Gray Water – Is it Safe for the Garden?

Home » Blog » Gray Water – Is it Safe for the Garden?

Robert Pavlis

Do you want to start an online fight? Ask about using using gray water in the garden (also spelled graywater, grey water and greywater). Opinions vary widely, and both users and non-users feel very strongly about their position. You will see lots of opinions, and many statements that are false.

In highly developed countries there is a definite distrust of gray water, but as regions experience more drought people are starting to consider it as a viable option. Clean water, also called potable water, is wasted on landscapes and as the cost of it goes up, more and more gray water will be used. In this post I will discuss its use in the garden.

Is Gray Water Safe for the Garden?
Is Gray Water Safe for the Garden?

What is Gray Water?

Such a simple question and yet we can’t agree on what it is. It does include water collected from laundry machines, bathroom sinks and showers. Some people also include water collected from the kitchen sink, while others don’t.

Water from the toilet is called black water and some include kitchen sink water in this category.

Why can’t we agree on a definition? Because politics is part of the story. Each government, federal, state, provincial and municipal wants to have their own definition. Government is not smart enough to standardize.

Plant Science for Gardeners by Robert Pavlis

In this post I will consider grey water to exclude the kitchen sink and call this ‘kitchen sink water’.

The Yuk Factor

Imagine eating vegetables being watered with the same water that you bathed in? Yuck! For some reason we find  this unacceptable.

But consider this. What gives that tomato its great flavor? Worm poop. Rotting vegetation. Bird droppings. Dead mice. Thousands of different dead insects.

Gray Water and the Law

Each level of government has different rules about graywater. In some places it is illegal to use it in the garden – dumb, I know, but that is the way it is. In other places you are not even allowed to collect rain water for use in the garden.

Other areas allow you to use gray water, but only after it has been treated. This seems to be common, here in Canada, where the focus is on adding large gray water cleaning systems to homes. The only problem is that they are too expensive, especially since our water is still cheap. So nobody uses them. But I doubt police will be checking how you use gray water.

In some places you are free to use gray water anyway you want, but most jurisdictions have some regulations that you should check before using it. Or at the very least – don’t tell your neighbors what you are doing. 🙂

Understand your local laws.

Concerns About Gray Water

There are two main concerns about using gray water in the garden; chemicals and bacteria.

Gray water contains numerous chemicals but the main ones will be the cleaning products added during the washing process.

Bacteria are a concern because they can cause infection and disease. It is unlikely that they will affect the health of the plant, but if these bacteria return to humans or pets they could conceivably cause problems. Remember that our bodies are covered with bacteria and fungi. As we wash, some of these are washed off into the gray water. If you wash some fruit, you will add microbes from the fruit. Even standing water collects microbes from the air.

Building Natural Ponds book, by Robert Pavlis

A main concern for bacteria comes from the fact that fecal matter entering gray water could cause diseases.

A third concern is the pH of graywater.

Chemicals in Gray Water

Certain chemicals will harm plants. The ones of concern with gray water include sodium, boron, and bleach.

Sodium and boron are commonly found in soaps and detergents and even low levels are toxic to plants.

A lot of on-line advice suggests that you use gray water in the garden, you should only use soap and not detergents. This advice doesn’t make any sense since soaps such as dish washing soap are actually detergents.

Both soaps and detergents can contain sodium, which will harm plants. The best option for a cleaning product is one that does not contain sodium or boron. It could be a soap or a detergent.

Keep in mind that we don’t normally use a lot of cleaning chemicals. Consider how much you use when washing your hands compared to the amount of water. These chemicals get diluted quite a bit.

Consistent use of grey water does show an increase of boron and surfactants in soil. Excess sodium tends to wash away with rains, but can be a real problem in some types of soil in arid regions.

Most other chemicals in cleaning products are organic in nature, meaning they contain carbon – this has nothing to do with organic gardening. This means bacteria in the soil will decompose them and turn them into CO2 and water. In low concentrations they are not an issue.

Since chemicals can harm plant leaves, the gray water should be applied to the soil and not directly on the plant.

Bacteria in Gray Water

All gray water will contain a wide variety of bacteria. Most of these will not harm animals or plants. A few can make us sick, but will probably not harm plants.

When soil treated with clean water and treated gray water was analyzed, various  pathogens and fecal indicator bacteria, including Escherichia coliKlebsiella pneumoniaeSalmonella entericaPseudomonas aeruginosaEnterococcus faecalis, and Shigella spp., were found. The reality is that these pathogens already exist in your garden even if you don’t use gray water.

As grey water stands, the bacteria will start to multiply making a potential disease issue worse. Many gray water regulations require you to use the gray water within 24 hours, if it is not treated.

If gray water is added to ornamental beds there is no health risk. Any added bacteria will need to survive their new environment and compete with the bacteria already there. Human pathogens don’t live that long in soil. Since we don’t eat ornamental plants, there is no risk to us.

When lettuce, carrots, and peppers in a greenhouse were treated with gray water containing high levels of both fecal coliforms and fecal streptococci (averaging 4 × 105/100 mL and 2,000/100 mL, respectively), “no significant difference in contamination levels was observed between crops irrigated with tap water, untreated greywater, or treated greywater. Contamination levels for all crops were low and do not represent a significant health risk”

Adding gray water directly to soil will not cause a pathogen problem in plants like tomatoes or beans which have their fruit above ground. Pathogens will not enter the roots, and migrate into the fruit.

There is a potential problem with pathogens attaching themselves to root crops. When you eat the root crop, you could be ingesting some pathogenic bacteria unless you wash and cook it before eating. If you eat the crop right in the garden, or prepare it fresh there is a very minor potential problem.

But consider this. Soil contains thousands of different types of bacteria and we don’t even know what they all are. We do know pathogens are present. People have been pulling carrots out of the ground, wiping them on dirty pants and eating them raw for thousands of years. We are still alive.

Authorities can’t take any risk so they tell you not to use gray water on root crops, but the reality is that the risk of doing this is very low and it is common practice in many parts of the world.

The pH of Gray Water

Gray water tends to be more alkaline, mostly due to the soaps and detergents in the water, but that is not always true. In one study the clean well water had a pH of 7.7 and the gray water had a pH of 8.2. In another study gray water had a pH of 6.7 with clean water at 7.6.

Several studies, but not all, show an increase in soil pH when only gray water is used. The degree of change depends on the pH of the gray water and on the type of soil being tested. Changes in pH will be less dramatic where rain adds a significant contribution to watering. Higher microbe activity in the soil will also mitigate such pH changes.

What About Kitchen Sink Water?

The problem here is that this water contains more bacteria and food particles than normal gray water. The bits of food make bacteria grow faster so there is more potential for pathogens.

Where do the bacteria come from? Some come from your hands, but these are also found in other types of gray water so we can dismiss them as an issue. The rest come from the food we wash. Running water over a carrot or apple before you eat it will add various microbes to the water. But these will be mostly plant microbes – not human ones, so they are less likely to be human pathogens.

When you water a vegetable crop from above it also dumps huge numbers of plant microbes onto the soil. Is this any different than washing some lettuce leaves in the sink and dumping the resulting water onto the soil?

The other issue with this water is that it contains fats, oils and grease. These decompose very slowly in soil, and too much in the garden could be a problem. A lot depends on how much grease you dump down your sink and how big your garden is.

The concerns over kitchen sink gray water are highly exaggerated provided the water is used right away. Storing this water without cleaning it first is more of an issue.

Other Myths About Gray Water

Laundry Machine Water Should Not Be Used

Some people recommend that you don’t use water from laundry machines because the plastic microfiber from the synthetic clothes causes drainage issues in soil. It is true that micro-plastics are being found everywhere and it would be best to keep them out of the environment. But our sewage cleaning systems do not filter them out very well and they end up in rivers and oceans where they probably do more harm than in soil.

They degrade in soil very slowly and plants will not absorb them. They won’t end up in your food.

I doubt that they will change soil drainage conditions, except maybe if they are used over a very long period of time. The plastic is consumed by things like earthworms, which are then eaten by larger animals. We still do not know what harm they do once in animals.

Microfibers are an issue – we just don’t know how big of an issue they are.

Biodegradable Soap is OK

To many people ‘biodegradable’ means that it is perfectly safe. That is simply not true. Biodegradable means that microbes decompose it until finally it is in its basic elements. But as it is decomposed, it can actually be converted into harmful chemicals.

Almost all the soaps and detergents are biodegradable – even if it does not say so on the container. Even crude oil is biodegradable. Products that are labeled biodegradable and contain a lot of sodium, are worse for the soil than non-biodegradable ones with less sodium.

Washing Soda is a Safe Soap

Washing soda is not even a soap – it is a salt.

Washing soda has been used for many years and considered by many to be a very safe product, and it is. The problem is that it is sodium carbonate (sodium, carbon and oxygen) and sodium is very toxic to plants. So this is not a good product when collecting gray water.

Don’t Spread it on the Surface of the Soil

One person commented that you should not spread it on the surface of the soil because you will get flies.

Some regulations require the gray water to be applied below the surface of the soil. I guess this might reduce the chance of some gray water splashing onto plants, but that is hardly a concern. Mulch will also prevent this.

It won’t create flies. They might come for a drink if you spread it on the surface, but so will bees and butterflies.

Gray Water Does Not Contain Fecal Matter

A common definition of gray water is, “all wastewater that is generated in household or office building sources without fecal contamination“.

That is not true. Shower water and washing machine water both contain fecal matter from our bodies and our dirty cloths. In one study, faecal coliforms (CFU 100 mL– 1) had a value of <1 in clean water and 1,000,000 in gray water. You are not as clean as you think.

The same study found that faecal coliforms did not survive long in soil, so it is not really a big issue.

Don’t Use it on Acid Loving Plants

It is a common belief that gray water is alkaline and therefore it should not be used on acid loving plants, like rhododendrons and blueberries.

Gray water is not always alkaline and even if it is, it does not mean it will make the soil more alkaline. If it does not make the soil alkaline it is perfectly fine for acid loving plants. Exercise some care and consider measuring your soil pH.

How Does Gray Water Affect Plant Growth?

Gray water contains nutrients that plants need. Many soaps contain phosphorus. Organic matter will contribute nitrogen, sulfur, calcium, magnesium, iron and other nutrients. Soaps that don’t contain sodium will most likely contain potassium. Gray water is a fertilizer.

How does this affect plant growth?

A study that looked at this issue found that gray water improved growth and productivity of carrots, peppers, spinach and beets compared to clean water. In some cases growth in gray water was as good as or better than with fertilized water. This testing was done in pots.

gray water grows more peppers than clean water, image from reference 1
Gray water grows more peppers than clean water, image from reference 1

Field testing in Texas showed that gray water (laundry water) increased production in bell peppers, Chile and tomatoes when compared to clean water. Gray water did not increase salt accumulation, but did increase pH.

Greenhouse testing showed tomatoes grew better in gray water than in tap water, and those grown in gray water had higher levels of P, K, Ca, Mg, Na, Fe and B.

Other studies have found no increase in productivity, but none reported a decrease in productivity.

Should Gray Water Be Used?

From both a healthy garden perspective and from an environmental perspective, the answer is clearly yes.

Gray water, as well as kitchen sink water, should be used on ornamental beds and lawns. Gray water can certainly be used on non-root crops and can probably be used on root crops without concern. It is certainly safe on root crops that get cooked.

Follow these suggestions.

  1. Don’t store gray water for more than 24 hours.
  2. Don’t use gray water on vegetables if someone in the household has a serious pathogenic infection.
  3. Use cleaning products with lower sodium and boron levels.
  4. Use smaller amounts of cleaning products.
  5. Spread the gray water around your whole yard – don’t keep dumping it in the same spot.

Tips and Tricks for Watering Plants

Here are some more posts about watering plants.

Watering Houseplants – Top or Bottom? Which is Best?

Watering Plants in the Sun – Do Water Droplets Damage Leaves?

What is the Best Watering Schedule for Your Garden

Watering Plants Correctly – When and How to Water

Watering Orchids With Ice Cubes – Does It Harm orchids?

Best Way to Water Indoor Plants

References:

    1. Agricultural Greywater Use for Agricultural Irrigation in urban Areas; https://ocw.un-ihe.org/pluginfile.php/682/mod_folder/content/0/14_Greywater_reuse_South_Africa_Salukazana_Donsheng_conf_2007_.pdf?forcedownload=1

 

 

If you like this post, please share .......

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!

11 thoughts on “Gray Water – Is it Safe for the Garden?”

  1. Robert
    Thank you for taking the time to put together an excellent article. As we all need to do our bit to save water, I have a practical question that I hope you can answer.
    I would like to capture our bath water which should have a low amount of soap in it, but then store it in an a large water butt to use on our potted plants when required. As it is not recommended to keep gray water longer than 24 hours? can I filter the soapy water with something before entering the water butt or maybe add something in the water butt so that I can keep it longer?
    Many thanks in advance. Ian

    Reply
    • It is difficult for the home owner to filter out bacteria, but you could treat it with something like chlorine to kill them.

      Reply
  2. How do I neutralize the acids in gray water is there a natural way or anything I can add to help neutralize the water to irrigate paddocks

    Reply
  3. Need some advice! I live in an area of annual rainfall below 500ml. Rain water is catched from the roof and into a well which is around 6000lts. I use this water for flushing and irrigation pot plants (both ornamental and edible plants) using drip line. I’m thinking of diverting gray water from shower and sink directly in the well. Taking in consideration all the advices you gave us in this article, my really concern is to use the gray water within 24hrs since this is not possible in my case (Gray water is diluted with rainwater into the well). What is your advice please?

    Reply
  4. Robert, would you be willing to write about using reverse osmosis water in the garden? We have well water that’s too full of iron, sulfur, and calcium carbonate to drink. So we use a reverse osmosis system with UV light to destroy any potential harmful bacteria. We give this water to our plants, too. And after reading through your site I am beginning to wonder if the information online, which often warns people to remineralize the water to avoid deficiencies in us and plants is less than useful. Thank you for your consideration.

    Reply
    • I’ll add it to my long list of future topics.

      Reverse osmosis produces very pure water – which could actually pull nutrients out of plant roots, especially on potted plants – less of a concern outside in real soil. Same with distilled water and water from a dehumidifier. I would add some regular water back to the osmosis water to add a few salts – I assume that is what you mean by remineralize.

      Not sure if it makes much difference to humans, since we can’t lose the ions into the air/soil.

      Reply
  5. We live in an area not served by a mains sewerage system and because we are in a river catchment zone have tomhave a “biocylce swptic system”. The system treats all our effluent and pumps out ‘grey water’ dor use in and around the garden. It has to be chlorinated by law, and we use it on our truitbtrees, and am about to use it to iirigate all our non root crop vegetables. As long as it is applied direct to the soil and not via overhead sprinkler systems there is not any issue.

    The reason for us to use it, is we live in an annual rainfall zone of around 300ml (12 inches) and our rain water is used in the house 1st. So in effect we get to use all our water twice.

    Reply

Please leave a comment either here or in our Facebook Group: Garden Fundamentals