Myths About Rainwater Harvesting Systems for Gardens

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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”.

Plant 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.

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!

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

  1. Pine trees, and pine cones oaks, bee hives on trees , importance for wildlife In New York how to help or grow , need education

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  2. 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.

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  3. PLEASE FILTER THE RAIN WATER HARVESTED THROUGH RIVER PEBBLES,,CHARCOAL AND FINE RIVER SAND WITH FREE BOARD TO AVOID AIR LOCK…ALSO PROVIDE AIR VENT PIPE WITH COWL COVERED WITH COPPER COATED SS WIRE MESH,,,THIS WILL KEEP THE QUALITY OF RAIN WATER FOR 3 TO 4 MONTHS,,PROVIDED SUN LIGHT SHOULD BE AVOIDED,,,FOR THAT COVER THE TANK OR DRUM WITH A LID,,

    Reply
  4. 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.

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  5. 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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  6. To clarify: Tap water, at least when it’s stored in bottles (like screw-top Nalgeen bottles used in camping and backpacking), does indeed go stale and becomes unpotable after only a few days, such that if one drinks it there’s a good chance they’ll fall ill, as the chlorine and other additives that municipalities add to their tap water begin to evaporate or deteriorate after the tap water leaves your home’s copper plumbing. The water typically spoils, or becomes stale and shouldn’t be consumed without sufficient boiling or adding things like iodine tablets. I’m speaking from personal experience as an Eagle Scout whose lead crews of novices on extened back packing and conoe trips to remote wilderness areas where you’re supposed to obtain your daily water from either wells or, more typically, from streams, swamps, etc and use iodine tablets and water filters (or boiling) before consumption; many times I’ve had novices try to sneak along enough tap water from home to go the full 1-2 weeks without resorting to the disinfected water the rest of us are getting from the rivers and treating (since the treated water is an acquired taste). By day three, we always know who’s been living off their hidden stash of tap water from home, because they’re the ones making the whole crew stop every half-hour because they’ve got the runs from their tainted tap water. In bygone eras, travelers, voyagers, frontiersmen, etc. did come to find certain rivers and streams in the water known to provide “sweet water” that could keep in storage much MUCH longer, as the water from those sources was often loaded with tannic acids from fallen leaves from certain trees, and so such water would remain potable far longer than usual.

    Sorry fr the length of the post—going batty in isolation I guess—but I didn’t want any casual reader to apply what was said in the article far more broadly than the author obviously intended; I’m fairly certain he meant stored rain water is fine for watering gardens, but a few of the comments that followed made me wonder whether some fellow enthusiastic over rain barrels and such might misinterpret the point that water doesn’t go stale or spoil, which is true for gardening, but don’t go drinking it, as it does spoil as far as human consumption is concerned.

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    • Stale and contaminated with organisms is not the same thing. Stale just means it tastes bad, or has some chemicals that make it taste different.

      If people get the runs from drinking 3 day old tap water, then there is a microbe contamination.

      I have a hard time believing what you say. We keep water in the fridge for days, even weeks, so that we have it cold for drinking. And this water comes from a well – it was not even treated.

      I have spent a lot of time on canoe trips, including several month long trips in the Arctic. This is the first time I have ever heard that carrying water for more than a few days becomes contaminated.

      Is it possible that the water got contaminated along the trip because people were drinking from the bottle?

      But I would be interested in seeings some scientific support for the statement.

      Reply
  7. When I was much younger I was living in Indonesia (equatorial climate). The building had a brand new galvanised roof, gutters and brand new galvanised water tank. As an experiment on the first day after this construction several of us tried drinking the water unboiled, and we all got diarrhoea, but nothing serious. I never really knew whether it was mild zinc poisoning or just E coli

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      • Dr. Oz is unquestionably a quack but what he says about spinach and strawberries may well be true and is, in fact, corroborated by many other, more reputable sources. Just Google ‘spinach and strawberries contamination’.

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  8. I believe that such rainwater systems give very expensive water if you just count the $ cost. And in my area I have the same problem as Anne – ie the tank will be empty for months when you most need water. I suppose in an area where rainfall is low and evaporation is high they’re poor use of money and materials, especially for people growing vegetables.
    However it is still good to learn about the myths you mention.
    Thanks,
    David

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  9. RAIN IS THE PRIMARY SOURCE OF WATER,,DUE TO WATER SCARCITY BECAUSE OF GROUND WATER LEVEL DEPLETION AND SURFACE WATER SCARCITY IN MANY COUNTRIES LIKE INDIA DUE TO ERRATIC RAIN BECAUSE OF HYDRAULIC CYCLE AFFECTED,,
    AT THIS CRITICAL STAGE LIKE IN INDIA WE HAVE TO CONCENTRATE ON RAIN WATER HARVESTING FOR DRINKING,,COOKING,AND RECHARGING INTO OUR MOTHER EARTH,,
    PLEASE VISIT ” ARUNACHALAM RAIN WATER HARVESTING,,,IN GOOGLE,,
    FOR GARDENING PURPOSE WE CAN CAN FILTER THE BATHING, KITCHEN WATER THROUGH SAND- CHARCOAL- RIVER PEBBLES ( NATURAL FILTER) AND MAKE IT AS SECOND USE FOR GARDENING,,TRY IT AND FIND VERY GOOD RESULT AND HARVEST VERY FRESH VEGETABLES,,
    WISH YOU ALL HAPPY RAIN WATER HARVESTING,,,
    N.ARUNACHALAM..
    FORMER CHIEF ENGINEER,,PWD,,

    Reply

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