Myths About Rainwater Harvesting Systems for Gardens

Robert Pavlis

Rainwater harvesting systems are a hot topic in gardening circles. As we become more aware of the value of water, gardeners want to collect rain and use it to water their plants. Not only does this make financial sense, but in many cases the quality of the water is much better than tap water. And it is great for the environment because it sends less water to the city for processing.

Along with any good idea comes a number of myths. In this post I will look at myths pertaining to the use and harvesting of rainwater.

Myths About Rainwater Harvesting Systems for Gardens
Myths About Rainwater Harvesting Systems for Gardens, photo from Buzzword

Rainwater for Gardens

Let’s be clear about the scope of this review. I am strictly dealing with use of the water in the garden. This is not about using it for human or animal consumption. Having said that, collection for human consumption is done all over the world, its just my experience.

Stored Rainwater Goes Bad

A recent discussion on our Facebook group had several people comment such as “stored water deteriorates” and “standing water will go stale and should not be used in the garden”.

Compost Science for Gardeners by Robert Pavlis

If you take a glass of tap water and let it sit out overnight, it does taste funny in the morning.  This fact might lead people to think water goes stale.

The reason drinking water tastes different after sitting out is partially due to temperature. Most people like the taste of cold water better.

A lot of tap water has been treated with chlorine and humans like the taste of water better when it has a bit of chlorine added, but too much is not good. Chlorine is quite volatile and off-gases overnight, so water does not taste as good in the morning because it has lost its chlorine edge.

Water also absorbs gases from the air and these change the taste.

We think water is going stale because we don’t like the taste, but chemically it is just as fresh as when it was collected. Sitting water does not get stale or go bad on its own.

Stored Rainwater Needs to be Treated

There is the potential of getting some E coli in the water from roof runoff, mostly due to animal and bird feces. This is of no concern for plants except maybe vegetables, and then only if you apply the water to the part you eat. Watering the soil, as recommended, is no problem since your soil already contains E. coli.

Treating rainwater with chlorine has some limited value if you water the edible part of vegetables. This advice is mostly precautionary and probably makes very little difference. Picking veg with hands that have been working in the garden is just as big a problem and we don’t wash our hands with chlorine before harvesting.

Building Natural Ponds book, by Robert Pavlis

You also have to remember that the same animals and birds that poop on your roof, also poop in the garden.

Harvested Rainwater Contains Heavy Metals

It is true that rainwater contains heavy metals, but the all important question is, what are their concentrations? After all, your soil also contains heavy metals, as does the fruits and vegetables you eat. Heavy metals are only a concern at higher levels.

One study that looked at backyard rain barrels found that “Heavy metals were well below federal irrigation standards for reclaimed water and posed minimal risk for irrigating a vegetable garden. ”

A study that looked at runoff from different kinds of roofs found very low levels of contaminates.

Another study, in Australia, grew various vegetables using artificially contaminated water and soil to see how various chemicals were absorbed by the plants. Some of these contaminates had a concentration that was 5,000 times higher than environmental levels. They found, “French beans and beet leaves, but not the beet root, had lead levels that exceeded Australia’s health guidelines. The kale in particular had lower levels of all of the metals, illustrating the wide variability in metal uptake among crops, and even into different parts of the same plant.” Most of the vegetables were still safe to eat. Keep in mind that they were testing an extreme case, far worse than the water from your roof.

There is little concern about heavy metals in harvested rainwater.

Rainwater Harvesting is Illegal

This idea was making the rounds a couple of years ago. Numerous people claimed harvesting rainwater was illegal in their state. When asked to provide evidence of this law, they couldn’t.

It turns out that Colorado did have a rainwater harvesting law on the books. That law was changed in 2016 so that homeowners are now allowed to have two rainwater harvesting barrels.

Other US states also have regulations in place for controlling and regulating the type of systems used, but most of these are for health and safety reasons. They don’t make water collection illegal.

Many jurisdictions are now encouraging the installation of rainwater harvesting system, because it is good for the environment, and it reduces their water processing costs. My town even sells rain barrels at a discount.

Storing Rainwater in Plastic Causes Toxins to Leach Into the Water

This is true. All plastic leaches toxins, even the plastic drinking bottles everybody loves. The UV light from the sun speeds up the degradation of plastic and exasperates the problem. The amount of chemicals leached by most plastic is minimal.

I have review the safety of using plastic as containers for growing vegetables. If a plastic is safe for growing them, it is also safe for collecting and storing water.

Stored Rainwater Breeds Mosquitoes

A properly designed rain barrel will be fitted with screens so that adult mosquitoes can’t lay eggs in the stored water. Breeding mosquitoes are not an issue in this case.

If you have an open barrel, consider using mosquito dunks, or simply add a couple of goldfish. They will not only get rid of the mosquito larvae, but also fertilize the water. If you do this, it is a good idea not to drain the barrels completely 🙂

Is Harvested Rainwater Safe?

Harvested rainwater is perfectly safe to use on all ornamental gardens, trees and shrubs.

If you are using it on vegetables, all the science indicates it is safe to use. There are some limited potential problems and most of these can be minimized or eliminated by doing the following.

Add a Diverter

rainwater diverter
rainwater diverter

A diverter is a device that diverts the initial wash-off from the roof away from your barrel. This water contains most of the pollutants from the roof and by keeping it out of the collection system, you end up with cleaner water.

The downside is that you collect less water.

Water The Soil, Not the Plants

Keep water off vegetable plants, and water only the soil. This will reduce any chance of pathogen contamination of the food.

Treat Reservoirs with Chlorine

Add one ounce of household unscented chlorine bleach to 55 gallons of water and wait 24 hours before use.

Use Containers That Are Opaque

Transparent containers allow too much light to get through, which can cause blue-green algae to grow. These plants can add toxins to the water. Opaque containers keep the light out and prevent them from growing.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

33 thoughts on “Myths About Rainwater Harvesting Systems for Gardens”

  1. Thanks for such an informative and rational article, Robert.

    I have just moved into a new house and there are two very large plastic watertanks. The tanks are full but the water inside has been sitting in there for at least years. Would it still be safe to use on the grass and trees? They both have mosquite proof mesh at the inlet, so I’m pretty sure there’s be no debris in the tanks.


  3. I have performed geochemical analyses on numerous samples on rainwater and my rain barrell/roof water as part of a study on rainwater composition. Compared to virgin rainwater gathered in a special apparatus,the rain barrel water was enriched in numerous major nutrients, such as calcium, magnesium, phosphorus,potassium, sulfur and numerous micronutrients, including copper, zinc, and manganese. Potentially toxic elements, such as cadmium and arsenic, were well below actionable levels. My wife and I have stored the water from the rain barrel in white food grade 5 gallon buckets in our basement furnace room (75 degrees F mean temperature) for the last 10 winters for our house plants, they thrive on it. We have never had a problem.

  4. Generally, a very good article. The only thing I would add it that I would start with about a 10,000 liter tank for a small 1600 square foot garden. Want to know details on water system check out the American Rainwater Catchment Systems Association (ARCSA) manual- it has everything.


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