One popular myth about houseplants is that you have let tap water rest for 24 hours before use to prevent damages caused by the chlorine it contains. But this is a waste of time for two reasons.
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 That technique was popular into the 1990s, but has largely been replaced by chloramine treatments (chloramine is a compound containing chlorine). Chloramine, unlike chlorine, does not evaporate when you let water sit out, at least, not to many great degree. That’s why municipalities use it: its antimicrobial effect is much more durable than that of volatile chlorine.
In fact, letting the water sit out will often 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 chloramine to evaporate, but still, some plants are sensitive to the chlorine compounds in tap water, including chloramine. This includes dracaenas (Dracaena), ti plants (Cordyline), spider plants (Chlorophytum comosum), prayer plants (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 that?
It’s important to note, though, 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, and in fact damage due to chlorinated water is a fairly minor factor. 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 use rainwater or distilled water. That would solve the chloramine problem, but rainwater can be hard to come by and is often inconvenient, while distilled water is expensive. Again, letting the water sit overnight is of no help whatsoever.
The Easy Way Out
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 anyway) 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 just apply the Laidback Gardener’s “15 Pace 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!