Cattails and water primrose are emergent plants, so they are rooted in the water and extend above the surface. These plants can reach nuisance levels if neglected. Cattails may “ring” your pond, preventing anglers from casting from the shoreline. Primrose may cover the surface and entangle fishing lures. This dense growth of primrose may provide too much protection for small bluegills, making it difficult for bass to obtain enough food for good growth. Bluegill also grow slowly in this environment due to excessive populations created by overly successful reproduction.
Once cattails and primrose have established strands, they are very difficult to control. Prevent cattails and primrose from gaining a foothold by constructing a 3-to-1 slope along the shoreline of new ponds. This usually discourages the development of these two emergent plants. If this is not possible, then diligently removing all cattail and primrose sprouts will save money and hours of effort later.
This method only works with cattails. Cutting cattails with a sickle bar mower or a hand held “weed eater” provides temporary control, but the plants will still spread through rhizomes or seeds. If the cutting effort is continued and plants are never allowed to grow more than a foot tall, seeds will not be produced. Eventually, the plant will die as the stored food in the roots and rhizomes is depleted.
This is an effective temporary measure for initial control. Stretch black Mylar plastic (8 millimeters thick) over patches of growth and fasten in place with weights. Puncture the sheet in multiple places to allow gasses to escape. This virtually destroys the existing plant growths, but unfortunately, the plants will return from seeds and rhizomes requiring repeated or different treatment. Shading is, however, an excellent method for getting a nuisance population under control in preparation for other treatments.
Cattails can be initially loosened with a potato fork, but they must be pulled by hand. Water primrose can be removed by hand or with a rake. Constant weeding will be necessary for both species. If weeding is done faithfully, eventually the available seeds will be eliminated. Aquatic vegetation makes good compost!
Create a 3-to-1 slope to 4 feet of depth to limit the amount of shallow water available for aquatic vegetation growth. Use a long-armed backhoe to remove large populations of cattails (their roots, rhizomes, and seeds) and to create the proper slope to control future regrowth. Weeding will be necessary to control new growth.
If the pond is old and has become shallow due to accumulation of black muck on the bottom, it may be necessary to drain, dry, and deepen the pond. The black muck stores the nutrients that fuel the excessive growth of aquatic plants. All excavated material should be removed from the pond’s watershed so that unwanted seeds, nutrients, and sediment do not wash back into the pond.
Prevent excess nutrients (nitrogen and phosphorus) from washing into lakes and ponds. These excess nutrients quicken aquatic plant growth. These nutrients accumulate naturally as the pond ages, but some localized nutrients enter the pond through runoff.
Sources of nutrient runoff:
Filter excess nutrients from runoff water by maintaining a 100-foot or wider buffer strip of grass and trees around the pond’s edge. Construct small silt retention ponds in the watershed to settle out nutrients before they enter the pond. Avoid localized nutrient inputs by tiling or constructing a water diversion terrace below the nutrient source to direct its runoff away from the pond. Also, fence livestock away from the pond’s edge and water them from a tank below the dam.
Grass carp are generally not effective at controlling cattails, water primrose, and other emergent plants.
Before using chemicals, you should consider potential contamination of domestic water supplies and the waiting periods for watering livestock, eating fish, swimming, and irrigating. Aquatic plant control with chemicals works best when the water temperature is above 60 degrees Fahrenheit. To avoid oxygen depletion and a possible fish kill, avoid treating water primrose when the water temperature is above 80 degrees Fahrenheit and treat only one-third of the vegetation at a time.
Chemicals can be expensive and do not provide permanent control, so repeated treatments are usually necessary. Please remember that the long-term effects of most herbicides on the environment are not well known.
Download the Aquaguide below for additional information on chemical recommendations and determining the amount of herbicide needed. For more information, contact your local MDC office.