Softened Water Can Be Hard On Indoor Plants

If your indoor plants or plants brought in for the winter seem to be faring poorly, take a hard look at softened water as the culprit.

"There isn't any research that tells us which plants can be injured, but plants do vary widely in their tolerance for softened water," says J. Robert Nuss, professor of ornamental horticulture in the College of Agricultural Sciences at Penn State. "In many cases, water from a mechanical softener will harm plant growth."

Hard water contains large amounts of calcium and magnesium, which are plant nutrients. However, these minerals reduce how well soap lathers when washing clothing or other items. "The minerals in hard water often combine with soap to form the ring you see in bathtubs or wash bowls," Nuss says.

Many homeowners soften their water by using a filtration system to exchange the calcium and magnesium in the water for sodium, creating a softer water that allows soap to function better.

"Sodium is used by plants only in very small amounts," Nuss says. "Over long periods of time, too much sodium becomes toxic..

Nuss says excess sodium will damage the soil quality around the plant's root system by breaking down the soil structure, thus reducing drainage.

 Savvy plant lovers can save their plants in several ways.

  • Leaching. Using rainwater or unsoftened water to water your plants will leach high levels of sodium out of the soil.
  • Elevation. By placing your plant on gravel or a similar material, a reservoir is formed to collect water passing through the soil. "Never allow plants to stand in softened water," Nuss warns.
  • Repotting. Fresh potting soil will provide a new environment for the roots, free of excess salts.

Other water additives used by municipal water systems, chlorine and fluorine, also can harm some plants. Nuss says they are trace elements that are needed in very small amounts to keep plants healthy. "In excess, like sodium, they can be toxic," he says. Chlorine generally presents less of a headache for houseplants, Nuss explains.

"Chlorine can be eliminated from water by heating or aeration, or by letting the water stand in a container overnight before watering the plants."

Fluorine can adversely affect plants at concentrations as low as .1 part per million. In some municipal water supplies, 10 times that amount is added to prevent tooth decay. "A good indicator of fluorine content is the spider plant, or Chlorophytum, which will show spotted leaves or burned tips when flourine is present," Nuss says.

Houseplant gardeners can deter the effects of fluorine by adding two teaspoons of limestone per six-inch pot of soil. Lime raises the pH of the soil and combines with fluoride in a chemically insoluble form that plants cannot take up through the roots, according to Nuss.

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