Both types of aquatic floating plants, duckweed (Lemna spp.) and mosquito fern (Azolla spp.) prosper when the freshwater pond surface basks in sunlight, doesn't have waves and water temperatures are warm. If conditions are right, these plants multiply quickly, spreading their leaves across a wider area in the pond, potentially covering all exposed surface. Winter cold typically kills some foliage and other plants drop to the pond bottom to lay dormant until spring. As long as the pond isn't too large, skimming off the plants and allowing them to dry on land kills them and frees up water surface to air and light.
- Skill level:
Things you need
- Swimming pool skimmer net (sometimes called a "rake")
- Rubber boots
- Harvesting container
Put on rubber boots to ensure you can walk safely and comfortably around the edges of the water feature, especially if it's a natural shoreline. If the water is deep and pond large, hip-waders may be better because they allow you freedom to step into shallow water and manoeuvre without worry of getting wet.
Skim the water surface with a wide, tight-meshed swimming pool skimmer net. Scoop up plants in the net in a slow motion to get as many plants captured in the net as you lift it out of the water.
Fling the captured duckweed or mosquito fern plants onto the dry land on the shore to dry out in the sun and die. Alternatively, invert the skimmer net into a large bucket to hail away to the compost pile. A wide plastic bin floats on the water and glides easily with you in the area you are removing the floating plants.
Repeat the skimming of the water surface until you remove a substantial amount of duckweed or mosquito fern. Don't try to remove all of the plants, as some inevitably float away from your skimmer and collect in tiny areas in minuscule numbers.
Wait one to three days before skimming the water surface again. The change in wind or addition of water from rains re-congregates the floating plants into new clumps. Skim the water surface again in increments to continue to remove and air-dry (kill) plants to keep plant populations low and manageable.
Tips and warnings
- Work with the wind. As you skim the surface, the motion you make by stirring the water relocates the floating plants. If it's windy, these plants tend to move with the wind and eventually congregate in the far end of the pond and are trapped against the shore for easier harvest.
- The Ohio State University notes that goldfish/koi eat duckweed and moving water diminishes the amount of duckweed in a pond. Also, reducing the nutrients in pond water from lawn fertilisers or animal waste runoff reduces the rate of growth and spread of floating water plants.
- Don't use chemicals to kill the floating plants. The rotting vegetation in the water adds nutrients, reduces oxygen levels and encourages algae bloom, an unhealthy ecosystem, and increased water temperatures since no leaves shade the water surface.
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