First, there is not enough chlorine in tap water to affect most plants. Water from the tap, preferably tepid water, suits them perfectly. That’s good, too, because with some 300 indoor plants to water, I would have to leave at least 75 watering cans out the day before watering! It is also worth noting that chlorine is one of the minor elements that plants require for good growth. So they actually need a small amount of chlorine.
Secondly, letting tap water sit overnight will not reduce the quantity of chlorine it contains. That’s because few municipalities still use volatile chlorine to treat their water. That technique was popular in the 1950s and 1960s, but has since been replaced by chloramine (a compound containing chlorine). Chloramine, unlike chlorine, does not evaporate when you let water sit out. In fact, letting the water sit out will concentrate the level of chloramine in the water… although only very slightly.
Problem solved, right? Not so fast!
OK, letting water sit overnight doesn’t allow chlorine to evaporate, but still, some plants are sensitive to the chlorine compounds in tap water, including chloramine. This includes dracaenas (Dracaena), cordylines (Cordyline), the spider plant (Chlorophytum comosum), marantas (Maranta), calatheas (Calathea) and carnivorous plants. If these plants are watered with heavily chlorinated water, it can cause the tips of the leaves to die and turn black. What can you go about them? Note too that chlorinated water is only one of the causes of black leaf tips: dry air, drought stress, and excess fertilizer also “burn” the leaves of these plants. So you can’t just blame the tap water…
In an ideal world, you wouldn’t water these plants with tap water at all, but instead with rainwater or distilled water. That would solve the problem, but the first can be hard to come by and is often inconvenient, the second is expensive. Again, letting the water sit overnight is of no use whatsoever.
Here’s what I do: I water plants subject to black tips (except carnivorous plants, which aren’t “regular houseplants” and need special care in so many ways that you really have to treat them separately) with regular tap water along with all my other houseplants and have learned to just put up with the occasional blackened leaf tip. I’ve also learned that if I keep the air fairly humid, leach these plants occasionally to remove excess mineral salts in their potting mix, and don’t stress them with underwatering, there is very little leaf tip damage anyway. Only the very tippy tip of some leaves dies back a bit.
Essentially, I apply here the Laidback Gardenersn”15 Steps Rule”. Back up 15 steps and if you can’t see the problem, it’s not one worth getting upset about. That’s certainly the case here.