Aquatic Plants in Ponds
Aquatic plant vegitationMany different aquatic plants can grow in ponds. These plants range from algae, which drifts suspended in the water, to plants floating on the water surface or rooted in the pond bottom.  The rooted plants grow either entirely under the water, have floating leaves, or grow with stems above the water surface. Some have both underwater and floating leaves. Both algae and the rooted plants will grow in all ponds.  It’s maintaining a balance that is sometimes difficult.  A pond plant identification guide can help you determine what plants are in your pond, and how to manage those plants (printer friendly pond plant guide).

Single-celled algae are usually not visible to us, form the base of the food chain, and produce much of the oxygen necessary for other life in the pond.  Filamentous algae, sometimes called moss or grass, is more visible and most easily becomes a nuisance.  When there is a sudden growth of either type of algae it is called a “bloom”.

Rooted aquatic plants are important to the overall health of ponds and lakes.  They stabilize the shoreline and pond bottom, tie up plant nutrients thereby reducing algae blooms, help the water to clear faster after a rain, produce oxygen, and provide food and habitat for the many inter-related forms of life that reside in and around a pond.   Plants also provide nursery habitat for many fish, and moderate levels are important for good growth, condition, and abundance of sport fish.  We know from our research here in Iowa that lakes without rooted plants may not produce balanced, desirable fish populations.

Plant Introduction
Though aquatic plants will usually become established in a pond without special effort, some kinds of plants are less likely to become a nuisance for anglers or other pond uses. Introduction of a variety of these plants may allow you to more quickly meet your pond management goals.  Consider a new pond like a newly tilled field or garden, something will grow there given nutrients, light and seed.  Shoreline species that grow above the water surface recommended for ponds include:  arrowhead, pickerel weed, spike rush, sweet flag and water iris.  The floating-leaved lilies may be a nice addition to ponds without extensive shallow areas. (Don’t confuse the preferred lily with lotus, which  spreads aggressively and would not be a good addition to a pond.)  Wild celery and water stargrass are candidates to add to the diversity of plants that grow completely under the water.   Largeleaf and longleaf pondweed are additions that have both underwater and floating leaves.  Plants to avoid introducing include:  milfoil (could be the invasive Eurasian milfoil), naiad (invasive brittle naiad), curly-leaf pondweed (invasive), cabomba (invasive submersed plant common in water gardens) and coontail (will colonize on its own).

Recommended Not Recommended
Arrowhead (E) *Phragmites(E)
Pickerelweed (E) Cattails (E)
Spike Rush (E) Lotus (E+F)
Sweet Flag (E) *Naiad (S)
Waterlily (F) *Milfoil (S)
Longleaf Pondweed (F+S) *Curlyleaf Pondweed (S)
Largeleaf Pondweed (F+S) *Cabomba (S)
Wild Celery (S) Coontail (S)
Water Stargrass (S) Elodea (S)

Growth form:  Emergent (E), floating-leaved (F), submersed (S)
* Is itself an invasive species or closely related

When purchasing plants, it is recommended to purchase from a local nursery or one in a climate zone similar to your pond’s location. Many guides, both printed and online, exist to assist with plant identification and pond management. Words of caution: Any plant can become a nuisance with the right conditions.

Plant introduction in new ponds is a little easier than in existing ponds that already have established plants.  In existing ponds, it is recommended that you remove the aquatic plants that are growing in the area where new plants will be introduced (see mechanical or chemical removal below). If plant-eating or burrowing animals are present (grass carp or common carp, muskrats, turtles, waterfowl or deer), it may also be advisable to protect your plantings behind a small, protective fence/cage until they become established.

Plant establishment success will be higher when planting from 5-20 plants per area.  Water depth is also a consideration.  Shoreline plants do best when planted at the water’s edge or in very shallow water.  Floating-leaved and underwater species can be planted in water from 6 inches to 2-3 feet deep.   There is no need to plant deeper than you can easily reach.   

Plant Control
Although rooted plants are required for a healthy pond, too many can upset a fishing pond’s balance and become a nuisance.  Everyone has different tolerances to pond plant life. Most biologists agree that once the plants cover over 40% of the surface area of a pond, they are over-abundant.  Plant control methods that can be used fall into four classes: preventative, mechanical, biological and chemical.

Watershed management and shoreline deepening are preventative and are easiest and most effective to implement prior to pond construction.  Conservation practices like silt retention structures, wetlands, buffer strips and grass waterways in the pond’s watershed keep soil and plant nutrients from reaching the water.  Shoreline deepening creates a smaller area where sunlight can reach the pond bottom, effectively reducing the area that rooted plants will grow.

In cases where preventative measures are not practical, mechanical methods like hand removal, bottom blanketing, shading, and water draw-down can be effective for short-term control.  Removal can be done at any time of year.  Plants can be removed by hand, with a rake or by dragging a light wire lattice or steel frame.  Many aquatic plants can root from fragments, so be careful to remove all plant stems that float up.  Once plants are removed from high-use areas, the pond bottom can be blanketed with sand or gravel to slow plant re-growth.  Small areas can be blanketed for 30 days with thick black plastic (punctured to allow gasses to escape) or commercial weed barrier products held with a frame or weighted at the corners.  Shading the whole pond can be done with pond dye products that reduce light penetration.  These products are best applied early in the growing season but often need re-application later in the season.  A shallow water, whole-pond treatment to kill underwater plants is over-winter drawdown, with the hope of exposing these dewatered areas to freezing and drying for several weeks through the winter to kill underwater plant roots.  Not all kinds of plants can be controlled by this method, is most effective at controlling species like Elodea, milfoil, coontail and lilies.  Pondweed (i.e. Potamogeton) naiad, hydrilla, and seed-producing species are not effectively controlled by drawdown.  Do be careful to leave a minimum of eight feet of water depth in the pond to avoid killing fish over a long, snowy winter.

Biological control with grass carp (white amur) can provide effective, long-term control for underwater plants, but will not control algae or shoreline plants like cattails.  A word of caution; stocking too many grass carp can actually increase the growth of algae and eliminate sport fish habitat.  This happens when grass carp are over-stocked and eat all rooted plants that would normally compete with the algae for nutrients.  Because grass carp can live well over 20 years and are difficult to remove once stocked, an over-stocking can lead to problems for many years.  For this reason, the automatic stocking of grass carp is not recommended for aquatic plant control.  However, a conservative stocking of 1 to 2 fish per acre can be used for heavy plant growth.  These fish should be 10 inches or longer to avoid being eaten by largemouth bass.  Some initial vegetation control should be noticed 2-3 growing seasons after stocking, so you must be patient.  Additional stocking may be needed if no control is seen, or if plant growth increases again to nuisance levels.

Chemical control with herbicides is another method for seasonal control of aquatic plants.  There are five steps with any herbicide application: 
  1. Correct identification of the plant to be controlled. The Texas AgriLife Extension Service has an interactive website that is useful for identifying aquatic vegetation:
  2. Measure the area to be treated (surface area and depth).
  3. Read the herbicide label to determine the correct timing and amount to apply.
  4. Identification of potential restrictions on uses of the water, e.g., irrigation or watering animals.
  5. Apply according to label directions. 
There are several drawbacks to plant control with herbicides.  First, control can be negligible if treatment is at the wrong time of year (read the product label for best application times).  If there is a large area of vegetation to kill, the decay of all these plants can use much of the oxygen in the water and cause a fish kill.  For this reason large areas should be divided, and these smaller areas treated several weeks apart, especially in summer months when plant decay is faster and the water holds less oxygen due to warmer water.  Control can be temporary and expensive and repeated applications may become necessary.  Some herbicides require that selected pond uses cease for a period of time after application.  A listing of approximate costs of herbicides can be found with a cursory web search.  An excellent overview of which herbicides will work for different plant types can be found in the Iowa State University Fisheries Extension Service publication Aquatic Plant Management or at the Aquatic Ecosystem Restoration Foundation website. If these sources are unavailable to you, a phone call to your local fishery management biologist can also provide direction and guidance.

Depending upon the level of plant growth, the types of plants present and the intended use of your pond, it may be beneficial to use several control methods to achieve desired results.  For example, hand removal or herbicide control with bottom blanketing in high-use areas with a light stocking of grass carp may be more effective than any one of these methods alone.