Is Rain Water Good for Houseplants? Or Is Tap Water Better?

Can rainwater be used for houseplants? Is this a good alternative to the more expensive tap water? After all, plants live and thrive in the wild with just simple rainwater. So can they survive with just that?

In general, rainwater is good for watering houseplants as it causes no harm to them. It is equally beneficial as tap water, except in areas with acid rain. The only case where rainwater is actually better than tap water is in areas where the water supply has high hardness and low pH.

What should you look for in water for your plants? Are there certain minerals you want to have and chemicals you want to avoid? Let’s find out the answers to these questions as we move forward!

Is Rainwater Good for Houseplants?

Rainwater is generally good for houseplants and it will not harm them. Rainwater also contains a small amount of nitrogen that might promote growth. However, this effect could be counterbalanced in case of acidic rain that might wash away the nutrients in the soils of houseplants over time.

For the most part, rainwater is not enriched. We can’t readily control its content and quality either. More specifically, the mineral content in rainwater depends on the zone you live in—close to the beach, an industrial area, or the city center—and the time of the year.

Although the mineral content of rainwater can vary, research has found that nitrogen and calcium are among them. Nitrogen promotes better growth. Also, rainwater is defined as “soft water” (as opposed to hard water) due to the lower concentration of calcium.

Of course, you need to remember that rainwater is not a substitute for fertilizer. It has some nitrogen but the amount is close to nothing to what a plant needs. Nevertheless, the nitrogen in rainwater can be easily absorbed by the plant straight away.

Besides that, the pH value of rainwater is closely related to the level of pollutants in the air. Normal clean rain, as stated by the United States Environmental Protection Agency (EPA), has a pH of around 4–5, which means it’s acidic.

In other words, rainwater is more acidic than tap water (remember, tap water is on the high pH side, so alkaline). So keep this in mind when watering your plants as they have different preferences when it comes to the pH level of their growing medium—which may be affected by the type of water used.

How Does Acidic Rainwater Affect Plant Growth? (2 Effects)

A low ph level (high acidity) in rainwater, as discussed by the EPA, can negatively affect your plants in two ways:

1. Makes aluminum available in larger quantities to the plant. In effect, will slowly poison stunt, and affect their growth by reducing their ability to develop roots. Normally, aluminum is locked into the soil and not easily obtained by a plant.
   
2. Washes away nutrients from the soil. This can cause a large variety of problems for plants. However, browning tips and yellowing leaves are the most common issues.

Pro Tip: If the rainwater in your area is highly 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 can add some extra nitrogen in doing so.

Learn more about determining whether your soil is acidic or not in our article on the Best pH And Moisture 3 Way Tester.

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, rainwater mostly falls 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.

Tap Water for Houseplants: 3 Aspects to Check

Let’s clarify one point before moving on. 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 in terms of content and quality. Here are the three features that might differentiate tap water from rainwater and how they impact your potted plants

1. Mineral Content

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.

Calcium and magnesium are minerals that plants need for their development. However, having too much can be a problem.

This is even more important for potted plants. Flushing water enriched in calcium can cause the soil of potted plants to get saturated. You see, unlike other minerals, calcium does not get flushed away. It sticks into the soil.

As discussed by experts from the University of Nottingham and James Hutton Institute, an excess of calcium (although rare) might cause a large variety of symptoms from yellow leaves and wilt among the most common.

Excessive calcium causes plants to be deficient in other important nutrients. They will attempt to absorb all the calcium available (way too much), becoming unable to absorb other minerals like nitrogen, phosphorus, and potassium—all of which are very important for 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, or phosphorus), the symptoms can vary greatly.

In tomatoes, for instance, calcium toxicity causes, among others, tiny yellowish spots on top of the tomato.

A Word of Caution: If your plant looks wilting and has yellow leaves, it might be due to calcium excess. However, be mindful. Most of the time, the problem is way simpler like overwatering, or lack of light! Check our articles 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 to know is to perform a test. You need to measure the hardness. For simplicity, I suggest testing your tap water on your own.

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

Tap water can be defined hard if its concentration of calcium is higher than 120g per liter of calcium (or 7 grains per gallon). Of course, the test discussed above is by far the most reliable way to understand if your water is too hard (and how much).

Below are 5 more signs that your tap water is hard:

1. 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.
   
2. Clothes are never soft: When you wash them with tap water, in case of high mineral content, your clothes will feel hard even when using a softener.
   
3. 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 look opaque.
   
4. Hair feels sticky: Your hair, similar to your clothes will always be sticky if the water you use to take a bath is rich in calcium.
   
5. The white crust on kettle: In some countries (like the UK), kettles are widely used to boil water—tea anyone? 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

Water with a pH far from neutral will affect, in the long term, plants and herbs, causing a pH imbalance rather than calcium excess. The pH level in your soil affects the capacity of your potted herbs and houseplants to absorb the nutrients in their growing medium.

Tap water pH is something that is not strictly regulated in many countries, the USA and the UK included. Oftentimes, however, tap water pH should be between 6.5 and 9.5 (it can often be on the more alkaline side). Take note of the “should be.” Moreover, the range is also quite wide.

For the most part, your soil’s pH value should be around 5 and 6.5 for most plants to thrive. There are a few exceptions, of course, including blueberries and a few others. So adding alkaline water (pH above 7) will slowly and surely change the soil pH. The higher the water pH, the quicker the detrimental effects will be.

When your water’s pH value is far from what your plant desires, your plant can be affected for a long time—months or years. Do not forget that 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 in a bad way.

How Do You Know if Your Soil pH is Adequate?

Using tap water for a while might have already affected your soil’s pH value. In this case, you might need to give it a go thorough test. Nowadays, this is not a huge problem. You can perform professional-level tests for just a few dollars in the comfort of your home or garden.

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

3. Chlorine in Tap Water

The presence of chlorine in tap water is shown to have minimal beneficial effects on plants by limiting the population of spoil-promoting bacteria. However, due to the low concentration of chlorine in tap water and its high volatility, these effects are very limited.

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

As they said the population of two harmful bacteria while leaving unaffected the others.

Moreover, 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 apples, mango, peach, and grape 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?

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 it a step further, just leave it for a day, and you will be almost chlorine-free.

Some people in forums ask 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.

Rainwater and Tap Water Test?

There aren’t that many 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 dug quite a bit and was able to find an interesting study from Michigan State University. They took a looked 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 counterparts (8.3 inches 21.36 cm) but with an overall lower weight. Of course, this experiment was performed with radish only so we can’t be sure if the same will happen with other plants.

They hypothesize that acidic rainwater with a pH value of 4.5 didn’t promote the same overall growth as a more neutral water for plants that sits closer to a pH level of 7.0.

Conclusion: What Should You Use?

Here are two simple questions for you:

1. Is your tap water too hard, or it has a high pH level? Test it. 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 a proper pH testing kit. If you live in the western 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 issues will not have enough time to build up.

Here is my personal takeaway: From my experience in gardening both indoors (mainly herbs) and outdoors (from squash to zucchini and roses), I used mainly rainwater in winter and autumn (it rains a lot here) and tap water for the rest of the year. I never had any problem using either!

Further Questions

Sources

• “Composition of Atmospheric Precipitation” by Erik Eriksson in Wiley Online Library
• “Effect on pH value of rain water and soil pH in River State Nigeria.” by Osang, J. E., Uquetan, U. I., Oko, P. E., Egor, A. O.1, Ekwok, S. E., and Ekpo, C. M. in ResearchGate
• “pH Scale” by N/A in the United States Environmental Protection Agency
• “Effects of Acid Rain” by N/A in the United States Environmental Protection Agency
• “Calcium in Plants” by Philip J White and Martin R Broadley in ResearchGate

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