Managing "Pond Moss" and Aquatic Weeds
Jeff Seiler: July 27, 2021
Excessive aquatic plant growth is a common problem for ponds in Sedgwick County. This can be unattractive and can inhibit the use of a pond. Over the last few weeks we have received a high number of calls on "pond moss" and pond weeds, which is common for this time of year. While completely eliminating pond plant growth is difficult, there are strategies that can be utilized that can help.
First off, does the aquatic plant growth need to be removed? Actually, some weed and algae growth is healthy for the pond because as they photosynthesize they produce oxygen. Once you get into a situation where the growth restricts your use of the pond, that can be a problem. Everyone’s tolerance is a little different for how much weed and algae growth they allow. A rule, I have heard, is that you should treat if 20%, or more, of the pond is covered in weeds/algae. Some people’s tolerance is 0.
The "pond moss", people commonly call about, is actually filamentous algae. The primary cause of an abundance of pond weed and algae growth is an excessive amount of nutrients in the water. Another cause is sedimentation. To help slow eutrophication, you want to limit as much nutrients as you can from entering the pond. Often, this can be difficult so we have to settle for treating the symptoms. Once a pond has excessive nutrients it can take quite a bit of time and management to combat it.
To limit the amount of nutrients entering a pond, one thing that can be done is installing a 10 feet wide, or more, native plant vegetative buffer around the outside of the pond. Essentially, what this does is filter nutrients entering the pond through runoff. This buffer can include grasses, broadleaves, flower, etc. The strip should only be mowed once a year, in March, and allowed to grow the rest of the year. The thicker the growth the better. Native plants have expansive root systems that will also help to stabilize the bank and absorb some nutrients from the water. Another benefit, is the waterfowl are less attracted to ponds with thick growth around the outside. They would much rather hangout where a lawn is mowed to a ponds edge. The problem with waterfowl is their droppings add to the nutrients in the water. While waterfowl may not like the taller thick vegetation, some animals may find it more attractive, like snakes. A concern, for some pond owners, with the vegetative buffer is it can make pond access more difficult. In that case, mowing some access points may be necessary. (Riparian Plants List)
A lot of the ponds I deal with are in HOAs with a bunch of lawns that are over-watered and over-fertilized. Whatever you have surrounding the pond, crops, pasture, lawns, over-fertilizing should be avoided. It’s important for every house in the neighborhood to practice responsible nutrient management for their lawn. Regardless of whether or not your house is on the waterfront, most likely the runoff from your lawn ends up in the pond through the storm drain. That’s what these ponds are designed for, stormwater retention. To fertilize properly, have your soil tested. Know what your soil can handle. In some cases, it may be necessary to split apply your fertilizer to help minimize nutrient loss from your field/lawn. You shouldn’t be mowing your grass down to the pond bank and should avoid blowing grass clippings into the lawn. For information on proper lawn maintenance refer to these publications:
Other means of fighting pond weeds and algae: pond dyes, aerators, copper algaecide/aquatic herbicide, grass carp, digging out the pond, or physically removing them.
Pond dyes help suppress the growth of weeds and algae. Some people do not like how it changes the color of the water. They are not toxic though. The dyes work by shading the pond. Algae and weeds use photosynthesis so by reducing the amount of sunlight reaching the depths of the pond, their growth is slowed. These should be applied in the spring before you get a bunch of algae and weed growth.
Aeration. Everyone I have talked to that has installed an aerator, and a lot of the videos I have seen, mention how the fish are doing better after installing one and they also may see a reduction in algae. Our ponds are stratified with different layers. The top layer is oxygenated. Deeper layers do not have as much oxygen. That top layer gets thinner as we get hotter in the summer. Pond aerators pump air to the bottom of the pond. As the air rises to the surface, the water is mixed. That mixing helps to oxygenate more of the water profile. The oxygen promotes good bacteria that compete with algae and weeds for nutrients. Some people install fountains, which do help some, but you are only agitating the surface, so not as much oxygen is added. I have seen ponds covered in algae besides the 15 feet surrounding the fountain. Any means of getting the water moving can help break up some of those blooms, but what I hear is aerators do more than fountains.
Chemicals. Regardless of whether you use all these methods to help prevent algae and weeds, you may still end up with a mess of pond weeds, especially if you are just starting out prevention measures. Once you have them, chemicals can be used to control them. For filamentous algae, copper algaecides are the best products to use. Filamentous algae begin their growth on the bottom of the pond. As they grow the mat fills with gases and floats to the top of the water. This algae should be treated once you start seeing it growing on the bottom of the pond/rocks in the water. Once we get into summer and have warm water, or if you have an excessive amount of algae growth, the pond should be treated in sections according to the label. Usually it says to treat ½, 1/3, or ¼ at a time and then give the pond a rest period before treating the next section. If you purchase a copper sulfate product that is in pellet form, the pellets should be dissolved in water for application. This solution will give you better control than just spreading the pellets over the pond. Treating an entire pond that has warm water and a bunch of weed growth can lead to fish kills. Ponds excessive in nutrients are already lower in oxygen. Hot water doesn’t hold as much oxygen. Once you kill the algae, it is broken down by bacteria. That bacteria uses oxygen in the water and can lead to low dissolved oxygen levels, killing fish.
Copper algaecide is not effective on other pond weeds, so depending on what particular weed you need to get rid of, you’ll have to use an aquatic herbicide. Some of these are cheaper than others. The Aquatic Plants and Their Control publication has a good chart on page 7 for helping select an herbicide. Chemical means of control does not remove any nutrients from the water so they are recycled back into the pond.
All applications of copper algaecide and aquatic herbicide should be made following the product label!
Grass Carp can be used to control some pond weeds but are ineffective at controlling filamentous algae. If we ID the pond weed, we can figure out if they will be useful in the pond. One problem with them is they can increase pond turbidity.
An expensive way to help against weeds and algae is to dig out the pond and make it deeper. Shallow water usually is hotter and sunlight penetrates to the bottom easier, so increasing the depth helps. This doesn’t work if there isn’t enough water to fill the pond. Although it comes with high cost, this method is often one of the longer lasting solutions.
Lastly, you can always physically remove the growth. You can do this with a rake and rope, a seine, by wading into the water, or from a boat. Make sure you are removing the weeds and algae from the pond shore or they can breakdown and their nutrients are back in the water. While this is the most labor intensive measure, it usually has the highest success rate and fewest downsides. Also, rather than recycling nutrients you are removing them from the pond. The easiest time to physically remove pond weeds and algae is early in the season.
Multiple strategies, over a period of time, will need to be utilized to combat excessive pond and algae growth.
Hope this helps!
For questions contact Jeff Seiler at: 316-660-0153 or firstname.lastname@example.org.