Is it harmful to water plants with chlorinated water?

If you are not an avid reader of this blog and want the straight answer, you have it at the end of the article. If you want to stick around and learn more about what chlorine means to a plant, we recommend going line by line.

Each plant, whether indoor, garden, potted or otherwise, has had a natural habitat in which it grows more or better depending on its agronomic characteristics.

Rainwater, in principle, falls pure and crystalline and reacts with various compounds in the atmosphere. Thus, for example, acid rain is formed with the various pollutants (sulphur, among others) resulting from human and non-human activity. And it also contains chlorine. In fact, one of the sources of chlorine in soils is rainwater, in coastal areas mainly seawater, dust and human activity.

Chlorine as a nutrient (chlorides)

Be careful. NOTOr is the chlorine used to make water safe to drink is the same as chlorine as a nutrient in the form of chlorides. They have nothing to do with it. Chlorine as an atom in the periodic table is not found in a pure form (dichloro) because it reacts quickly with almost everything. It is found in the form of chlorides, chlorites, chlorates, dissolved…

The name chlorine comes from the Greek khloros which means greenish, yellowish because chlorine in pure (gaseous) form is greenish yellow in color. By turning the wire a little more, we can then explain why chlorophyll (this pigment which gives the green color to plants) is so called. Chlorosis in agriculture should also not be confused with a lack of chlorine. It refers to the yellowish color that the leaves acquire due to the lack of other minerals such as iron (ferric chlorosis).

Chlorine is considered a micronutrient in plant fertilization. In plants, it occurs as Cl- or chloride. It is an osmotic solute of great importance in plant cell vacuoles that helps maintain plant tissue turgor and osmotic regulation. It also influences certain enzymatic reactions. It regulates the potential of the cell membrane, it also acts as a pH regulator inside the cell.

Chlorine enters the plant through the root, which will come mainly from the soil and moved by water. That is, the entry of chlorine into the plant would normally occur under humid conditions. Water mobilizes chlorides and these pass to the plant from the root.

It is generally not considered in fertilization plans because the vast majority of irrigation waters have sufficient concentrations of chlorides to supply the plant with sufficient amounts of this micronutrient. It is also present in the soil and there is usually no deficiency of it.

Being a nutrient, can it be toxic?

Yes, any chemical element in excess can become more or less harmful in any organism. In fact, in the fertilization plans of years ago it was not even considered because it was considered toxic for many crops. And today it is a parameter that is measured in any soil or water.

And now we enter the vast world of plant biodiversity. There are species more or less tolerant to “high” levels of chlorides in soil or irrigation water. There are more tolerant plants and more sensitive plants. Normally, plants that are more resistant to salinity (coastal for example) tend to have a higher tolerance to chlorides. In fact, there are specific classifications based on a plant’s tolerance to the presence of chlorides in the soil:

  • 1. Extreme Halophyte Plants. Those whose growth is stimulated with concentrations of 100-200 mM [Cl-]ext.
  • of them. Halophile tolerant plants. Their growth is not stimulated by the presence of salts, their growth is slightly reduced but with the possibility of development at high salt concentrations.
  • 3. Low tolerance or sensitive plants that their growth is significantly reduced in said concentration ranges. This group is very large.
  • Four. extremely sensitive plants to salinity. Those who suffer with minimal concentrations of chlorides.
Relative growth of 4 groups of plants as a function of chloride concentration.

In group 1, we can place coastal plants that tolerate saltpetre well, and therefore the presence of chlorides in high concentrations. In group 2 we would have plants such as sugar beets. It is quite tolerant to salinity. In group 3 we can include some species of grasses of the genus Festuca, cotton, tomato as not very tolerant and soybeans or beans as not very sensitive. In group 4 we have citrus, other fruit trees, tobacco or hops.

So that we can get an idea. 1 milliequivalent/L of chloride equals one mM/L, which means an amount of approximately 3.55 mg/L. That is to say, slightly tolerant plants can tolerate up to 177 mg/L of chlorides in the medium. And the most sensitive could even withstand a few tens of mg/L.

The other chlorine. That of making the water drinkable.

City water purification treatments in any city in the modern world are designed to bring totally healthy water free of contaminants and microorganisms. This is achieved with various methods which, depending on the administration, country, etc. they will be each other. They are used The UV radiation as a disinfectant, ozone and chlorine mainly, the latter being the most common, mainly due to price and efficiency. There are many chlorine compounds, but the most common is in the form of sodium hypochlorite (NaClO). It is also chlorine that is used in swimming pools to prevent the formation of algae and the proliferation of micro-organisms since the water in the swimming pool is stagnant water.

How much chlorine does the water supply network have?

This is already well researched and regulated. The amount of residual free chlorine in all drinking water should be less than 1 ppm, less than 1mg/l of water. One thing needs to be clarified. These values ​​are what is supposed to stay as residual chlorine after reaction. Chlorine reacts with micro-organisms (germicidal) with organic matter, with other inorganic compounds (forming salts) or by hydrolysis with the water itself.

Consequently, the quantities of chlorine discharged upstream (into the treatment plant) are greater. We always talk about residual chlorine, the one that remains after all these reactions along the route of the network to our taps, which, after all, is what we or our houseplants are going to drink after all. And how is it calculated?

It is quite complex because the concentration of this chlorine varies with pH water, the Temperaturethe quantity of limefrom organic matter, salts…it’s not something static, although sewage treatment plants usually control it fairly computerized. The supply networks have measurement and rechlorination points in case the limits are lower than what is considered healthy.

And I say generally because in small rural towns with a spring water tank (the town tank) that they treat with chlorine, there are times when for some reason the water is turbid with chlorine, whitish. And I say that knowingly. I have seen pylons in towns with all the flora (algae) totally burnt off by excess chlorine upstream in the affected reservoir.

Obviously, the concentrations are quite low if dosed with fully computerized controllers and pose no health risk. Yes, it is true that at some point a certain smell of chlorine may emanate from the fresh tap water, but this is usually not a problem.

Is this residual chlorine harmful to plants?

Normally no. This amount is so low that it usually does not pose a problem in the growth of our indoor or garden plants. There may be plants that are sensitive to it, such as certain tropical plants such as calateao or dracaena.

We found only one paper that specified some phytotoxicity at the cellular level in the root caused by sodium hypochlorite in onion cultivation in which they concluded that with amounts of 1ppm (1mg/l ) nothing harmful is caused. With higher concentrations, there seem to be effects. We leave you the reference at the end of the post in the bibliography.

Either way, if you find that a houseplant in your home is sick or something is wrong with it, think of chlorinated water as the least of your problems. There may be a thousand other more common reasons than possible phytotoxicity due to residual chlorine in drinking water. Reasons such as excessive irrigation, lack or excess of direct light, change of substrate, root rot due to waterlogging of the substrate, lack of fertilizer…

In any case, chlorine can always be eliminated by letting the water sit for a few hours in an open container. The chlorine will eventually leave and this tiny part of residual chlorine will eventually disappear.

Bibliography

White, PJ and Broadley, MR (2001). Chloride in soils and its uptake and movement in the plant: a review. Annals of Botany, 88(6), 967-988.

Causil, LA, Coronado, JL, Verbel, LF, Vega, MF, Donado, KA and Pacheco, C. (2017). Cytotoxic effect of sodium hypochlorite (NaClO) in the apical cells of onion roots (Allium cepa L.). Colombian Journal of Horticultural Sciences, Eleven(1), 97-104.

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