Chlorine In Your Water: Does It Harm Plants?

Anyone on a public water supply in the United States is almost certainly drinking and watering with disinfected water. Utilities add chlorine, or chloramine, a compound of chlorine and ammonia, so that the water stays free of disease-causing bacteria and viruses all the way from the treatment plant to the tap. That is good news for the household. For a gardener who has just paid for worm castings or compost precisely because they are full of living things, it raises a fair question: does the hose undo the work?

How much is in the water

Not much. The Environmental Protection Agency allows a maximum of 4 milligrams per liter, or 4 parts per million, of chlorine or chloramine in drinking water, and most systems deliver well below that at the tap. The Centers for Disease Control and Prevention estimates that about one in five Americans now gets water treated with chloramine rather than chlorine. Utilities have been switching because chloramine lasts longer in long pipe networks and forms fewer unwanted byproducts.

Every community water system has to send customers an annual water quality report, usually by July 1, and it names the disinfectant in use. The utility’s customer service line can also say which one comes out of the tap.

The plants themselves

At drinking water levels, chlorine does not harm garden plants, and generations of lawns, vegetable beds and houseplants have been grown on municipal water. Chlorine is in fact an essential plant nutrient, taken up as chloride, though plants need only small amounts of it and get them easily from soil and water. Salt damage from chloride is a real problem in some places, but it comes from salty well water, road salt and water softeners, not from the trace of disinfectant in city water.

The soil

The microbes are the more interesting question, and Colorado State University Extension summarizes the research. Chlorinated water can kill some microbes in soil or in a compost pile, but they reproduce so quickly that populations bounce back within a short time. In one study, soil was irrigated continuously with heavily chlorinated water for 126 days; two days after the treatment stopped, microbial numbers were back to their earlier levels at every depth.

Soil also disarms chlorine. It binds to soil particles and loses its ability to kill, so little of it travels far. Water carrying 5 parts per million killed microbes only in the top half inch of soil, and it would take about 65 parts per million to kill microbes 6 inches down, far more than any tap delivers.

When it is worth removing

There are a few cases where it makes sense to take the disinfectant out.

Compost tea and worm tea

Brewing a tea is an attempt to multiply microbes in a bucket, with no soil to buffer them. Starting with water that still carries a disinfectant works against the whole idea.

Ponds and water features

Both chlorine and chloramine are toxic to fish. Topping up a garden pond with tap water calls for a dechlorinating treatment that removes chloramine as well as chlorine, the kind sold for aquariums.

Seed starting and cuttings

Most seeds and cuttings are fine with tap water, but some growers prefer rainwater for very tender material, and it costs nothing.

Removing chlorine and chloramine

Free chlorine is volatile. Water left to stand in an open container for a day or so, or stirred to add air, loses most of it. Chloramine is far more stable and survives standing for days. Ascorbic acid, or vitamin C, neutralizes both quickly, and some utilities use it to dechlorinate water flushed from their own mains. Some carbon filters remove chlorine, although chloramine needs a filter designed for it and a longer contact time.

Rainwater carries no disinfectant at all. A rain barrel under a downspout supplies water for teas, ponds and seed trays, and every gallon collected is one less drawn from the tap.