You have probably heard that chlorine in tap water is harmful to plants. Some cities have switched to using chloramine in tap water and that is also a concern for plants because it is much harder to remove before watering indoor plants.
What is the difference between chlorine and chloramine? Will they harm your plants? How can you remove them from tap water? These are all good questions gardeners should be able to answer and in this post, I will help you do just that.
Chlorine in Tap Water
Chlorine is added to municipal tap water to kill microbes and make the water safe to drink, but chlorine can also be toxic to plants. As with all toxicity, the dose makes the poison. At low levels chlorine will not be toxic, in fact, it is a required nutrient of plants. At high levels it becomes toxic.
The key question is, are the levels in tap water toxic to plants?
Chloramine in Tap Water
Chlorine is effective at killing microbes but it does not work for very long. Some municipalities have now switched to chloramine instead. Chloramine treatment adds both chlorine and ammonia to the water, which forms chloramine. Treatment plants that use chloramine, also flush their systems with chlorine from time to time.
About 25% of municipalities in North America now use chloramine. Here is a list for Canada.
The key question here is the same, are the levels in tap water toxic to plants?
Toxic Level of Chlorine for Plants
A lot of greenhouse horticulture uses municipal water and chlorine toxicity is a concern for them. They have determined that levels under 150 ppm are not a concern for toxicity, in potted ornamental plants. Hydroponics might be different.
Levels of Chlorine in Tap Water
The World Health Organization suggests using no more than 5 ppm (mg/L) of chlorine in drinking water. “Most people will smell or taste the chlorine at higher concentrations.” The Center for Disease Control suggests a limit of 4 ppm.
Most municipalities in North America have values below 4 ppm. The values in my home Town of Guelph, for 2023, were in the range of 0.40-1.46 ppm.
Levels of Chloramine in Tap Water
The Centers for Disease Control (CDC) suggests a limit of 4 ppm for chloramine. Most municipalities in North America have values below this level.
Chlorination Toxicity in Plants from Tap Water
Given the above values, it becomes clear that neither chlorine nor chloramine in drinking water will be toxic to indoor potted plants. The CDC agrees with this statement for both chlorine and chloramine. The University of Nebraska found no harm from chloramines.
When eight different bedding plants and nine shrub species were sprayed repeatedly with 100 ppm chlorine dioxide, there was no significant damage to the plants. In another study, chlorine had no effect on the growth of radish and lettuce. A study of houseplants and seedlings found that the growth of geranium and begonia declined at 2 ppm, but all other potted plants and seedlings were unaffected at that level.
One study found that chloramines caused root browning in hydroponically grown lettuce plants. A follow-up study found that even 1 hour of exposure to chloramine at 0.5 ppm affected lettuce plants.
Some specialty plant groups, like African violets, report problems with chloramine, but the data is anecdotal.
Pool water has higher levels of chlorine and can harm plants.
Does Chlorine Kill Soil Microbes?
Since chlorine is used to kill microbes, will it also harm the microbes living in soil or compost piles?
Chlorine tends to bind to clay particles and organic matter which makes it much less toxic to microbes. Soil and compost piles contain huge amounts of microbes and even though some are killed they quickly repopulate. Any effect that does occur is short-lived.
“In one study, researchers continuously applied highly chlorinated water to the soil for 126 days. Two days after they stopped, the soil microorganism populations reached pre-treatment levels at all depths of soil.” This is just one example of how quickly microbes grow in soil and why adding things like biostimulants, humic acids, and mycorrhizal fungi has no effect.
Another study measured the chlorine concentration that is needed to kill all the microbes in the top 6″ of soil. A concentration of 65 ppm would be needed, a value that is more than 10 times higher than any drinking water.
Chlorine in tap water does kill a few microbes, but it has little effect on the total population, and numbers quickly rebound.
Chlorine and Aquatic Plants
Chlorine and chloramine are of much more concern in ponds and aquariums since aquatic organisms have very low toxicity levels for chlorine. It is not clear to me, how this chlorine affects aquatic plants that are mostly submersed. Since all of their leaves are immersed in the water you can expect them to pick up more chlorine than plants with their roots in water or in soil but I found no evidence chlorinated tap water is an issue.
Fish and other aquatic life have very low toxic levels of chlorine. Topping up a pond with water should not be a problem. It is probably best to keep fish out of a newly filled pond for a week or so.
Chlorine and Seeds
Testing of chlorinated tap water on cowpeas showed no detrimental effects on germination or seedling growth.
Seeds are routinely treated with bleach as a way to kill pathogens. Cornell University recommends the following treatment, “To Clorox treat seed, prepare a solution with 10,000 ppm available chlorine.” Clearly, the 4 ppm in tap water will hat harm seeds.
Removing Chlorine and Chloramine from Water
Both chlorine and chloramine can be removed from water by boiling. Removing half of the chloramine (half-life) takes 30 minutes, while doing the same for chlorine takes 2 minutes.
Chlorine will also offgas from water just by letting it sit, but chloramine doesn’t in any reasonable period of time. The latter can be filtered out with a special charcoal filter.
The Bottom Line
Contrary to popular concerns, neither chlorine nor chloramine from municipal tap water is a concern for plants, provided the World Health Organization Recommendation of less than 5 ppm, is followed. A few types of plants may be harmed if the level is above 2 ppm.
Chloramine may be harmful at lower levels in hydroponics. Levels as low as 0.5 ppm have been reported.
The many reports on social media of plants being harmed by chlorination are based on anecdotal information and could be the result of all kinds of issues. The symptoms of chlorine poisoning, browning of leaf edges, can be caused by numerous things including improper watering and overuse of fertilizers. This is a good example of why anecdotal information is of little use in trying to understand the science.
Chlorinated tap water is safe for both indoor plants and gardens.
Can you link the study this is in reference too?
“In one study, researchers continuously applied highly chlorinated water to soil for 126 days. Two days after they stopped, the soil microorganism populations reached pre-treatment levels at all depths of soil.”
The link just takes me to another website with no source?
Or is it anecdotal?
Do you know why my garden is greener and plants grow faster after rain as opposed to being watered then? I assumed it was because the rain water was free from chlorine and chloramine. But it sounds like that is not the reason. Thanks
No, but it is not due to chlorine. I doubt they grow faster. Who knows why we think they are greener?
If you measured the color you might find they are not greener.
Depending on where you are located there could be a high level of sodium, chloride, anfd other salts in your water source. These can slow growth, reduce uptake, and create discoloration (yellowing or browning). Rain water is low or nearly zero conentration of these salts which is why you see improved growth and health
A late possible explaination to your question.
It could be more related to pH level of your water.
I’ve had the same issue as you describe.
Please, have a look at this article it will describe the pH issue in reasonable depth.
If anecdotal evidence is not acceptable—- then please provide more than 1 link (that takes you to a non academic web page referencing the same same non present “one study”) regarding the effect of chlorine on microorganisms in the soil. I have a hard time believing that substances used to kill microorganisms on drinking water have “little to no effect” on the organisms in the soil because “most of it binds to clay particles”. Where is your proof for such claims? The burden of evidence is on your shoulders, not those advocating for the removal of chlorine.
You are free not to believe what science knows. We are not talking about new science here.
Can you eat vegetables from plants whose leaves show signs of chlorine burn from pressure house with chlorine?
Yes.
I use potassium chloride as a fertilizer supplement, mostly due to low cost as it is sold as a water softener, and find that the advantage of larger fruits greatly outweighs any possible damage to the greenery.
It’s nothing new that plants need Cl (it’s considered essential micro) but I’ve been searching for like 2 hours and still couldn’t been able to find how much ppm of Cl they need in HYDROPONICS. Literally zero mention of ADDING Cl in all the nutrient setups I’ve seen in hydro which of course mostly uses RO WATER that has no Cl in it. Isn’t that strange? What is even more F’d up is that HYDRO specific nutes don’t have Cl!
Anyway, RO systems remove 99%+ of Cl. So… I suppose that 1% is enough for plants.
We have to shock or well with chlorine to remove bacteria. After 24 hours, we have to run the chlorine out of the system onto the soil in a fallow field of swamp grass. What damage will be done to the soil? Also could we run the water onto a blackberry patch to kill the blackberries?
Depends on how chlorine you added and the volume of water it is mixed in.
Many botanists agree that using tap water is not an issue for typical house plants, but it’s also easy to remove the Chlorine and Chloramines. And why not remove them, especially when growing with a “super soil” that has a lot of added microbes? Better to be safe than sorry if you ask me.
Chlorine and Chloramines can easily and instantly be removed/neutralized by using pure Ascorbic acid (vitamin C). It’s pretty cheap too. 2lbs on amazon for about $20 US. Enough to treat roughly 25,000 gallons.
In short, about 50 milligrams (0.05 grams) of ascorbic acid or 57 milligrams of sodium ascorbate is enough to treat one gallon of water with a chloramine concentration of 3.8 PPM (check your local water municipality website for your areas concentration levels). Use the calculator found below to make adjustments.
*note, using ascorbic acid will lower the ph of the water and rebalancing pH levels can be done using pH Up and pH Down solutions.
Dechlorination calculator:
https://www.reddit.com/r/aquaponics/comments/1qpumz/here_is_a_water_dechlorimination_calculator/?utm_source=share&utm_medium=ios_app&utm_name=iossmf
High times Article
https://www.dudegrows.com/remove-chloramines-tap-water/
Scientific studies addressing Chlorine and Chloramines
https://www.epa.gov/dwreginfo/chloramines-drinking-water
Why do more work to fix something that is not a problem?
Excelent research, thank you!
Don’t forget to mention fluoride, though. Since the USA puts it (unlike Europe, afaik) into tap water and it’s supposed to be bad for plants, it really is important to know if it’s a problem. If both aren’t (chlorine and fluoride) then great. If fluoride in tap water is a problem then… not so great.
Also, chlorine in tap water when added to a pond will probably kill insect larvae (e.g. dragonflies) and such. If you have data that shows otherwise, though, that would be good to see. I don’t think it’s wise to add untreated tap water to ponds that are designed to be for wildlife.
The one element for plants no one could explain, finally.
I live in the Adelaide Hills of South Australia where chloramine is used in the tap water. In the summer, it can be used at a quite high rate.
If tap water it put into ponds, without removing the chloramine, the fish, tadpoles and most (not all) types of aquatic animals die. The 𝐩𝐥𝐚𝐧𝐭𝐬 normally show no obvious effect from the chloramine.