United Kingdom 
Since the hot weather started i have been saving the washing up water in a water butt, the problem is when i water the garden with it, it has a terrible smell. Is there anything i can add to the water in the water butt to hide or freshen the water??
Many thanks for your help, will try the peroxide if it does not work on plants i can always dye my hair blond with it. :)
27 Jul, 2012
Answers
All kinds of bacterial and fungal growth can be expected if you keep gray water for any length of time. It doesn't usually hurt the plants, but it can lead to your neighbors chasing you around with torches and pitchforks. Anything you could add to help the smell would also hurt the plants.
27 Jul, 2012
If you add a small amount of hydrogen peroxide at regular intervals, when it starts to pong, it will kill the bacteria that causes the smell. You don't need much but give it at least 24 hours after treatment before using the water. I have been using grey water from our shower for years in this way. Oh, and don't add household bleach. It will kill the smell but makes the water unusable for months.
28 Jul, 2012
Previous question
« why have there been no flowers on my lavender this year? (not English lavender)
I always thought that when using ex-washing up water, it had to be used fairly quickly. I don't know of a way to store it satisfactorily, I'm afraid. But there ought to be a way.
27 Jul, 2012