Is chlorine-treated water harmful to houseplants? In a word, no. Still, many people attest that chlorine-treated water is harmful to plants. This maybe due to the concentration of chlorine early in the morning, when water has been standing all night in the pipes. To be safe, allow the tap to run a few minutes prior to gathering water. Its good to know that chlorine disappears rapidly upon aeration. If you really want to be sure on the safety of water, let the tap water sit overnight in a pail before applying to plants. Nevertheless, fluorine does not go away.
There areplants that exhibit some reaction to fluorine in tap water. In some areas of the country, foliage shows tip burning when the soil or water carries as little as 1/10 ppm of fluorine. Because some municipal water supplies contain as much as ten times this amount (added to reduce tooth decay) the use of rainwater, well water, or water from a defrosted refrigerator is highly encouraged for these plants. Make sure your rainwater is not acidic.
If you suspect that water treated with fluoride is causing problems or if your soil mixture containsfluoride, there is no need to be alarmed since it's easy to correct. Just add some limestone or gypsum to counteract its effects. When using gypsum, add 1 teaspoon per 5-inch pot of soil. If you prefer using lime, put 2 teaspoons per 6-inch pot. Offsetting fluoride in tap water is also quite simple. Use a "Hydrion" paper dispenser to test the pH or acidity. When the water tests acid, just add a little lime to bring it up to around pH 6 or 6.5 (nearly neutral). At this level, fluorides are bound and plants won't absorb them. Doing a litmus paper test can also be used to find out the acidity of your water or potting soil.