How to Get Rid of Pond Weeds
When I used to work for a county agricultural commission in central California, I remember clearly how easy it was to define a weed. A weed is any plant you don’t want to be there.
Pond weeds can definitely be a problem. Definitely. Some are especially difficult to control once they get out of hand. A case in point is cattails. You have some weeds? Okay, that’s natural. But too many and you have a problem. They overtake your pond and reduce sunlight entering it. They reduce the pond’s dissolved oxygen. Cattails and tall reeds develop rhizome mats that work like roots and they must be kept under control or they will be nearly impossible to get rid of. You cut down cattails, but the mats are still there, growing new cattails. Some tall reeds also have these mats, making them still another problem to get rid of as soon as possible.
And of course there’s the problem with algae, which, in smaller amounts, is a good thing. Algae provides food nutrients necessary for a healthy pond. But when bad algae gets out of hand, the water can get murky and sunlight will be blocked and photosynthesis will be reduced, sometimes drastically. Eventually, fish and “good plants” suffer and your water quality gets terrible.
How to Control Algae
Algae provide nutrients for fish, insects, birds, and it produces oxygen through photosynthesis.
There are two main kinds we will discuss here, though really there are many, many more
There’s planktonic algae, which are miscroscopic plants which can bloom if they get excessive nutrients, like phosphorous and nitrogen. That is one reason why it is a good idea to have barriers around your pond so that these nutrients do not seep into the water from fertilizers that might be present on your property. Filamentous algae, also known as string algae, is also microscopic but gets “stingy” as it mats together and floats on the surface. Again, barriers need to protect the pond from stormwater. If a pond gets too covered by string algae, fish kills are more likely and the rotten egg smell can develop as the pond becomes stagnant. In addition, more muck is going to develop at the bottom of the pond, increasing the likelihood of eventual dredging.
To prevent algae. you need to reduce nutrients seeping into the pond or lake. Be careful with fertilizers! You should not apply them on slopes that could leak down into the ponds. Don’t discard lawn clippings and leaves into the pond. You can grow plants at the shoreline to absorb the runoff of nutrients. Adding pond dye can reduce algae growth by blocking sunlight.
To control the algae that is already there, there is also a tropical fish, blue tilapia, that eats filamentous algae and could help control the growth. There are also chemicals that can be applied but typically they should be applied by licensed applicators. If done incorrectly, fish kill could result. If algae is killed too quickly and decomposes too quickly, fish will die.
Pond rakes can be used to rake algae to the shore, where it can be disposed of. Yes, it’s a messy job, so it is easier to try to get control early. Also, in small, backyard ponds, you might try using a UV sterilizer.
How to Control Pond Weeds 
Besides algae, weeds can grow around the sides of a pond, emerging in just a couple of inches of water. They are half based in soil and half based in water, essentially. Weeds like cattails can reproduce very quickly, and it is a good idea to take control at first sight. For cattails and other weeds with rhizome mats, merely cutting the top of the weed will not work. You need to pull out as much of the weeds’ rhizome mats as you can to minimize regrowth. If there is a way to lower the water level in the pond, weeds at the edges can die from lack of water as the water drains from the edges. When you dispose of the plants, make sure the seeds don’t spread and get right back into your pond. You can also use herbicides, but again it is best to have a pro do the work, but at the very least, make sure that you follow the instructions carefully and don’t over-apply. Just try out part of the pond first to see how effective they are and to make sure you don’t have fish or other wildlife dying.
Pond aeration can help control the growth of unwanted plants, as it churns the water and dissolves oxygen. You might add a pond fountain or a bottom aerator for deep ponds, over seven or eight feet average depth. Small backyard fountains can also benefit from a waterfall if you like.