‘Dear Garden Doctor,
Is it safe to re-use greywater from my washing machine on the garden and would it have any beneficial effects on my plants. A lot of the literature I have read gives conflicting reports, what is your opinion?
Regards Jim, Sanur’
Watching the news recently it seems odd that Jakarta is in the midst of a drinking water crisis when the city is usually underwater at this time of year. Likewise Bali has water supply issues of it’s own, meanwhile every morning I see people out hosing the street!
Using greywater is a thoughtful idea not only for the benefits to the garden but also as a way to conserve potable drinking water that seems to be rapidly disappearing by the day. And this doesn’t even take into consideration the beneficial effect of withdrawing toxic chemicals from the aquatic environment into which our waste water is typically dumped. The rivers around Denpasar unfortunately became algae choked sewers a long time ago, lets make sure it doesn’t happen elsewhwhere on the island. The average household produces hundreds of litres of greywater each day – why not use it productively on the plants? A lot of that water that goes down the drain is essentially wasted, and would be put to better use as irrigation in the garden.
To clarify, greywater refers to waste water from the laundry or bathroom sinks and showers, but definitely not the toilet which is termed ‘black water’ for obvious reasons. Yes of course you can use greywater in the garden for the benefit of plants. Look for the most ecofriendly detergent that you can find or even develop your own – now there’s a new years resolution for you! There may be a lot of trial and error involved, but that’s half the fun anyway isn’t it? That’s why gambling is so addictive for some, they never know for sure that they will win…even though it’s always a sure thing, right? I digress.
Never rely solely on greywater to irrigate the garden if you are new to the concept. Greywater quality will vary dramatically depending on the types of detergents and additives used which will have a major impact on soil and plant health. It is advisable to use detergents that are biodegradable and low in salts. Detergents are alkaline and over use may increase soil pH affecting plants by limiting their uptake of essential trace elements.
The main concern with using greywater on the garden is the build up of salts in the soil, increasing alkalinity and resulting high soil pH. Spread your grey water around, don’t always water it in the same spot. Use greywater before rain to take advantage of the dilution effect. Take into account your soil type when using greywater. The odds of detrimental compounds building up in clay is greater than that of sandy soil.
All of these concerns can be disregarded if you simply find the most efficient ecofriendly method to wash. Low phosphate and even phosphate free detergents are available and these will lower the amount of harmful residues in greywater from the laundry. Laundry detergents in developing countries such as Indonesia with lax environmental regulation are typically heavily laden with phosphates. Australia phased out phosphates in laundry detergents back in 2014, but over here big business will get away with whatever they can while they can.
Typical laundry detergents are highly alkaline. If you use a large amount of detergent it’s better to use only the water from the last rinse cycle which will have both a lower level of salinity and lower pH. Better still figure out how to get the cleanest clothes with the least detergents. Become a minimalist in life and laundry – great things will happen!
If detergents are used at such a high potency that it can kill your garden then think about what it’s doing to the rivers and oceans where the waste water ordinarily flows directly without filtration. On the other hand, detergent laden water is really good to water over the aphids and scale that may be attacking your favorite plants, I know that it removed the mealy bugs from my citrus with no harmful effects to the plant 🙂
Common laundry detergents typically contain sodium salts such as sodium nitrate, sodium sulphate, sodium phosphate and sodium silicate. They are highly saline, and frequent use will harm your garden long-term, unless it is used sparingly and distributed over a large area.
Look for biodegradable, phosphate free and low concentrate. Forget about softeners, whiteners, bleaches and oxyaction blah blah blah…yes for a great garden we have to make sacrifices!
Now let’s assume that you have got your washing routine down to the fine art of extracting the most dirt with the least detergent, then you can really start watering. With a simple greywater system laundry to landscape is as easy as removing the discharge hose from the drain, connecting it to an extension and running it out into the garden. This will enable you to manually reuse grey water every time you do the laundry. It is best discharged onto a surface mulch, which filters the particles enabling greywater to soak into the soil more evenly.
In general plants that are adapted to a coastal environment will do much better on grey water as they will cope with the higher salt content, plants such as frangipani, palms, hibiscus, hydrangea, most succulents and herbs such as rosemary and lavender will always do well. Avoid using greywater on acid loving plants such as citrus, tomatoes and azaleas. You can safely irrigate edible plants as long as the greywater doesn’t contact the edible portion (no root crops and salads). Fruit trees will do well.
If you overdo it and your plants are looking sick add a general trace element mix every few months until the problems clear up. Addition of gypsum will also help to neutralise the increase in salts. Load the organic matter into your soil because compost will naturally buffer the pH and also improves soil health. Many have reported only positive effects from greywater – once again this all depends on the quality of the greywater in question.
It all seems a bit concerning that you could damage your garden with greywater but it’s really not that complicated – it’s just that I prefer to err on the side of caution when advising others to use it. I have only had good results when using greywater, except for the time I wiped out a few tomato plants. It all depends on the detergent and ratio used. If in doubt start using it on your more hardy plants such as the frangipani, succulents or a patch of grass up the back of the garden first, and then go from there if all is well.
Greywater is a valuable resource when used correctly, not only saving potable drinking water from being wasted on garden beds but also withdrawing toxic chemicals from aquatic ecosystems.
And just think…we’ll have all that extra water so that we can keep on hosing the street!
Copyright © 2018 Dr. Kris
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