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Graywater for Gardens


Gardens do not need clean, potable water to flourish. The vast majority of plants thrive on graywater, and the nutrients contained within.

This page outlines how graywater effects plants, and precautions that should be taken to ensure your garden becomes an oasis relying solely on graywater.

  • Do NOT store graywater for more than 24 hours.
             
  • Only use laundry detergents that are very low in sodium (salt).
             
  • Distribute graywater evenly across the garden - don't kill plants with kindness by over watering.
               
  • Take care with graywater distribution if using movable hoses connected to the washer machine outlet.
              
  • Avoid any cleaners containing Boron - this is toxic for plants.
               
  • Remember that what goes down the drain is going onto your garden!


Shower Water

Shower water contains soap, shampoo and conditioners. The amount of fats, sodium and phosphates in shower graywater is negligible, and has virtually zero impact on plants and lawn.

A common misconception is that all graywater is alkaline, and that acid loving plants do not grow as well with graywater. Azaleas are often quoted as a plant that does not enjoy graywater.

Many graywater gardens include Azaleas, and have experienced as good growth, if not better than irrigating with potable water.

The PH of shower water is largely dependent on the type of shampoo used. While most shampoos are near neutral PH, some fruit based shampoos can be slightly acidic. Again the effect on PH overall is neglible due to dilution.


Washing Machine Water

Salt:
Non concentrated soap powders typically contain high levels of sodium (salt), used as fillers. Use only concentrated powders that claim to be low in salt, or use liquid detergents.

Alkalinity:
Almost all detergents will produce highly alkaline graywater (PH approximately 10.5) when releasing the first wash water.  Repeated application of concentrated highly alkaline water to one or two areas of the garden will kill plants.

This is of particular concern when DIY connecting a 1" hose or similar to the washer outlet, and leading the hose to a garden area.

A common problem in Australia is the use of a manual gravity fed hose connected to the washing machine - people would move the hose from one area of the garden to another between wash cycles. Being creatures of habit, many people always moved the hose the same way each wash. As a result the first (dirty and high PH water) ended up in the same area each time, raising the soil PH level and killing plants in the area.

We fear this will occur in California due to their unusual requirements for a 'simple' system in the recent CA code update, which appear to encourage this style of irrigation.

However, if graywater is irrigated evenly across the garden, the high PH water will not concentrate in one area, and is in fact partially balance by the following rinse cycles (the overall PH level of washer machine water (including rinse cycles) is typically in the range of 8.0 - 9.0).


Water Softeners
   
Water softening agents use large amounts of salt. Softened water should not be used in the garden.

Graywater for Fruit and Vegetables


Fruit trees and vegetable plants grow well with graywater. The issue is that graywater may contain bacteria, and if you eat fruit and vegetables covered in graywater (and therefore bacteria), you may become sick.


If the fruit or vegetable is going to be cooked, any bacteria present will die during the cooking process. This removes the risk.

Because the fruit on established trees is much higher than ground level, there is very little chance the fruit will come into contact with the graywater.

Graywater and vegetables / low fruit

Where the edible part of the fruit or vegetable is above ground, and it is eaten raw, the graywater must NOT come into contact with the edible parts.

This can be done by bucket (very carefully) but is labor intensive. High flow rate dripperlines (such as Irrigray) apply the graywater directly to the soil zone, and seep into the soil immediately. So long as a layer of mulch covers the dripperline and soil, there is virtually no chance for the edible component to come into contact with the graywater.
Where the edible part of the vegetable is below ground (eg. potato, carrot, onion), graywater will come into contact with the vegetable. If the vegetable is cooked before eating, the risk is removed by the cooking process.

If the vegetable is eaten raw, the risk can be removed by peeling the washing the vegetable, peeling, then washing again.

We do not recommend growing the following types of vegetables, if they are to be eaten raw:

  • Radish. Because the skin is rough, radish is difficult to clean. Commonly served unpeeled, there is a risk of bacteria remaining on the surface.
              
  • Salad onion (including scallions etc). Due to the onion's layered structure, there is a chance (albeit small) of bacteria being caught within the onion, presenting a risk if eaten raw.

lettuce.jpg                 

Lettuce is an excellent example of why the first reaction of authorities is to prohibit irrigation of vegetables with graywater.

If the lettuce is irrigated by sub-surface dripperline (under a mulch cover), the lettuce leaves will not come into contact with the graywater.

However if the lettuce was irrigated with graywater by a watering can, bacteria would collect on the leaves, and present a health hazard as lettuce is rarely cooked, and the cleaning process is unlikely to remove the bacteria.

Common Questions

What about soap in the fruits and vegetables?


We are yet to find an appropriate scientific study that has measured how much soap is actually contained inside the fruit or vegetable.
         
The consensus view is that only tiny amounts could be present and only at a molecular level.
         
Keep in mind how much the soap / shampoo / conditioner has been diluted by the time it gets to the plant.
         
Consider how much your body is ingesting through other daily activities (brushing your teeth; direct skin absorption of soap, shampoo and conditioners; soap residue on plates that you eat off etc. Another consideration is how much pesticides do your store-bought fruit and vegetables contain (pesticide residual washed into the soil and then absorbed through the roots).
         
Rain water leaches the soap down into the subsoil, preventing accumulation over the years.

Yuck! Eating vegetables with water I have showered in or washed my clothes in!

This is a common response - and quite natural. Think about what your vegetables are growing in now; decaying compost, worm castings (and droppings), fertilizer, bird droppings, all manner of less pleasant ingredients than body oils and dried skin.


Graywater irrigation of fruit and vegetables is of course personal choice. No one is forcing you to grow your fruit and vegetables with graywater. However, on balance it would appear home grown foodstuffs will have less contaminants than commercially grown produce.


My local regulations prohibit or recommend against the use of graywater for vegetables.

Contact your local health department and find out why. Many regulations / recommendations are written to cope with the most common practices, such as people considering bucketing water onto their lettuce and strawberries etc. This type of irrigation with graywater is risky, because of the bacteria.
 
Graywater dripperline technology has evolved enormously over the last 5 years in countries such as Australia. The new dripperline technology has only recently arrived in the US, and health department regulations or recommendations take many years to catch up.

We are not recommending that you break the law. We are simply suggesting thinking about why the law was written in the first place.

This section (fruit and vegetables) is written as general advice, based on our own research and experience. We recommend conducting your own research and / or consult local authorities before deciding your own actions.

 

tomatoes.jpg

 

Tomatoes are an interesting case. Even Arizona, which generally has excellent graywater laws, only permits graywater irrigation of citrus and nut trees. 

So, juicy peaches can be irrigated with graywater but tomatoes (also fruit) cannot? This doesnt make sense to us - if the fruit is irrigated by subsurface drip irrigation, covered at the minimum by 2" of mulch.