Using Grey Water to Help Plants Survive a Hot Summer

, written by Jeremy Dore gb flag

Sprinklers are a wasteful way to water your garden

When summer temperatures start to soar many people reach for the hosepipe and sprinkler to keep their lawn and garden looking good. It is convenient to have endless water 'on tap' but it is also very ecologically expensive – clean drinking water takes energy to produce and a sprinkler can use 1000 litres (200 gallons) an hour. That's the same amount of water that a whole family uses in two days. Multiply that by millions of homes and it soon becomes apparent why this can be bad news for the regional water supply, often resulting in a hosepipe ban to conserve water. So what alternative exist for keeping your fruit and vegetables well watered?

There are four main areas to consider when keeping plants hydrated:

  1. Helping Plants Cope: There are plenty of ways to reduce the amount of water that is lost through evaporation and prevent heat-stress to plants. You can read all about these in our articles on Mulching and Dryland Gardening. In the longer term, adding plenty of moisture-retentive organic matter such as compost will help, particularly if your soil is sandy or you use raised beds.
  2. Targeted Watering: Sprinklers distribute water evenly over a wide surface area but gardens very rarely require this. Some vegetables such as corn, tomatoes and chard can survive with a lower water ration for a few days while others such as lettuce, celery and developing onions need to be kept well watered. Using a watering can takes longer but helps you to apply the water where it is most needed.
  3. When to Water: It is tempting to water plants during the hottest part of the day. Much more efficient is to water them when temperatures start to drop in the evening as less is lost to evaporation and the plants have the whole night to take up the moisture.
  4. Recycling Water: Using 'grey water' (also called graywater) from your house is an excellent way to recycle water to your garden.
Grey water from washing up
Gray water from washing up can be used for garden vegetables and fruit if a few simple precautions are followed

Grey Water in the Garden

Grey water is any used waste water from your house that is not considered sewage (which is referred to as 'black water'). Common sources of grey water are washing up water, water from washing machines/dishwashers and water from washing in showers, baths and sinks.

Using grey water is a very controversial topic when it comes to gardening. Why? On one side horticulturalists and health experts worry about the potential for spreading contaminants or disease while on the other side many gardeners argue that they've poured grey water over their crops for years without problems, with some even citing the fact that the soap helps control aphids.

The truth is that grey water is fine as long as you follow some sensible guidelines:

  1. Use ecological plant-based cleaning products. A typical washing powder contains around 25 ingredients, some of which will be petroleum based and these can accumulate in the soil. Conventional products contain surfactants, phosphates or phosphate replacements, chelating agents, salts, thickeners, fragrances, colourants etc. Some of these are likely to be non-degradable and others can have negative effects on plants (such as EDTA or NTA which assist the uptake of heavy metals). The easiest way to avoid these problems is to stick to companies that only make ecological products (e.g. Ecover or Bio-D in the UK) as it's all too easy for other manufacturers to slap deceivingly 'eco-friendly' labels on a product without using all plant-based materials.
  2. Be careful about using 'fresh' grey water on plants. Even ecological products will leave a surplus of active ingredients in grey water which could weaken plants if applied directly. This is where grey water filtering and storage systems really come into their own. By allowing the water to stand microorganisms can start to degrade these active ingredients and contaminants can sink to the bottom (a rough guide is to let it stand for 1 day in summer, 2 in spring/autumn and 3 in winter). Just be careful that young children and pets don't have access to the grey water during this period.
  3. Equally, try not to store the water too long. In hot weather pathogens can develop in the grey water unless it is treated (see below for details of suitable storage systems). Particles of meat and dairy products from washing dishes or bacteria from bathrooms and washing machines can all be risks if the water stands for more than a day in hot weather, so don't store it too long and use it for ornamental plants rather than edibles if you are unsure. Common sense rules here – water that you used for washing vegetables will be no problem, whilst washing up water from a barbecue needs caution.
  4. Don't apply grey water directly to plants. Sink a plant pot into the soil and pour the grey water into it so that microorganisms in the soil can further break down any remaining substances. An added bonus is that this will keep the plants drawing water from deeper ground sources rather than the surface which quickly dries out. Never run grey water through hoses or sprinkler systems.
Watering plants

Ideally we would all have grey water filter and storage systems built into our houses. Large grey-water storage tanks build up a population of microorganisms to quickly break down active ingredients and reduce the potential smell. In fact a grey water collection and storage system is surprisingly easy to implement and there are some excellent articles for cheap home-built systems on the Renewable Energy UK website.

However, for most people the motivation to use grey water only comes when we hit a drought period and water restrictions are in place . I remember collecting water from a standpipe in the road when I was a child during one such long dry summer and we had to be careful with every last drop. It's unlikely that it will get to that stage this year but there is already talk of a hosepipe ban in parts of England. By following the steps above we can all help to conserve water and still reap the benefits of a productive edible garden. I'm certainly going to try to be more water-conscious in my own garden this summer.

[My thanks go to Peter Malaise, a keen gardener and expert adviser at Ecover, and Michael Barwell, founder of Bio-D, for information used when preparing this article]

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Show Comments


"We are outside city limited and have a sewage tank that all grey and black water drains to. Then, after the microorganisms have a chance at the material, it get sprayed on the lawn. I wish I could direct more of that water onto my garden, but on to the lawn is better than nothing!"
Rachael on Friday 2 July 2010
"I was always told that watering in the evening would promote mold and mildew in the garden. Now you are saying to water in the evening. That is better than in the cooler morning when the plants can use the water during the growing time of the day."
Dan on Friday 2 July 2010
"Actually plants do a lot of their growing during the night, hence the advice to water in the evening. I think moulds and mildews are only usually a problem in damp or cloudy weather, rather than hot dry summer weather. I am more cautious about evening watering during spring and autumn/fall when things feel 'damper', so there is probably truth in both points of view but during hot summer periods watering in the evening seems to work the best."
Jeremy Dore on Friday 2 July 2010
"When I initially started the garden I was watering both in the morning and early evening. My Uncle, who has gardened all his life told me to stop watering in the morning because it's to stressful on the plants. So I only water around 6PM when the sun is not so strong and the garden is growing like crazy. And the pepper plants look so much better now that I am only watering once a day."
Ann on Saturday 3 July 2010
"Pardon me, but some of this advice is a public health disaster waiting to happen. Grey water should never be kept more than 24 hours and should not be used to spray. And, re the first poster, never EVER use black water, which is untreated sewage. As for worrying about detergent - well the build up of that in your soil will kill you more slowly than the salmonella/e coli/ giarda infection you're breeding. The micro-organisms you all mention will not have any effect of reducing the chemicals in the water but will, within 24 hours, have reproduced to levels harmful to humans. Grey water should be put directly onto soil, not the leaves, within 24 hours. Disperse it over a large area and rotate with fresh water to avoid build ups of sodium salts in the soil. Keep it away from leaves and anything eaten raw eg salads and root crops - the soil will act as a filter and won't provide a good home for pathogens to grow but spraying can cause pathogens to become airborne and be inhaled. The answer to the detergents worry is - use eco products and never dishwasher water or anything containing bleach, borax or softeners. "
Emy on Monday 23 July 2012
"Great Article! Grey water should definitely be re-used at least to water the gardens."
max on Friday 6 September 2013
"thank you so much "
aloosh.lafta on Tuesday 25 August 2015
"We know that slugs and snails are more active at night and i presume need damp conditions, so maybe watering in the morning would be less helpful to the slugs and snails. If they are out and about in daylight they will get eaten by the birds?? your thoughts?"
andy M on Sunday 1 July 2018

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