Is Chlorinated Water Safe To Put On A Garden

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, Chloramine and Plants - Everything You Need to Know

Chlorine, Chloramine and Plants – Everything You Need to Know

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, 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.

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Most municipalities in North America have values below 4 ppm.

Levels of Chloramine in Tap Water

The Center 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 plants. In another study, chlorine had no effect on the growth of radish and lettuce.

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One study found that chloramines caused root browning in hydroponically grown 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.

Chlorine Toxicity in Soil and Compost

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 once it does this, it is much less toxic to microbes. Soil and compost piles contain huge amounts of microbes. When the water initially hits, it will kill some microbes, but they quickly repopulate. Any effect that does occur is short lived.

"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."

This is just one example of how quickly microbes grow in soil and why adding things like biostimulants, humic acids and mycorrhizal fungi have no effect.

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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 for 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.

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 does'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 or chloramine from municipal tap water is a concern for plants, provided the World Health Organization Recommendation of less than 5 ppm, is followed.

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 over use 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.