Duckweed is one of those aquatic plants that can spread quickly and overtake the whole water body in a short span. One of the smallest among flowering plants, a single duckweed plant consists of one or two plate-like structures and a single rootlet. These plants are found to thrive in still water bodies. They either float over the water or just beneath the surface. These plants are said to have various ecological as well as commercial benefits. Duckweed is a food source for various aquatic as well as land animals and birds, including fish, duck and waterfowl. These plants are also said to purify water, as they absorb nitrogen and phosphates. They are also said to prevent mosquito breeding by covering the water surface. It is also said that duckweed is useful in water conservation, as they prevent evaporation to a small extent.

However, these plants have their downside too. If the conditions are favorable, they can spread and cover the whole water surface in a very short span. This results in oxygen shortage in the water, leading to the death of other aquatic animals like fish. Even underwater aquatic plants may die without oxygen. Water bodies filled with duckweed can be unsightly and are usually not deemed fit for swimming. So, it is always necessary to control the growth of duckweed.

How to Control Duckweed in Ponds

As mentioned above, duckweed growth must be controlled to avoid this plant from spreading and taking over the entire water body. This can be done in many different ways. The following are some tips and guidelines for duckweed control.
  • One of the easiest and commonly adopted methods is manual removal of the weed. You may use nets for periodic removal of duckweed. Raking is another method that can be adopted for removing duckweed manually. Some people opt for a floating boom that can be used to sweep the water surface, so as to control duckweed growth.
  • Once removed, you must dump these plants at some location that is far from the pond. Otherwise there are chances that the plant gets transported back to the pond in one way or another. Duckweed is one of the best options for composting.
  • Biological control of duckweed is not a good idea, as it will not be very effective. Even though certain types of fish (grass carp and koi) water fowl and duck are found to feed on this plant, it will not be effective in controlling its rapid growth.
  • Another method is to use chemicals. There are certain specific herbicides (like Diquat and Fluridone) that are used for this purpose. Adding a small amount of chelated copper to the chemical may increase the effectiveness of the latter.
  • Use of such herbicides must be done strictly as per the instructions of the manufacturer. Another point to note is that you should avoid using the water of that pond for at least three days, so as to avoid poisoning.
  • You may also resort to a duckweed control aeration device. In other words, an aeration device (like a bubble aeration device) will cause surface agitation of the water and this will control the spread of duckweed, a plant that prefers still water to grow. The same result can be achieved by using a fountain.
  • This will also prevent proliferation of duckweed to some extent. You may also try reduction of nutrients, as it is one of the main factors that triggers the growth of duckweed in water bodies. Nutrients in the pond can increase with fallen leaves and small amounts of lawn fertilizer.
  • Another method that can be adopted for controlling duckweed growth is to block sunlight that is needed for the growth of duckweed. Use some shading on the southern side of the pond to prevent maximum amount of sunlight from falling on the pond.
It is always better to start with manual and biological control methods rather than resorting to chemicals. If you want to try chemicals, make sure to use them as per expert guidance. In such case, you have to take sufficient precautions to prevent problems like poisoning. So, choose duckweed control products wisely. You may seek advise from a horticulturist about the same. If properly controlled, duckweed can be a beneficial addition to the pond.