Aquatic ecosystems are each incredibly unique and incredibly sensitive, and around the world are facing threats from habitat destruction via human development as well as the introduction of invasive weeds. Your pond or lake is no different, and is in just as much danger of being colonized by invasive plant species.
However, you can’t use the same herbicide treatments in water as you can on land, as water bodies and their inhabitants all have precise parameters they must stay within to remain healthy – standard herbicides, and even specially formulated aquatic herbicides, can disrupt this balance and cause harm to fish and many other organisms if not used carefully and precisely. Even when used properly, these chemicals have the potential to persist in the soil and seedbank, as well as being distributed elsewhere via the water cycle.
With this in mind, there are a number of more natural, alternative methods to traditional chemical treatments that, depending on the situation and the aquatic weed species that are present, may be a better option for you. Though it has its own drawbacks, utilizing salt is one of these alternative options.
How Does Salt Work to Kill Aquatic Weeds?
Despite the fact that aquatic weeds are obviously found in water, salt kills them by disrupting osmosis and drawing water out of them – just as too much sodium dehydrates us, it does the same to plants, but only some of them.
There are some plants, known as halophytes, that are specially adapted to saltwater or brackish water in that they have thick, waxy leaves to protect their pores from salt absorption, and undergo osmosis very quickly. This means that they’re able to move salt through their roots and stems and out of their pores before the salt has a chance to damage the plant. Most plants, like those that are likely found in your pond or lake, are glycophytes, meaning that they are not tolerant of salt.
The chart found at this link is helpful by showing the salt tolerance levels of many common pond and lake plants so that you can better gauge whether or not you should use salt, and which concentration is too high in terms of plant health (though do keep in mind that the salt will impact fish as well).
Is Salt Suitable for All Aquatic Weeds?
Salt can of course damage other plants aside from the weeds you’re trying to get rid of. Salt is best used with plants that are extensive and rhizomatous, meaning that they grow in thick mats via rhizomes. Examples of rhizomatous plants are cattails, lily pads, sweet flag, equisetum (horsetail and scouring rush), the invasive reed phragmites, and many others. By using salt blocks only in the center of these rhizomes, you help ensure that the salt goes directly to these plants via the thick and extensive rhizomes, while minimizing how much salt is able to dilute into the water and travel to other plants in the pond or lake.
As with incorporating anything into your pond or lake, dosing is exceptionally important. In natural lakes, it’s best to not use salt at all as these not only usually have a more established ecosystem, but it’s also difficult or next to impossible to remedy the water if you make a mistake, such as adding too much salt due to the size. Backyard ponds of course also have their own ecosystem as well, but it’s much easier to perform water changes if needed to remove excess salt if required, or make use of salt in the absence of fish.
Salt Dosing & Aquatic Application Considerations
Note: Salt for weed control should always be applied directly to the plants rhizome, and not simply dissolved into the water, as the concentration needed would be so high it would kill off all aquatic life. If you have fish, extensive weeds, or a large lake, salt is not recommended and you should consider different methods of control.
A commonly cited dosage is using 1 gram of salt per liter of water, but studies have found that 860 mg (which is less than 1 gram) per liter is considered toxic and stressful to aquatic life, including fish. Therefore, it’s exceptionally important that you add salt in small amounts directly to the rhizome over an extended period of at least a few hours, making sure that after a few days you don’t exceed a concentration of 860 mg per liter as continued exposure of a couple of days and beyond to sodium means that lower and lower concentrations become detrimental to aquatic life.
Further studies conducted in Minnesota on several lakes that experience spring runoff rich in road salts (which are utilized all winter to help de-ice roads) have found that salt concentrations at or above 1,000 mg per liter have immediate lethal effects on freshwater life, while exposure lasting more than a few days of as little as 250 mg per liter is still quite harmful.
Fish, frogs, and even exceptionally important diatoms can be severely damaged or even killed with excess salt. Be sure to purchase and use a test kit to make sure that salt concentrations do not exceed 0.1% (outside of the plant rhizomes) – below this and pond life should not be harmed, above this and you can add water to dilute the salinity. An added note here: do not use road salt in your pond or lake! High quality rock salt, sea salt, or aquarium salt will work just fine. It may also take a fair amount of time to kill the plants via using salt – for example, placing a salt block amongst cattails can still take as long as two months to kill them off. With this in mind, don’t jump the gun and keep adding more salt!
Alternative Methods to Kill Lake & Pond Weeds
Alternatively, you can remove aquatic weeds by a variety of other, potentially more safe, ways. Pond dyes work by blocking some of the sunlight and therefore can help curb algal and weed growth. Cutting and raking can help to control weeds, but using this method in lakes means that you likely won’t be able to remove all of them – at least not without a significant amount of time and effort.
You can also purchase a shade sail, making sure that you only shade the portion of the pond or lake where there are the most weeds. This way, you help ensure that you kill off or stunt the growth of only those, while allowing the native and desirable plants to continue to receive sunlight. You can of course also utilize aquatic herbicides, though using any chemical should always be your last resort as they have the potential to damage other organisms as well and persist in the environment.
My Son in Law, who is a big coarse fisherman has just completed thinning out mace, and Reed from one of my natural ponds, he wants to now add salt, which I am not altogether happy with, it is stocked with Rudd, Roach and Tench, all healthy, is it really necessary please?
Thank you in anticipation.
Hi Paul,
Thanks for reading and commenting!
We understand and echo your concern. We definitely don’t recommend using salt when fish or other aquatic creatures are involved. Fish, amphibians, etc. are incredibly sensitive to water quality changes, and salinity in particular can kill them if it’s too high. Roach and tench can handle brackish waters to a certain degree, but changing the salinity suddenly (as would be the case with a salt block) can harm or even kill them, too. Rudd are completely freshwater fish and won’t tolerate it well at all.
The best bet would be to just continue manually thinning out the mace and reeds on an as-needed basis (probably once every year or two). If they’re really a problem, removing the roots may be necessary, if that’s possible for you to do. If you or your son-in-law really want to curb their growth or prevent future growth, you can try using a systemic herbicide. Systemic herbicides work by gradually moving through the entire plant, including the roots, thus stopping future growth from occuring. Make sure it’s an aquatic herbicide marked safe for fish, and follow the label’s instructions for proper mixing and concentration. We recommend Rodeo, Aquapro, or Reward. Be aware that even chemicals labeled as fish-safe still have the propensity to harm them. Since mace and reeds are not entirely underwater, I’d highly recommend using a cut-stump method over just spraying the leaves. Cut-stump consists of cutting the reeds and mace to a few inches above water level, diluting the herbicide per the label’s instructions, and then (while wearing non-PVC gloves) using a sponge to apply it directly to the cut stumps. This greatly reduces the likelihood of any of the herbicide getting into the water and harming fish or other plants.
I do have one question: do you know what species of reeds they are? If they’re not invasive, there’s not much need to remove them unless they’re overcrowding the pond. Otherwise, native reeds actually help provide fish with habitat and food (via aquatic invertebrates), cool the water, and naturally filtrate the water.
I hope that this helps!
Hello, we bought a house with a 3/4 acre lake that wasn’t taken care of. It’s literally coated with mill weed and duck weed. We’ve used some of the best eco-friendly chemicals, the blue dye too, but it’s too pervasive. We have a pump in the center and run off drain… Yet the entire lake is bright green after 2 summers. Makes me sad. Can you help? Would a salt lick in small portions work? Would really appreciate some advice. We’ve stocked fish and the lake is surrounded by trees and lilies grow on one side… Still green and nasty.
Would water softener salt work for cat tail? Also most of my cattails are above the water line as we are experiencing a drought so would putting salt on the cat tail dry work? Thx.
Hi Michelle,
We would not recommend using water softener salt, as this has a different chemical makeup than standard salt. A salt block, like those used for deer, works best as it can be placed directly in the cattails with minimum impact to the rest of the pond’s inhabitants.
If you use a salt block do you break it up or just place it in water or sand due to drought?
Hi Michelle,
The salt block can be placed directly in the middle of the weeds that you’re trying to kill off. If you have multiple stands to kill, you can try breaking the block in half. Placing the block directly in the plants that you wish to kill is the best way to ensure that the salt is delivered right to those particular plants while minimizing impact to other plants in the pond. Placing it in the sand without water won’t do much, unfortunately.
Have cattails over taking a small retention pond (street run off) for rain. Pond is the home for a lot of ducks, frogs etc. Would salt(block) etc. be safe to use to control these cattails? What effect would the salt have on frogs, duck and diatoms?
We have cattails encroaching on our property from both sides. They are in a marsh area that is now dried up but the cattails are in wet mud. There are no fish in the water when the marsh fills up. What kind of salt block do you recommend. We simply do not want the cattails to continue to encroach and ruin our view. Now is the time that we can get to the cattails since the marsh is accessible due to drying up. Will the salt work without water present on the surface?