are modified into small rootlike structures that support chains of egg-shaped spore-bearing structures. The entire plant is only about 1 to 2 inches in depth. Giant salvinia is thought to reproduce only by fragmentation.

Fragments and s dormant buds that break off existing plants form new plants. Giant salvinia can double in size in 4 to 10 days under good conditions is an aggressive invader species. Giant salvinia is a List A noxious weed and eradication is mandatory.

Water Hyacinth (Eichornia crassipes)

Water hyacinth is native to Brazil and was introduced as an ornamental. It is still commonly used for water gardening and home ponds. Water hyacinth is a very aggressive invader and can form thick mats that cover the entire surface of the water and can cause oxygen depletion and fish kills. This species is notorious for clogging transportation systems and can colonize a wide variety of habitats.

Water hyacinth is a free-floating perennial plant that can grow to a height of 3 feet. The smooth, dark green succulent leave blades are circular to elliptical. The flowers are large (2 to 3 inches) and attractive. They can be pale blue, lilac, or white and bloom from June through October.

A thick, heavily branched, dark fibrous root system forms underneath the water. The roots are feathery and typically more than 3 feet in length. The mature plants are linked together by underwater stolons, which form daughter plants. Water hyacinth reproduces may also reproduce via seeds. Seedlings root in mud and then break free and float once mature. The water hyacinth is a vigorous grower known to double its population in two weeks.

Purple loosestrife (Lythrum salicaria)

Purple loosestrife is native to Europe and Asia, and was initially introduced to the northeastern seaboard of the United States in the ballast of ships in the 1800s. It has been repeatedly and continually introduced as a garden plant. Purple loosestrife invades marshes and lakeshores, replacing cattails and other wetland plants. In some locations, natural cattail marshes have been completely overtaken by loosestrife. The plant forms dense, impenetrable stands that are unsuitable as cover, food, or nesting sites for a wide range of native wetland animals. Loosestrife tolerates a wide range of environmental conditions. It favors fluctuating water levels and other conditions often associated with disturbed sites. It is shade intolerant and is apparently unable to invade saline wetlands.

Purple loosestrife is a perennial with long showy spikes of magenta flowers and a square stem. Usually under 4 feet in height, the plant may reach up to 10 feet tall in nutrient-rich habitats. The flowers have five to seven petals and bloom from June to September. The leaves are usually opposite, usually in pairs, or in whorls of 3. Leaves are lance-shaped, without teeth, and the venation has a peripheral margin which distinguishes it from other square stem mint species prior to flowering. Purple loosestrife is a List A noxious weed and eradication is mandatory.

Yellow Floating Heart (Nymphoides peltata)

Yellow floating heart is a perennial, waterlily-like plant that carpets the water surface with long-stalked heart-shaped leaves. The showy five-petaled yellow flowers occur on long stalks and rise a few inches above the water. Yellow floating heart is a native of Eurasia. Like other floating leaved plants, yellow floating heart grows in dense patches, excluding native species and creating stagnant areas with low oxygen levels underneath the floating mats. These mats make it difficult to fish, water ski, swim, or boat. Yellow floating heart prefers to grow in slow moving rivers, lakes, reservoirs, and ponds. It reproduces by water dispersed seeds and by new stolens. Broken off leaves with part of a stem will also form new plants.