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Irrigating with hard water

Q: I’ve heard that the hardness or softness of water can have an effect on your plants. Is that true?

Heather Raftery, St. Paul, MN

A: George Elliot, an associate professor of horticulture specializing in plant nutrition and water quality at the University of Connecticut, replies: Water hardness and softness can have a significant effect on plants, especially those growing in containers. Hardness is related to the content of calcium carbonate and magnesium carbonate dissolved in the water, often expressed in units of grains per gallon. Hard water has from 7.0 to 10.5 grains, and very hard water has in excess of 10.5 grains. You may recognize that calcium carbonate and magnesium carbonate are the components of limestone. When hard water is used for irrigation, it’s the same as adding a small amount of lime every time you water. Over time, this continual addition of lime will increase the pH of the growing medium.

An increase in pH is more pronounced in containers because they are watered frequently. Soils, however, are not as affected because rainfall helps to counteract it. Nevertheless, hard water can still cause some problems when irrigating outdoors. In overhead sprinklers, it can cause white, scalelike deposits to develop on leaves as minerals are left behind when the water evaporates. Similar deposits can clog drip-irrigation systems.

Many plants can tolerate a wide range of pH, although some plants—most notably petunias and acid-loving plants like azaleas—are sensitive to high pH levels. An increase in pH causes young leaves to turn pale and yellowish, while leaf veins remain green. This interveinal chlorosis is sometimes referred to as a lime-induced chlorosis and is actually the result of an iron deficiency due to high pH levels. It won’t kill the plant, but it will make it look unhealthy and reduce its vigor.

Hard water can be found in large areas of the United States (map, above). The only way to tell if your water is hard is to test it. Easy-to-use kits for do-it-yourself testing are available at most hardware stores.

If you have hard water, you can use a water softener to treat it for household use, but you should not use softened water for irrigation. Softened water has a high salt (sodium chloride) content, which causes its own set of problems.

Regrettably, there is no practical way to counteract hard water. The best solution is to refrain from using overhead irrigation, regularly check and maintain drip-irrigation systems, and avoid plants that are susceptible to lime-induced chlorosis.