Is Hard Water Good for Plants? The Surprising Facts

Hard water is good old H₂O with dissolved minerals.  Many gardeners wonder if it’s suitable for plants, so we’ll look at the facts to find the answer.  To give you a preview, we’ll discover that hard water is good for plants, but there’s a point where it can cause harm!  Let’s discover the effects of hard water on plants and how to fix any problems if we need to…

What is Hard Water?

We have to get into some of the basic science before looking at its effects on plants.  Hard water is widespread, so your supply will likely have some hardness.

It can have several kinds of minerals depending on where you live. These minerals actually improve water quality. If I had to choose between hard water and soft, I’d choose hard.

The main substances that cause hardness are calcium and magnesium bicarbonates. These minerals dissolve from limestone, chalk, and other rocks as water flows on or under the ground.  Some interesting chemical actions happen during this process, which you can learn about here.

Here’s a chart showing the relative hardness based on the amount of dissolved solids.  The information on this chart is from the U.S. Geological Survey:

Some important facts related to this table:

• Grains per gallon (gpg) is the most commonly used measurement for hardness.
• GPG is 1 grain (64.8 mg) of calcium carbonate in 1 gallon.  To help get an idea of the size of a grain, an aspirin tablet is five grains.
• When converting hardness from parts per million (ppm) to grains per gallon, divide ppm by 17.1.
• Mg/l is not always equal to parts per million (ppm), but it’s close enough to use in this situation.
• The hardest waters in the US are over 1,000 ppm.  The locations are southern Nevada and California, New Mexico, Texas, Arizona, Utah, Kansas, and parts of Colorado.
• Hardness is easy to check with home testing kits or digital meters.
• If you have tap water from a municipal source, contact the water company to find the hardness.

Now that we have all this information, is there any point where hardness starts affecting plants?  According to the Salinity Management Guide, plant harm occurs when the dissolved solids exceed 450 ppm.  This equals 450/17.1 = 26.32 gpg.

So now we can come up with some basic guidelines:

• If hardness tests at over 26 gpg, it will cause direct harm to plants.
• Hardness less than 26 gpg will cause mineral build-up in the soil over time.  This will eventually harm plants.
• Continue reading to learn how to solve this problem.

The Signs of Hard Water Problems

The minerals in water are good because they’re healthy for people and plants.  Magnesium is one example.   Most people are deficient in magnesium, and plants need it to produce chlorophyll. Hard water poses low to zero health risks for people.

But like the old saying, “You can have too much of a good thing,” and too much hardness can cause problems:

• It forms whitish deposits when it evaporates.
• After prolonged exposure, it damages plumbing fixtures and clogs pipes.  The heating elements for electric water heaters are well-known for having trouble with these deposits.
• It forms a scum or film with soaps and detergents, rendering them unable to clean.
• The hardness makes soil alkaline and affects plant nutrient uptake.
• Excess minerals in the soil also affect nutrient uptake in plants.  This problem is becoming serious if a whitish crust forms on the soil surface.

Despite this, very hard water is safe for drinking, even if it looks cloudy and tastes a little “funny.”  Lower concentrations of minerals give it a pleasant taste.

Some Personal Experience

I’ve spent most of my life in houses with wells.  I grew up in a house with well water that was tasty but hard.  Unfortunately, the hardness caused significant problems with soaps and detergents.

Everything we cleaned, including ourselves, ended up with soap scum or a film on it.  Yuck.  An ion exchange softener handily solved that problem.

The outside faucet had a direct connection to the pipe feeding the softener.  That allowed us to refresh our plants with raw well water rather than softened.

The first house my wife and I bought had a well with high iron content.  If you’ve ever had this problem, you know how unpleasant it can be.  Iron sulfide and hydrogen sulfide give the water an unpleasant taste and a smell like rotten eggs.

Because of the taste and smell, you couldn’t drink the water straight out of the well.  It would also damage any plants sprinkled with it.  Even though it had high iron, it had low hardness.

Since the house had an iron filter, we always had good drinking water, and our plants thrived with it.

Softening reduces water problems for people but causes problems for plants

Since hard water is safe for people, municipal systems rarely soften it.  The law requires water companies to make tap water safe to drink, not make it safe for pipes and cleaning.  Besides, softening water for an entire city would make it more expensive!

When the water supply is too hard, softening it is the user’s responsibility.  Softening has two major reasons:

• Protect the plumbing from scale. These mineral deposits build up in pipes and lower the water pressure.
• Make soaps and detergents more effective.

Some other characteristics of softened water cause problems with plants, especially the sodium ions.  See our article “Watering Plants With Soft Water” for more information. That article also has a short video that explains how ion exchange softeners work.

The Effects of Hard Water on Plants

These problems are more severe with high hardness.  Moderate hardness rarely causes any issues.

Plants outdoors that have exposure to rain typically don’t have these problems.  Rain is effective in washing away or diluting any concentrated minerals.  That keeps the minerals at a beneficial level.

Potted plants can have the most harm since the minerals keep building up in the soil with each moistening.  In this case, mix the hard water 50-50 with distilled, RO, or rainwater.

Gardeners with RO filters often install a raw water line before the filter.  That way, they can get mineral-rich water for their plants.  Then, they’ll mix it 50-50 with the RO water to dilute the mineral concentration.

Nutrient Absorption

Plants have trouble absorbing nutrients if the soil mineral level is too high. Too much calcium and magnesium can compete with other essential nutrients.  That makes it harder for plants to absorb what they need.

pH Levels

Plants tend to grow best when the soil is neutral to slightly acidic.  Because of the dissolved minerals, hard water tends to be more alkaline, which can shift the soil’s pH level.  This shift can affect plant growth and nutrient absorption.

Mineral Build-up

Over time, the hardness minerals can accumulate in the soil. This build-up can lead to a condition called “soil salinity.” High salinity can damage plants, causing symptoms like yellowing leaves and stunted growth.

The Benefits of Hard Water for Plants

While using hard water for plants can have some problems, there are also benefits:

Natural Source of Calcium and Magnesium

Calcium is a building block for the plant cell walls, and magnesium is a critical part of chlorophyll.  Hard water also contains many beneficial trace minerals.

Strengthening Plant Defenses

Some studies suggest that the minerals can strengthen plants’ natural defenses against diseases and pests.

How to Fix Hard Water for Plants

These solutions become more important for hard to very hard water (see the chart above).  I have used moderately hard water on plants with no ill effects.

Dilution

This is an easy method for potted plants.  Mixing the hard water 50-50 with rain, distilled, or RO water dilutes the minerals.

Filtration

Reverse Osmosis (RO) filters can filter water for the whole house.  Instead of a whole-house filter, smaller units are also available for branch lines on the supply.  In-line filters for hoses can reduce the mineral levels for irrigated plants.

Rainwater Collection

Rainwater is the best for plants, and collecting it can solve any hardness problems.  Use it directly on plants or mix it with hard water to dilute the mineral level.

Soil Testing

Regularly testing your soil can help you track the pH and nutrient levels. If the hardness makes your soil too alkaline, there are ways to correct it.  The most common way is to amend the soil with sulfur, peat moss, or shredded leaves to lower the pH.

In Conclusion

So, what’s the answer to our question, “Is hard water good or bad for plants?”  Well, it’s a solid, unwavering “It depends.”  I know; I don’t like that answer either.

As a retired electrical engineer, I can tell you I’ve had to use the answer “It depends” many times.  This answer is truthful because so many things depend on the conditions.

The dissolved minerals are beneficial for plants.  Using hard water on outdoor plants usually isn’t a problem.  That’s because the rain comes along and dilutes any minerals building up in the soil.

Hardness can eventually cause harm to potted plants that don’t receive rainfall.  Once minerals build up in the soil, the nutrient levels in the plants will drop.

The easy solution is to mix the hard water 50-50 with rain, distilled, or RO water.  Another option is to alternate hard water with any of those three.

Plants love rainwater, and it’s the best for plants.  It doesn’t have dissolved minerals but has other nutrients and characteristics that help plants.

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