Killing Lake & Pond Weeds With Salt – Is It Safe?

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An Overview of Salt For Lake & Pond Weed Control (Dosing & Safety)

Salt can be used to kill invasive weeds, but works best against rhizomatous types; such as cattails and lilies.

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?

Salt works by disrupting osmosis, effectively preventing the transfer of water through the plant.

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 is best suited to weeds with extensive rhizomes, as you can place salt blocks directly on the roots.

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.

Purified aquarium salt, sea salt, or rock salt blocks are the most common salts for killing weeds.

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.

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