Curly-leaf Pondweed (Potamogeton crispus)

Common names: Curly cabbage, crisp pondweed.

Location: Grows from the shore to depths of up to 15 feet.

Description: Leaves are somewhat stiff and crinkled, approximately 1/2-inch wide and 2 to 3 inches long; leaves are arranged alternately around the stem, and become more dense toward the end of branches; produces winter buds can be confused with claspingleaf pondweed.

Hints to identify: Has small “teeth” visible along edge of leaf; begins growing in early spring before most other pondweeds; dies back during midsummer; the flower stalks, when present, stick up above the water surface in June; appears reddish-brown in the water, but is actually green when pulled out of the water and examined closely. Easily confused with claspingleaf pondweed, which has leaves with no “teeth” around their edges.

Importance of plant: Provides some cover for fish; several waterfowl species feed on the seeds; diving ducks often eat the winter buds.

Management strategy: See DNR regulations. Like Eurasian watermilfoil, curlyleaf pondweed is not native to the United States and often causes problems due to excessive growth. When control is necessary, herbicides and harvesting can be effective.


Wild Celery (Vallisneria americana)

Common names: Water celery, eelgrass, tapegrass.

Location: Lakes in depths up to 15 feet and streams; prefers semi-hard bottom such as sand covered with a thin layer of muck.

Description: Leaves are ribbon-like, dark-green, and grow below the water surface; rooted in mud; in late summer, produces a small, whitish-yellow flower, supported by a coiled stalk; often grows in beds amid pondweeds and other submerged plants.

Hints to identify: Unbranched leaves extending from the lake bottom to the water surface; flowers (and occasionally some leaves) float on the surface; leaves are attached to a horizontal central stem right above lake bottom.

Importance of plant: Provides shade and shelter for bluegills, young perch, and largemouth bass; choice food of waterfowl, particularly diving ducks; attracts muskrats, marsh birds, and shore birds.

Management strategy: See DNR regulations. Because wild celery is an excellent wildlife food, it is usually best left alone. Abundant growth during July and August in shallow water may interfere with recreation. Herbicides don’t work well to control this plant. Hand-pulling or raking sometimes works, though floating, uprooted plants often re-establish themselves in shallow water.

Eurasian watermilfoil – Myriophyllum spicatum

Eurasian watermilfoil was accidently introduced to North America from Europe. Spread westward into inland lakes primarily by boats and also by waterbirds, it reached Midwestern states between the 1950s and 1980s.

In nutrient-rich lakes it can form thick underwater stands of tangled stems and vast mats of vegetation at the water’s surface. In shallow areas the plant can interfere with water recreation such as boating, fishing, and swimming. The plant’s floating canopy can also crowd out important native water plants.

A key factor in the plant’s success is its ability to reproduce through stem fragmentation and runners. A single segment of stem and leaves can take root and form a new colony. Fragments clinging to boats and trailers can spread the plant from lake to lake. The mechanical clearing of aquatic plants for beaches, docks, and landings creates thousands of new stem fragments. Removing native vegetation creates perfect habitat for invading Eurasian watermilfoil.

Eurasian watermilfoil has difficulty becoming established in lakes with well established populations of native plants. In some lakes the plant appears to coexist with native flora and has little impact on fish and other aquatic animals.

Likely means of spread: Milfoil may become entangled in boat propellers, or may attach to keels and rudders of sailboat. Stems can become lodged among any watercraft apparatus or sports equipment that moves through the water, especially boat trailers.

Functions of Aquatic Vegetation

Aquatic plants are beneficial in many ways. Areas with plants produce more food items for fish, such as insect larvae, snails, and other invertebrates. Aquatic vegetation offers shelter for fish, as well as spawning habitat. Many submerged plants provide food for waterfowl and provide habitat for insects on which some waterfowl feed. Aquatic plants further benefit lakes by producing oxygen and by absorbing nutrients from runoff, such as phosphorus and nitrogen. Emergent plants also protect shorelines and bottoms by dampening wave action and stabilizing sediments (Figures 1 and 2).


Figure 2. In many Minnesota lakes the littoral
zone contains all plant types: emergent,
submerged, floating-leaf, and algae.

Distribution of Aquatic Vegetation

Water bodies differ greatly in number and type of plant species. There are approximately 150 species of aquatic plants and macroscopic algae, such as Chara and Nitella, present in Minnesota. Aquatic plants are divided into four general classes. The free-floating plants including planktonic algae and duckweeds may occur anywhere in a lake. Vascular plants and macroscopic algae, collectively called macrophytes, are divided into the three classes of submerged, emergent, and floating-leaf. Class is defined by the leaf location in the water. The zone in which macrophytes occur is called the littoral zone, which in many Minnesota lakes extends to a depth of approximately 15 feet.

The distribution of plants within a lake is generally limited by light availability, which is controlled by water clarity. Water clarity is measured by lowering a Secchi disk (a plate-sized disk with black and white sections) through the water to the depth at which it can no longer be seen. The deeper the Secchi disk is lowered and still is visible, the clearer the water. As a general rule, the maximum depth to which plants grow is twice the midsummer Secchi depth. In lakes with a Secchi depth of more than 15 feet, rooted plants occur to depths of 15-20 feet.

In addition to available light, the type of sediment influences the distribution of rooted aquatic plants. Plants are more likely to be found in muddy or soft sediments containing organic matter, and less likely to occur where the substrate is sand or gravel. Lakes usually have more than one type of sediment. One edge of the lake might have a rich organic substrate, while the opposite edge of the same lake could have a sandy substrate because wave action has washed away the organic material. By examining the lake and lakeshore before you buy property, you can often tell how abundant aquatic vegetation will be. A muddy shoreline is a result of the wave action of water on the lake’s substrates. It is not true that if you remove vegetation a sandy beach will appear. Removing vegetation will create more mobile soils resulting in murkiness and turbidity.

Finally, water chemistry influences which plants are found in a body of water. Plant species vary in pH tolerance. Some species prefer alkaline lakes like those in southwest Minnesota, while others flourish in the more acidic lakes found in northeast Minnesota.


Identification of Submerged Plants

Exotic Eurasian watermilfoil (Figure 3) is a rooted plant with submersed leaves that occurs in 75 lakes and rivers in Minnesota, most in the seven-county metropolitan area. It is native to Europe and Asia and spread throughout North America by growing from small plant fragments carried by boats. It has a high growth rate and may quickly dominate the littoral zone of a lake. It can form dense mats on the surface, shading out other plants, impeding boating and other recreational activities (Table 1).

The native species Northern watermilfoil, Myriophyllum exalbescens, (Figure 3) is one plant easily confused with the exotic species Eurasian watermilfoil, and in some areas its abundant growth interferes with recreational activities (Table 2). The two watermilfoils are differentiated by pairs of leaflets in each leaf. Native Northern watermilfoil has five to ten leaflet pairs, while exotic Eurasian watermilfoil, Myrophyllum spicatum, has twelve to twenty-one pairs. Additionally, Northern watermilfoil is much more rigid when out of the water. Native coontail (Figure 4) is another common Minnesota species easily confused with Eurasian watermilfoil. Unlike the feathery leaves of Eurasian watermilfoil, the leaves of coontail are spiny and forked. Native Canada waterweed, Elodia canadensis, grows abundantly and in some situations, can interfere with access to water (Figure 5). Canada waterweed is dark green, has small leaves whorled around the stem, and is often kept as an aquarium plant.

Exotic flowering rush (Butomus umbellatus) is also from Europe and Asia and occurs in ten Minnesota waterbodies throughout the state. It grows submersed in lakes and rivers, but is emergent along shorelines. although it spreads vegetatively, this hard-to-control species invades sparse or vegetation-free areas and spreads slowly due to low seed production. Exotic curlyleaf pondweed (Potamogeton crispus) was introduced from Europe more than 100 years ago, and is now widespread in Minnesota. This submersed perennial is often the first plant to emerge in the spring. Amazingly enough, it actively grows under ice and snow. The stems reach the surface and flower in June. Curlyleaf pondweed forms mats that are often mistaken for Eurasian watermilfoil. After flowering, these mats break apart and decompose, often becoming a nuisance around the first of July. Exotic curlyleaf pondweed is difficult to control because it forms vegetative propagules which remain in the sediments like seeds. Native species of Potamogeton are beneficial and need not be controlled. Be sure plants are correctly identified before a control program is begun. Contact the MNDNR if you need help identifying an aquatic plant.

Identification of Emergent Plants

Purple loosestrife, Lythrum salicaria, (Figure 6) is an emergent wetland plant from Europe and Asia. It is common in the Upper Midwest and was originally sold as a garden plant. This perennial has escaped and naturalized to many aquatic and marshy habitats. It is an attractive plant with brilliant purple flowers, grows in large stands, and produces enormous quantities of persistent seeds. After invasion, it outcompetes native vegetation, clogs drainage ditches, and destroys spawning areas for northern pike. Because of these undesireable characteristics, it is now prohibited from sale. Native swamp loosestrife, Lysimachia terrestris, has flowers in whorls at leaf axis (Figure 7). Cattails, Typha spp., (Figure 8) are a familiar sight in ponds and lakes in Minnesota. These native plants are up to 10 feet tall and have a cigar shaped inflorescence that forms in the summer. Cattails provide cover for many game fish and nesting habitat for waterfowl. They also help stabilize shorelines and bottom sediments. Emergent plants like cattails (Typha latifolia, Typha angustifolia), bulrushes (Scirpus spp.), and others are problems when the plants clog small waterbodies (Table 2). However, control of emergents is usually strictly limited because they stabilize sediments and shorelines.

Identification of Floating-Leaf Plants

Native waterlillies, Nymphaea spp., (Figure 9) are common floating leaved plants in Minnesota. The leaves are up to one foot across, may have purplish undersides, and have a slit running from the edge to the center. The flowers are white with a yellow center, opening in the morning and closing in the afternoon. Waterlillies provide excellent cover for game fish.

Native watershield, Brasenia schreberi, (Figure 10) has small oval leaves without a center slit. The leaf petioles are covered with a clear gelatinous material. In June they produce small purple flowers.

The yellow lotus, Nelumbo lutea, (Figure 11) has large, grey-green leaves growing up to two feet across, with fragrant pale-yellow flowers opening up to 10 inches. Yellow lotus is a protected species in Minnesota and cannot be removed.

Cow lilies, Nuphar spp., (Figure 12) are found in sheltered areas with shallow water. The heart-shaped leaves are 8 to 16 inches long and float on the water surface. It has bright yellow flowers, and produces fruit eaten by muskrats and waterfowl.

Identification of Algae and Free-Floating Plants

The many types of algae in Minnesota lakes are sorted into three groups (Table 3). Filamentous algae, also called pondscum, look like masses of green cotton or wool on the surface of the water, and grow attached to logs and rocks. They provide cover for the prey of small fish. Planktonic algae are free-floating algae growing near the water’s surface (Figure 13). Some types form large colonies that accumulate on windward shores and in backwater areas and provide food for some small fish. Chara, also called muskgrass (Figure 14), is another type of algae, though it more closely resembles larger plants. Chara is commonly two to three feet long, and provides habitat for the prey of small fish, and stabilizes sediment.

The duckweeds (Figure 15) and watermeal (Wolfia columbiana) are free-floating plants that occur primarily in small ponds or sheltered bays. All of these plants provide habitat for insects that feed fish, but in dense concentrations will shade out submersed plants below them.

Managing Aquatic Vegetation

By limiting exotic introductions: Perhaps the most important way to control invasive aquatic plants is to prevent their spread. By being aware of the problem and acting accordingly, expensive and difficult corrective actions are not necessary. Before leaving a lake or river with a boat, drain all water out of the live and transom wells. Inspect the boat and trailer, paying particular attention to the lower part of the motor and propeller, the trailer axle, rollers, and the hitch. Remove any attached vegetation, even if you do not think it is an exotic species. Transportation of any aquatic plant on public roads is illegal in Minnesota.

Some exotic plants were introduced by the aquarium and landscaping trade. Do not dump aquarium plants into lakes, streams, stormwater basins, or created ponds. Also, when landscaping a waterside, make certain the plants you use do not contain restricted exotics.

By limiting nutrient enrichment: Limiting the amount of nutrients entering a lake is a good way to reduce long-term plant growth. If you live on or near a lakeshore, use only the recommended amount of fertilizer and apply it only in the fall. use a no- or low-phosphorus fertilizer. Leave a buffer zone of unmowed, unfertilized lawn between the yard and the lake, or establish a filter strip of native vegetation. Never fertilize right up to the lake edge. Collect and compost lawn clippings and fallen leaves. Do not rake them into the lake or burn them near the shore. Finally, be sure that your septic system is operating correctly and not draining into the lake. For other ideas on how to limit nutrient input into bodies of water, refer to the Shoreland Best Management Practices packet from the Lake County Soil and Water Conservation District and the Minnesota Arrowhead Water Quality Team.

By sustainable lawn care: Manage lawn and garden through sustainable management practices which promote low inputs of fertilizers, herbicides, and insecticides into your yard. For instance, some grasses do not grow well in shady areas and the gardener might be tempted to use fertilizers and herbicides to encourage growth. Instead, replace grasses in shady areas with shade-tolerant ground covers. If weeds become a problem, spot spray weeds or remove them by hand. For more information, refer to Sustainability in Urban Ecosystems, a bulletin (FO-6709) and video (VH-6639) available from the University of Minnesota Extension Service at (612) 625-8173.

Leaves, grass clippings, and granules of fertilizer left on paved surfaces all find their way into your lake. Compost the leaves and grass clippings, and use a drop spreader for better control when fertilizing. If a buffer zone of native vegetation separates your grass from the lake, leave grass clippings on the lawn, whether or not you have a mulching mower. Lawn clippings do not contribute to thatch build-up, because clippings decompose rapidly and add nutrients to the soil. Leaving the clippings all season is equivalent to one fertilizer application, saving money and time. When mowing, keep the turf height at about three inches, mow frequently, and take off no more than one-third of the leaf blade at a time so as not to stress the plants. All these methods promote turf growth and health in a more sustainable way.