The various types of plants in a pond are generally separated into three categories — submersible, emergent and floating. Plants in the submersible category grow completely under the water, but their leaves may stretch out and float on the surface. Canadian waterweed (Elodea canadensis) is a desirable submersible plant that grows in U.S. Department of Agriculture plant hardiness zones 4 through 10.
The roots of emergent plants are attached to the soil located at the bottom of ponds but grow high above the surface of water. Sedges (Carex spp.) and rushes (Juncus spp.) are two common emergent plants that grow in USDA zones 3 through 10. Floating plants feature foliage that sits on the water surface and floats with the current. Waterlilies (Nymphaea spp.) are one of the most well known floating plants and grow in USDA zones 3 through 11.
Desirable Pond Grasses
Pondweed species growing in and around the pond generally have an aggressive nature that can take over the pond, suffocating desirable plants and wildlife. Although algae are actual plants, they are one of the most common weeds found in ponds. These plant-like organisms covers the surface of ponds, growing rapidly and threatening the life of desirable aquatic plants and wildlife. Another common pond weed is duckweed (Lemna minor), which grows throughout USDA zones 4 through 10. This annoying plant is aggressive and even though it acts as food for ducks, it will quickly spread, covering the surface of mud puddles, slow-moving streams and ponds.
Ornamental grasses — such as golden sedge (Carex elata), longleaf woodoats (Chasmanthium sessiliflorum) and northern sea oats (Chasmanthium latifolium) — can prevent soil erosion, provide food for wildlife and improve the look of the landscape. Golden sedge grows in poorly drained areas located throughout USDA zones 5 through 9. Longleaf woodoats thrive in wet soils in USDA zones 6 through 9 and northern sea oats grow in either full or partial sun in USDA zones 4 through 9.
Types of Pond Grasses and Weeds
Pond grass and grass-like plants can improve the look of ponds while providing much-needed habitat for beneficial insects and wildlife. Pondweed species, on the other hand, can negatively affect the appearance of ponds and can disrupt other plants and wildlife. Knowing which grasses and weeds are undesirable and how to control them will help you keep your pond looking its best.
It is very important that weeds removed from ponds or lakes are composted, buried or burnt. On no account should they be transferred to rivers, other ponds or lakes. Several introduced pond weeds, widely available from garden centres, cause enormous problems where they escape or are introduced into the wild.
Invasive water weeds are troublesome in various ways:
Alternanthera philoxeroides (alligator weed)
Azolla filiculoides (fairy fern)
Cabomba caroliniana (Carolina fanwort)
Crassula helmsii (New Zealand pygmy weed)
Eichhornia crassipes (water hyacinth)
Elodea nuttallii (Nuttall’s waterweed)
Gunnera tinctoria (Chilean rhubarb)
Hydrocotyle ranunculoides (floating pennywort)
Lagarosiphon major (curly waterweed)
Ludwigia grandiflora (water primrose)
Ludwigia peploides (creeping water primrose)
Lysichiton americanus (American skunk cabbage)
Myriophyllum aquaticum (parrot’s feather)
Myriophyllum heterophyllum (broadleaf watermilfoil)
Aquatic weeds are usually a problem only during the warmer months of the year when water temperatures rise above 6°C (43°F). Many plants grow rapidly in the warmer temperatures and can quickly take over garden ponds.
There are no weedkillers approved for the control of aquatic weeds in gardens, but there are a small number approved for use by professionals. Because of the danger of water pollution their application is very carefully controlled and prior approval for their use must be obtained from the Environment Agency or equivalent authority.
Submerged plants (aka ‘oxygenators’)
Aquatic weeds (or pond weeds) can normally be tolerated in small numbers, but it is when they make excessive growth that they become a nuisance, particularly in summer. In garden ponds control is relatively easy, but in larger ponds and lakes it is more difficult.
Different approaches will be needed depending on the type of aquatic weed: