The answer is a little less simple than yes or no. The answer depends on the quality of your tap water. As you may have heard plants enjoy a certain quality of water. Slightly acidic, more oxygen, and without contaminants. This perfectly describes rainwater, which is ideal for plants. Rainwater washes salts out of the soil that may inhibit plant growth, and also dissolve nutrients in the soil and make them more available to plants. This makes rainwater the gold standard for plant water.
However, not all of us have access to rainwater at all times of the year, if at all! So can we use tap water? Well if your tap water happens to be under a certain number of Parts Per Million (PPM), it might be alright for your plants! If you are wondering what PPM is: put simply it is the number of solid components in your water and is a measure of purity, the lower the number, the purer your water. One option to measure your water is to use a TDS meter. This is a tool that measures PPM. Another option for more of a general measurement is your municipality’s website! Often published are water testing reports that give you a general idea of how pure your water is.
In general, you should look up a specific plant’s tolerance of impurities in water. For carnivorous plants, Nepenthes are often tolerant of PPM less than 100, whereas Venus Flytraps are a little pickier with a tolerance of 50 PPM.
Here is the TDS meter we use:
TDS Meter Digital Water Tester – ppm Meter, EC & Temperature Test Pen 3-in-1
What do I do if my water doesn’t pass the test? Well, if rainwater is available, you can use that, otherwise, you may want to consider buying distilled water from your grocery store. If you are really serious about your water you can even invest in a reverse osmosis filter for your home!
Here in Burnaby, British Columbia we have measured our tap water to be at 14PPM and our rainwater to be at 4PPM these are both adequate ratings for most plants. However, there are a few important things that are more difficult to measure: i.e. level of oxygen in the water, acidity, and non-solid contaminants.
You may want to keep in mind that even if your tap water passes the test of total dissolved solids, it may still contain things other than water: chlorine and other chemicals or bacteria that cannot be easily detected. If you like you can sit out your tap water for 24 hours or more and the chlorine will evaporate off somewhat. All these things contribute to plant health and may make you think more about the quality of water in your area.
Sources:
https://en.wikipedia.org/wiki/Rainwater_harvesting
https://www.who.int/water_sanitation_health/gdwqrevision/rainwater.pdf
https://www.burnaby.ca/services-and-payments/water-and-sewers/drinking-water