Does Using Grey Water to Water One’s Plants Affect Their Growth?




Scientific explanation of grey water's affects on plant growth. Quick and easy to follow.

Step 1: Background Information:

Today we are faced with two significant issues in society: the overuse of the finite supply of water in the world and the cost of living. Many households save both water and money by recycling grey water on their garden. This means watering ones garden with old dishwashing water, shower water or laundry water.

However, does the soapy water negatively affect the plant’s growth and development. Some sources say that “plants have been scientifically proven to grow better with grey water” [1] . However some other sources suggest that it could harm the plant and that normal clean water should also be used.

“the potential harm is less to the plants than to the soil, with its complex bacterial and fungal make-up. Recycled soapy water (‘grey’ water) is perfectly usable for plants, but I would not use it exclusively. Rotate it with ‘clean’ water on an alternate basis and, as with all watering, direct it at the roots rather than the foliage.” [2]



Step 2: Aim

To find out whether using a solution of detergent and water to water plants will negatively effect the growth of that plant.

Step 3: Hypothesis

The more detergent used whilst watering the beans, the less these beans will grow. All the beans will show some growth.

Step 4: Equipment

- 9 Sauces

- 18 Lima Beans

- 72 cotton wool disks

- 20mL Syringe

- Cold Water

- Washing Detergent

- Ruler

Step 5: Method

1. Place all 9 saucers next to each other near a window (ensuring they will receive the same amount of sunlight)

2. Layer 4 disks of cotton wool on each saucer.

3. On each saucer, place 2 beans on the first layer of cotton wool approximately 3 cm apart.

4. Place another 4 disks of cotton wool to create a second layer of cotton wool on top of the beans.

5. Clearly label each saucer so that the concentration of detergent they will be watered with is easy to identify. There should be 3 saucers labelled 10mL, 3 saucers labelled 5mL and 3 saucers 0mL.

6. Immediately water the beans on the 3 saucers, labelled 10mL, with 20mL of water, and 10 mL of washing detergent measured with a syringe. When measuring with the syringe ensure that you are at eye level and measuring to the bottom of the meniscus.

7. Immediately water the other 3 saucers, labelled 5mL, with 20 mL of water and 5 mL of washing detergent measured with a syringe.

8. Immediately water the last 3 saucers, labelled 0mL, with 20 mL of water, measured with a syringe. These are the control.

9. Observe results daily. And when possible record length of beans.

10. Record all results

Step 6: Variables

Variables that need to be controlled and how I controlled them.

- Amount of sun/shade, I have placed all the saucers in the same area of the same room for the same amount of time. This ensures that all of the beans receive the same amount of sunlight as well as all the beans are grown in the same room temperature.

- Amount of water, Each beans on the saucers will receive 20mL of water daily with an additional 0, 5 or 10mL of detergent daily, this will be measured with a 20mL syringe.

- Amount and uniformity of cotton wool, I deliberately used cotton wool discs instead of cotton wool balls as they are all the same. I will use 8 discs of cotton wool per saucer. All the cotton wool is the same brand and from the same box.

- No air in syringe, before drawing up the liquid, I pushed the syringe all the way down as well as I held it underwater to let all the air escape. This ensures that the amount of liquid that the syringe says is drawn up is that same amount as the liquid in the syringe.

Step 7: Observations

The cotton wool absorbed water incredibly quickly, and after my first day I had decided to double the amount of water, and concentration of detergent to make sure the cotton wool was soaked. After 2 days the colour of the cotton wool on the saucers labelled 5mL and 10mL started to go yellow. The colour of the cotton wool with 10mL of detergent was undoubtedly stronger than the colour on the cotton wool with 5mL of detergent. My control beans grew much faster and stronger than the beans watered with 5mL of detergent. The beans watered with 10mL of detergent did not grow at all. At the end of the experiment the beans on the saucer labelled 5mL grew small white stems, just like my control bean had done after 1 or 2 days. I also observed that the beans watered with 5mL of detergent grew mould, whilst the others did not.

Step 8: Results

I used the above image as guidelines towards the growth of my plant.

Control (0mL of detergent) : Stage at beginning of experiment = 0

Stage after 5 days of experiment = 2

Stage after 11 days of experiment = 4

5 mL of detergent : Stage at beginning of experiment = 0

Stage after 5 days of experiment = 0-1

Stage after 11 days of experiment = 2

10 mL of detergent : Stage at beginning of experiment = 0

Stage after 5 days of experiment = 0

Stage after 11 days of experiment = 0

Image without stage labels found at

Step 9: Further Results

Step 10: Total Length From the Roots to the Leaves of the Beans at Conclusion of the Experiment

A) Total lengths of Lima beans watered with 20mL water were, 9cm, 2cm, 3cm, 7.5cm, 3.5cm and 8.5cm.

Therefore the average length for the control beans was 5.6 cm.

B) Total lengths of Lima beans watered with 20 mL water and 5 mL liquid soap were 1.5cm, 0.5cm, 0cm, 1cm, 0cm and 0cm.

Therefore the average length of Lima beans with 5mL added liquid soap was 0.5cm.

C) Total lengths of Lima beans watered with 20mL water and 10mL liquid soap were 0cm, 0cm, 0cm, 0cm, 0cm and 0cm

Therefore the average length was 0cm.

Step 11: Conclusion

I successfully followed my method but discovered that my hypothesis was only partially correct as I stated that all the beans would grow. Even the slightest measurement of washing detergent will negatively affect the growth of the plant. Once the concentration reaches 1/3 detergent the beans will no longer grow.

Step 12: Discussion

During my experiment I have found out that the more soap in the water the less absorbent the cotton wool was. This could have altered the results as the cotton wool was not absorbing as much water and therefore the bean was not receiving as many nutrients. If I were to redo the experiment perhaps I could put fresh cotton wool discs daily.

I also discovered that the beans watered with 5mL added detergent started growing a mould like substance although the beans watered with 10 mL added detergent did not. I do not know why it was only on the 5mL detergent saucers that grew anything whilst the other two remained clear. Further experiments may answer this. This is also interesting because the detergent is anti-bacterial and I thought that this would prevent the mould like substance growing. This is shown on the 10ml added detergent beans.

Even though I tried to control the amount of sunlight as my variables by placing all the beans in the same area, I did not think about the distance from the window would make such a difference.

The beans furthest away from the window grew the largest while the beans in between the window and furthest away from the window grew the shortest. I think this is because the beans received too much sunlight. However this did not affect the results of my experiment as I had three versions of each concentration of detergent. Layout closest to furthest from the window, so that when I take an average they all get the same amount of sunlight.

Step 13: Bibliography

All your plants, even your natives, love greywater. Available:

Last accessed 5th Mar 2014.

Don M. (2010). Is it Wrong to Hydrate my Plants with Soapy Water .Available: Last accessed 5th Mar 2014.

Elaine p. (). Information About Beans in Cotton Balls. Available: Last accessed 5th Mar 2014.

Scholasticus K. (8/13/2012). Life Cycle of a Bean Plant. Available: Last accessed 5th Mar 2014.

Williams J. (). How to Grow Bean Seed in Cotton & Water. Available: Last accessed 5th March 2014.

Scientific Method Contest

Fourth Prize in the
Scientific Method Contest

Spring's Coming Contest

Participated in the
Spring's Coming Contest



    • Growing Beyond Earth Maker Contest

      Growing Beyond Earth Maker Contest
    • 1 Hour Challenge

      1 Hour Challenge
    • Backyard Contest

      Backyard Contest

    14 Discussions

    Tecwyn Twmffat

    5 years ago

    Great stuff! I'd def try it again with more representative concentrations though. I water seedlings myself with grey water and it seems to be oh Kay. Not only is there detergent in my water – quite a small amount, but also a small amount of urine! Voted.


    5 years ago on Introduction

    Consider that plants, even germinating seedlings, need oxygen (and water) to carry on the process of respiration necessary for growth. The soap level may have acted as a surfactant, keeping any oxygen from being available for the bean seed. Just a thought.


    5 years ago on Step 13

    Years ago in Texas when potassium wasn't removed from everything, grey water was emptied from the washing machine onto the backyard canna lily patch. we also had canna's in the front yard, these grew to about 4 ft tall. The ones receiving the grey water grew to over 12 ft tall. Washing detergent is different than dish soap, even back then. Maybe you should do this again using washing detergent.

    1 reply

    Reply 5 years ago

    interesting ... Maybe I will do another experiment


    5 years ago on Step 13

    Oh and almost forgot, dish soap is used almost universally as a quick and cheap wetting agent for applying liquid fertilizer and weed killers, just a few drops in a gallon effectively improves the coverage and 'wets' the sprayed items so much better it can be seen with the naked eye.


    5 years ago on Introduction

    Excellent idea for an experiment. The only comments I would have about your procedure would be the concentrations of soap you are using. The question of the experiment is about grey water effect on plant growth. I'm not sure about how much soap they use when doing a load of laundry at your house, but I know at my house we are not using a couple gallons of soap per load of laundry, but that is the kind of concentrations you mentioned in your experiment. In your 10ml experiment you are talking about a 50% concentration of soap, and on your 5ml experiment you are still dealing with a 25% concentration. Still much much higher concentration than ANYONE would routinely use for household use.

    The reason grey water is thought to be beneficial to plants is due to the phosphate content of many soaps, which is one of the 3 major plant nutrients that are included in commercial plant fertilizers. The other two being potash and Nitrogen.

    But just like putting too much fertilizer on a plant out in the garden will burn it up and hinder growth, the same thing here. I would think you should try the experiment again, except this time instead of 25 and 50 percent concentrations, try something closer to 1 percent concentrations or even less.

    Also, you need to make sure that the soap you use does include phosphates, because I believe there is a pretty strong push on now to reduce the amount of phosphates used in soaps due to the effects of high phosphate waste water has on waterways when discharged, causing overgrowth of algea in the water, caused for the same reason that your plants like the grey water.

    Anyway, with all of that said, I think it is a great idea for an experiment and I believe you will get some different results with those changes in concentration.

    1 reply

    Reply 5 years ago

    Thank you for taking the time for reading and commenting.


    5 years ago

    Thank you


    5 years ago

    A well performed experiment.Yay!


    5 years ago on Step 13

    This was a very interesting read, and I am very grateful you shared your results with us. I think your methods were excellent and I was also surprised by the results you weren't expecting. Thank you for sharing this with us!

    1 reply

    Reply 5 years ago

    Thanks for the feed back?