Is Duckweed Good for Ponds & Fish? (Pros & Cons)

Often found floating in still or slowly-moving freshwater systems, duckweed is a group of notably small aquatic perennials. These plants are commonly referred to as water lentils or bayroots. They have extremely simple structures. Some species lack readily visible leaves and shoots and seem to have evolved just for the purpose of vegetative reproduction or budding. One could say they are the plant equivalents of yeast!

All duckweeds belong to the Lemnaceae family, which is comprised of the smallest angiosperm plants in the world. This family includes 5 genera of duckweed plants: Lemna, Wolffia, Spirodela, Wolffiella, and Landoltia. The largest species under these genera extend to a length of just 15 mm at maximum growth! These tiny plants are by no means impeded by their size, however, as they are distributed across the entire planet.

Commonly mistaken as a type of algae, duckweeds tend to form considerably thick mats on the surface of nutrient-rich water. They can withstand a wide variety of conditions, and can even bide their time during the coldest months by becoming dormant on the bottom of a lake, pond, or lagoon. Due to their mode of spread, ease of cultivation, and nutrient profile, duckweeds have become somewhat controversial! Although they are associated with many benefits, their rapidly spreading colonies can come with a cost.  

Common Types of Duckweed & Their Characteristics

1) Common duckweed (Lemna minor)

Also referred to as lesser duckweed, this species is the most widespread across North America and is also present throughout Europe, Africa, and Asia. It is characterized by having 1 – 2 pairs of minuscule oval-shaped leaves (more appropriately called fronds) and an even smaller root system that dangles just beneath the water’s surface. The seemingly weightless leaves have small air pockets that allow them to float, and the roots are known for being quite sticky. This “stickiness” aids in plant dispersion as the plant can become carried by waterfowl to other locations.

2) Dotted duckweed (Landoltia punctata)

Native to freshwater bodies in Australia and Southeast Asia, dotted duckweed is frequently carried from one location to another by the aquarium or pet store industry. It is now naturalized in several areas of the US. This species is characterized by individually occurring egg-shaped fronds that are vivid green (darker than Lemna) in color. Around 2 – 4 fine roots are attached to the base of each frond.

3) Greater duckweed (Spirodela polyrhiza)

S. polyrhiza is differentiated from other duckweeds by the reddish undersides of its fronds. This species is one of the largest of all duckweed plants. Each individual is characterized by a single disc-like thallus, which has a smooth margin and extends to just 2.5 – 7 mm across. Beneath the thallus, an abundance of rootlets (3 – 15) forms the root system of the plant. This species has a cosmopolitan distribution and favors nitrogen-rich water. It has also been popularized for indoor cultivation by the aquarium industry.

4) Rootless duckweed (Wolffia arrhiza)

Arguably the smallest of all duckweeds (therefore the smallest of all flowering plants), this evergreen perennial is so tiny that it is hardly visible unless found in colonies. At maturity, its sphere-like fronds extend to less than 1 mm across. As indicated by the common name, this species has no roots. It is able to obtain its energy requirements, in the form of dissolved carbon, via absorption. Undeniably prolific in terms of spread, rootless duckweed can double the size of its population in just a few days!

Benefits of Duckweed in Ponds

1) Natural water purification

When present in the right amount, duckweed can effectively strip water of potentially toxic compounds. Yes, those tiny leaves and even tinier root systems are so great at excess nutrient removal that they can leave wastewater significantly cleaner. Some species of duckweed are deliberately cultivated as a natural filter in countries that require a low-cost form of water treatment.

Duckweed has even been shown to remove heavy metals and pollutants from the run-off water of intensive farms and textile factories. This bioremediation potential would, of course, be extended to your own pond system if colonies of duckweed are present and well-maintained.

2) Prevention of algal growth

As duckweeds quickly produce mats that are able to shade the water column, they stifle the growth of algae and phytoplankton. Algae tend to compete with other plants for nutrients and space. When sunlight is readily available, especially during the summer, potentially harmful algal blooms can occur. Proper cultivation of duckweed throughout the season can definitely keep your water column clean and can reliably limit the spread of algae.

3) Refuge for small animals

Thin duckweed mats afford protection to many small animals, especially those that predators scanning the water’s surface would normally seek out. These include small fish, amphibians, and insects that feed on the plant. The mats can even disguise entire bodies of freshwater from fish-eating birds that are flying overhead, in search of any ripples or movements on the water’s surface. As long as they keep close to duckweed fronds, fish fry and juveniles of other animals may also be less visible to potential predators in the water itself.

4) Source of nutrition

Surprisingly, the fronds of duckweed are a great source of protein! Prized pond fish, including koi and goldfish, will happily consume duckweed. These fish can be used to control the coverage of duckweed, which they can shift to as one of their primary sources of nutrition if feeding times are reduced. Even tilapia and grass carp are able to obtain most of their dietary requirements and grow to be quite large when exclusively fed with mats of duckweed.

Even the smallest duckweed species, those belonging to the Wolffia genus, are known for being quite rich in protein and fiber. For this reason, the nutritional value and edibility of duckweed are currently being looked into for human consumption. Poultry and livestock have been already been shown to benefit from diets that contain either fresh or dried duckweed.

5) Reduced water evaporation

If your pond tends to lose too much water during the summer months, you can try growing duckweed to help reduce water evaporation rates. The mats are able to keep surface temperatures cool by providing shade and by reflecting light. Though some species can be quite sensitive to sunlight, duckweed fronds have a waxy surface cuticle that protects the plant from abiotic stressors, such as ultraviolet radiation.

Disadvantages of Duckweed in Ponds

1) Requires regular maintenance

Expect to either rake or net the surface of your pond fairly frequently if you have populations of duckweed. If you have a lot of omnivorous or herbivorous pond inhabitants, however, it is highly unlikely that duckweed mats will spread quickly enough to cover the entire pond surface. If your fish don’t readily take to duckweed, then you may soon find that your pond is completely overgrown with its colonies. In the worst case, you won’t even be able to see past the thick mats of duckweed. Luckily, you can recycle manually removed duckweed by adding it to your compost heap!

2) Blocked sunlight & competition with other plants

When duckweed populations are allowed to grow profusely, they can quickly exploit all resources that other pond plants would need to survive. As mats block out the sun, submerged plants may be unable to photosynthesize and produce the energy they need to keep growing. They can become stunted or cease to survive altogether.

3) Low dissolved oxygen conditions

Although duckweed fronds are able to produce oxygen, overgrown mats can cause anoxic conditions. This is more likely to occur in deeper, slow-moving ponds that may depend on submerged plants and algae to oxygenate the lower layers of the water column. When prevented from photosynthesizing, these plants will eventually decay and use up more oxygen as they decompose.

Moreover, extensive duckweed mats may actually consume instead of produce oxygen when sunlight isn’t available. The amount consumed may exceed oxygen production levels, resulting in a dangerous shortage. When mats are especially thick, they prevent oxygen from diffusing into the water from the air above as the interface becomes completely blocked.

4) Very difficult to remove

Keep in mind that a single square meter of water can be occupied by millions of duckweed plants. As some species grow to a maximum length of just 1 mm, it would be virtually impossible to remove all individual plants through manual means. To make things worse, any leftover individuals will simply form their own colonies through vegetative reproduction. A chemical means that could be harmful to other organisms may have to be employed to fully remove duckweed from a pond.

The Final Verdict – Is Duckweed Good or Bad?

The true cost of cultivating duckweed lies in its capacity to spread quickly and form mats. This is a somewhat misunderstood plant that does not truly deserve its ill-reputation, as there are means of managing its growth. Once you get the hang of controlling duckweed populations, it can truly be a neat plant to have in your own pond. Its numerous environmental services can help ensure that your pond’s water quality is kept in check and that its inhabitants are afforded protection and food. Not to mention, it is easy to obtain and undeniably cheap to cultivate.

If you’re convinced that this plant would do your pond a service, you should familiarize yourself with the many natural ways with which its spread may be controlled. Don’t forget that duckweed may be environmentally disruptive if it is allowed to enter public waterways, so avoid dumping your pond water close to a natural water source or canal.

Make sure that garden fertilizer doesn’t stream into your pond water because the input of excess nutrient levels can cause duckweed populations to multiply even faster. Use pond edge plants to create a buffer between your pond and the rest of your garden. Another highly effective means of controlling the spread of duckweed is by using a running water feature or a pump that can increase the water current. Remember that duckweed favors still or slow-moving water. Occasional use of a pump, water fountain, or waterfall feature can drastically reduce duckweed coverage.

Your pond’s inhabitants are also likely to help you prevent duckweed populations from growing out of hand. Reduce feeding times to encourage your fish to look for natural sources of food. They surely won’t mind having a diet enriched with some protein-rich floating plants every now and then!