Water is good for plants, right?

Tip burn on a tropical foliage plant is a sure sign of salt damage.

Photo Credit: David L. Morgan

Low salt levels should keep your houseplants blooming happily.

Photo Credit: Gerald L. Klingaman

This is actually a trick question, because all living things need water to survive and grow, but too much or too little can be harmful. And here’s the trickiest part: Even optimal amounts of water can be damaging – that is, if the water contains high concentrations of salt, because too much salt can kill or injure many plant species.

Salt appears in many forms. We’re all familiar with sodium chloride, or common table salt. And most of us know that salty water produces harmful effects in all but the most salt-tolerant plants. But there are many kinds of salts, and it may surprise you to learn that quite a few of the types we use are beneficial in certain concentrations but harmful in higher amounts.

What am I talking about? One word: fertilizers.

Fertilizers are compounds that contain plant nutrients. They dissolve in water into what we call ionic form, supplying nitrogen to the plant (ammonium ions) and carrying sulfur to it (sulfate ions). Together, these nutrients help the plant grow – when they’re applied in the correct amounts.

Of course, this information begs the question: At what concentration do these salts become toxic? Well, it depends on the ion. If fluorides are present in just a few parts per million (ppm), they can “burn” sensitive houseplants, like zebra plant (Aphelandra). When carbonates and bicarbonates are found in concentrations as high as 100-200 ppm – as they are in some city water supplies – they can severely affect many species.

So how do you determine if you’ve got salty tap water? The best way is to get a chemical analysis from your city water department. Many times the report will give you a very useful summary in “total dissolved solids,” or total salts in ppm (sometimes indicated as milligrams per liter, which is the same). As a general rule, if your water has 1,400 ppm, then the salts are too high to be used on many houseplants.

And if your salt levels are too high, you’ll definitely notice some salt damage symptoms on your plants, including:

• Yellowing leaves (and your plant won’t appear vigorous)
• Browning and dying leaf margins
• Stunted growth
• Reduced number of flowers (and they’re smaller)
• Dying plants

What can you do to prevent salt damage in your houseplants? If you’ve got salty water, the best thing you can do is stop using your tap water on your plants and start using bottled water – or at least use a mixture of bottled and tap water. (A friend of mine collects effluent from his air conditioner for his African violets.)

You can also use less fertilizer (remember, fertilizers are salts). When you do fertilize your houseplants, reduce the level of concentration you use. Also, consider repotting your plants in a soil mix with a lot of air space and a low water-holding capacity. Styrofoam pellets and perlite are often used to “loosen” growing media. Stay away from mixes with high concentrations of peat moss and fine bark.

When you water your houseplants, irrigate them thoroughly. Allow the water to wash through the medium completely and drain out the bottom to flush out offending salts that have accumulated. And water often. Keep your plants evenly watered, and don’t allow them to dry out (but don’t place them in standing water, either).

Finally, reduce transpiration – the loss of water through the leaves – by placing your plants in shady locations away from wind. That means putting your plants in sheltered locations if they’re “summering” outdoors, and keeping them away from open windows when they’re back inside.

And if your houseplants do succumb to all the salts despite your valiant efforts to resuscitate them, make sure you replace your plants with selections that aren’t as sensitive to salt. (Your local garden center can help you make the right picks.)

Most of all, don’t let salts ruin your beautiful houseplants! If you water them wisely, your plants will thrive for years!