Tap Water Or Rain Water For Your Indoor Plants? What is Best?


Wondering if tap water is of any good for your herbs and house plants compared to the more “natural” rainwater alternative? You are not alone. I was asking myself the same for a long time until I understood that they are not the same, and here why.

Tap water should be preferred to rainwater in those areas with acidic rains. However, if tap water has high hardness and/or low pH should be avoided/reduced as plants can suffer from calcium build into the soil or/and pH-related issues. Soil replacement can solve the issues making both types of waters generally suitable for indoor plants.

Tap Water And Plants: 3 Aspects To Look At

Let’s clarify one point before starting.

There is no such thing as “tap water”. The tap water of a flat from one of my readers in London is going to be different from those of you living in Chicago. The same applies to two people living within the same countries and, even more surprisingly, within the same neighborhood. Despite the fact that all tap water (hopefully for most of you) is safe for human consumption, it does not mean that it is always the same.

Three are the features that might differentiate tap waters and how they impact your potted plants

1 – Mineral Content in Tap Water

Tap water, compared to rainwater, is enriched with minerals such as calcium and magnesium. These minerals define the “hardness” of the water. The higher the hardness, the higher the content of such minerals.

What Are the Effects On Plants and Herbs?

Calcium and magnesium are minerals that plants need for their development. However, their excessive presence can be a problem. This is even more important for potted plants. Indeed, flushing water enriched in calcium can cause the soil of potted plants to get calcium saturated. Indeed, calcium, different from other minerals, it does not get flushed away. It sticks into the soil.

As discussed by other authoritative resources (article from the University of Nottingham for details), an excess of calcium (although rare) might cause a large variety of symptoms from yellow leaves and wilt among the most common.

Why? An excess of calcium causes the plant to be deficient in other important nutrients. Indeed, the plant will attempt to absorb all the calcium available (way too much), becoming unable to absorb further different minerals (such as nitrogen, phosphorus, and potassium) that are extremely important for its healthy development.

I cannot provide you with a complete list of symptoms for one simple reason. Even very experienced gardeners might be difficult to tell. Indeed, depending on what is the mineral that is absorbed the least (either nitrogen, potassium, and phosphorus), the symptoms can vary greatly.

In tomatoes, for instance, calcium toxicity causes, among others, tiny yellowish spots on top of the tomato (more in this scientific document).

A word of caution

If your plant looks wilting and has yellow leaves, it might be calcium excess. However, be mindful. Most of the time, the problem is way simpler like overwatering, or lack of light! Check the article on leggy mint and basil black spots if your plants encounter similar problems.

How Do You Know if Your Tap Water is Too Rich in Minerals?

Well, the easy and best way is to perform a test. You need to measure the hardness. For simplicity, I suggest testing your tap water on your own.

What to use? A very good test is the Health Metric water test, here on Amazon. Their colored test allows you, for a few bucks, to check the pH (more on this later, very important by the way), hardness and other parameters regarding the quality of your water. Considering that we drink our tap water, it is good to know what we put into our bodies (and plans).

Tap water can be defined hard if its concentration of calcium is higher than 120g per liter of calcium (or 7 grains per gallon).

Other signs of too hard tap water?

Of course, the test discussed above is by far the most reliable way to understand if your water is too hard (and how much). However, hard water leaves some signs that might ring you a bell:

  • White crusty patches on clay pots: these might also appear as white circles on the soil. Such patches are given by water evaporating and leaving behind calcium that (like salt) will crystallize (like salt) and stick to the planter or/and soil;
  • Clothes are never soft: when you wash them with tap water, in case of high content of minerals, your clothes will feel hard even when using softener;
  • Glasses are always opaque: calcium is white. Hence, when washing your glasses with hard tap water, a layer of calcium will remain on the glasses making them looking opaque;
  • Hair feels sticky: your hair, similar to your clothes will always be sticky;
  • The white crust on kettle: in some countries (like the UK), kettles are widely used to boil water (tea someone). If you have one of these, they are the best candidate to check as the tap water, evaporating over time, will leave behind the calcium that will then build up a crust within the kettle.

2 – pH Level in Tap Water

Tap water pH is something that, again, it is not strictly regulated in many countries, the USA and the UK included. Indeed, as it can be read in this document from a large UK water company, the tap water pH should be between 6.5 and 9.5 (it can often be on the alkaline side). Take care of the “should be”. Moreover, the range is also quite wide.

What Are the Effects On Plants and Herbs?

Water with a pH far from neutral will affect, in the long term, plants and herbs, causing a pH imbalance rather than calcium excess. Indeed, as discussed in detail in the article below, the pH level in your soil affects the capacity of your potted herbs and plant to absorb the nutrients in it.

In short, your soil ph should be around 5 and 6.5, as clearly mentioned in this study. There are a few exceptions, of course (blueberries and a few others). Hence, adding alkaline water (everything above 7) will slowly and surely change the soil pH. The higher the water pH, the quicker the detrimental effect will be.

Here is the catch

Even in the unlikely situation, your water pH is far from what your plant desires; this is going to affect your plant only for a long time (months or year). Do not forget that, as discussed in this article, you need to change the soil every few years if you want your plant to thrive.

Hence, the impact of a different pH on your potted herbs (if it is not crazily high, above the normal) will not significantly affect your plant.

How Do You Know if Your Tap Water pH is Adequate?

The Health Metric water test, here you can have a look at Amazon, also provides your tap water pH. Everything up to 6.5-7 does not represent any problem for your plants.

How Do You Know if Your Soil pH is Adequate?

Using tap water for a while might have already affected your soil pH. In this case, you might need to give it a go through a test. Nowadays, it is not a problem. Indeed, you can perform professional level tests for just a few dollars in the comfort of your home/garden.

How? One of my favorites is the Rapitest (here on Amazon or here on Walmart, where sometimes it is cheaper). Check this article for more info on how this tester works!

3 – Chlorine in Tap Water

Chlorine is a chemical that, thanks to its antibacterial property, makes the water safer to drink. Its concentration is around 0.2 to 1mg per liter, as indicated by the WHO guidelines.

What Are the Effects On Plants and Herbs?

When watering your plants with tap water, the chlorine is slightly reducing the population of harmful bacteria. This means that your tap water is likely to have a marginally positive effect on plants by limiting spoil-promoting bacterias. However, due to the low concentration of chlorine in tap water and its high volatility, these effects are very limited.

As discussed by the Spanish National Research Council, the presence of chlorine in water shown to reduce 2-3 folds the population of two harmful bacterias while leaving unaffected the others.

Moreover, as discussed in this scientific report, chlorine leaches very easily from the soil in case of neutral and alkaline ph (like your tap water), so there is no risk of accumulation here.

A final warming

Some plants like apple, mango, peach and grape that are sensitive to chlorine, However, this should not be a problem for you as you are not probably growing them in pots indoors!

What Can You Do To Reduce the Effect of Chlorine?

Some people in forums asked how they can reduce the chlorine concentration in tap water. As far as I know, this is not necessary. However, if you want to be 1000% on the safe side, then go for it, it is quite easy.

To remove chlorine it is sufficient to leave the tap water sitting in an open container for 8 hours. This is because the chlorine evaporates quickly. Of course, after 8 hours, depending on the amount of water, you will still have chlorine left. However, no worries, this is not going to be a problem. If you want to take a step further, just leave it for a day, and you will be almost chlorine-free.

Mineral Content and pH in Rainwater

As I mentioned earlier, rainwater is not enriched and finely controlled by us. On the opposite, the mineral content in rainwater depends on the zone you live (close to the beach, to an industrial area, city center) and time of the year.

Minerals – although it might vary, research found nitrogen and calcium among them. Of course, you need to remember that rainwater is not fertilizer. It has some nitrogen a very low amount. However, all the nitrogen can be absorbed by the plant straight away. This promotes better growth. Rainwater is defined as “soft water” (as opposed to hard water) due to the lower concentration of calcium.

pH – as many of you might have heard, it is indeed related to the level of pollutants in the air (here one of the countless studies proving this fact). Normal clean rain, as stated by the United States Environmental Protection Agency (EPA), has a pH of around 5 down to 4 in the case of acid one.

The take away for you?

In general, rainwater is more acidic than tap water (remember, tap water on the high pH side, so alkaline).

How does acidic rainwater affect plant growth?

A low ph level in rainwater, as discussed by EPA and in one of my previous articles below, affects your plants (negatively) in two ways:

  1. Makes aluminum available in larger quantities to the plant (normally is locked into the soil). This will slowly poison stunt and affect their growth by reducing their ability to develop roots as detailed discussed in this study.
  2. Wash away nutrients from the soil causing a large variety of problems where brown tips and yellow leaves are the most common, as mentioned here

What do you need to know?

If the rainwater in your area is heavily acidic (below 5), then do not use it in your potted plants and herbs. You are going to do more harm than good (even if you have some extra nitrogen).

Then how do you know if the rainwater is ok for your plants?

If you live anywhere close to the east coast of the USA, such as Portland and Boston, it is very likely that the pH level is quite low (around 4). In this case, I would avoid using rainwater for plants. However, from the center to the west coast, the rainwater was found to be very close to neutral, so no problem there.

What about your city? Check out the map here. Despite being the best is a bit outdated, but pH level patterns do not change quickly, so the data is still reliable.

Rainwater and Tap Water Test?

Not many are the studies in which the effect of rainwater and tap water are compared. Remember what we said before. There are so many variables involved in the characteristics of both that it is almost impossible to have a final answer.

However, I dig quite a bit, and I found a quite interesting study from Michigan State University. They took a look at the effects of tap and rainwater on the growth of radishes.

What they found will surprise you!

Plants that received rainwater were 10% higher (9 inches, 23 cm) compared to their tap water counterpart (8.3 inches 21.36 cm) but with overall lower weight. Of course, this experiment was performed with radish only.

They hypothesize that because they tested the acidity of the rainwater at a pH of 4.5, it didn’t promote the same overall growth as a more neutral comparison that sits closer to a pH of 7.0.

Conclusion: What Should You Use?

Here two simple questions for you:

  1. Is your tap water too hard, or it has a high pH level? You can have a look again using the Health Metric water test, here. If your water is too hard (more than 120g per liter of calcium), then you might want to use rainwater whenever possible. Otherwise, you might risk a calcium build up in the pot.
  2. Is your rainwater acidic? Have a look at this map. If your area is orange/red, then very likely your rainwater is not suitable. To be sure (as there might be differences locally), check with the Health Metric water test here. If you live in the west area, it is very likely your rainwater is ok.

Remember, if you transplant your plants every so often (1-2 years), there should not be problems in using either of the two as the (potential problems) will not have time to build up.

Here my personal take away

From my experience in gardening both indoor (mainly herbs) and outdoor (from squash to zucchini and roses) I used mainly rainwater in winter/autumns (it rains a lot here) and tap water and never had any problem!

Further Questions

Are there nutrients in rainwater? Rainwater does contain minerals (often called nutrients) that plants can be used. Often nitrogen and calcium are the most abondant

How long rainwater can be stored in a tank? Although there is no time limit it is recommended no more than a week. Indeed, in case of no protection from sunlight and the absence of any antibacterial chemical, algae might develop as well as attracting mosquitoes.

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Andrea

A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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