Written by Spirekassen 29 Jun 2023 08:31

Rainwater hygiene


The summer is upon us, and your greenhouse requires daily watering. Tap water can be expensive, but perhaps you already have a rainwater butt, to collect rainwater, or you are considering getting one. It makes a lot of sense, especially due to climate change. There is 25% more rainfall in winter than we are accustomed to. Additionally, the pattern of summer rainfall has also changed. While the overall amount of rainfall remains relatively constant, the manner in which it occurs has shifted. We now experience more frequent heavy rainfall with a significant downpour all at once.


Rainwater is not just water

Rainwater and groundwater are two very different things, and the ways in which these types of water can be used differ significantly as well. It is important to understand a few simple principles regarding how rainwater is managed before using it for your vegetables.



There are a few things you should know.

During the water's journey from the clouds to your roof, the water encounters particles from the atmosphere, such as pollution from car exhaust pipes. As the rainwater encounters the roof and flows into the rainwater tank, it becomes more than just fresh rainwater. The water that ends up in your rainwater tank should NOT be considered clean water.

There are often algae on your roof, bird droppings, and potentially lead elements in the roof construction. Additionally, there may be zinc on the roof or zinc roof guttering on your house. During rainy weather, the rainwater will collect unwanted particles from the roof, which will also end up in your rainwater butt. The Danish Environmental Protection Agency recommends avoiding the collection of rainwater in certain locations:

  • Roofs with a new bitumen coat (roofing felt).
  • Grass-, moss-, or thatched roofs.
  • Copper roofs or copper roof guttering.
  • Roof constructions containing asbestos.
  • Roofs that are particularly exposed to bird droppings.

(Source: Facebook of the Danish Ministry of Food, Agriculture, and Fisheries: Fødevarestyrelsen; please note that the post and video are in Danish) 


Remember the good hygiene

Our rainwater has the potential to contain unwanted chemicals that could end up in our vegetables when we water them with rainwater. Furthermore, rainwater can actually serve as a real source of infection for humans, primarily due to birds. Crows, seagulls, pigeons, and thrushes, in particular, are of interest regarding the pollution of our rainwater. Additionally, everyday items like leaves and other debris can cause a significant increase in microbiota within your rainwater butt. To ensure a good start, it is important for your rainwater tank to have an effective filter that removes debris before the water enters the tank.


The problem with infections

There are approximately 50,000 annual incidents of Campylobacter infections in Denmark and about 50,000-60,000 in England (Source: gov.uk). Several of these incidents (the exact number is unknown) are attributed to the use and handling of rainwater when watering vegetables. It has been proven that the bacteria typically have a lifespan of five to ten days in the rainwater tank, although longer lifespans have been observed. Additionally, there is an increase in bacteria, viruses, and parasites at temperatures ranging from 30 to 38°C. If your rainwater butt is exposed to the sun, it easily and quickly heats up to a temperature that bacteria thrive in.

Campylobacter infections often occur because many garden owners’ water their salad, radishes, peas, chilies, and tomatoes with water from their rainwater butt. It is actually recommended that you only use rainwater for watering flowers and vegetables like root vegetables and potatoes, which can be peeled and boiled.


Good advice.

Here is some good advice from The Danish Environmental Protection Agency:

  • Clean your rainwater tank once a year (preferably more often)
  • Water only crops, that can be peeled and boiled.
  • Do not let children handle or play with rainwater.
  • Wash your hands after handling rainwater.
  • Place your rainwater butt in the shade.

(Source: the Danish Environmental Protection Agency: Miljøstyrelsen; and their report from 2003: risk assessment of the use of water in private gardens collected from the roof (Risikovurdering af anvendelse af opsamlet tagvand i private havebrug 2003); please note that the sources are in Danish)


Collect water from the roof of your greenhouse

To avoid the risk of contamination, collect rainwater from the roof of your greenhouse where it won't encounter copper, asbestos, or roofing felt. According to the Danish Ministry of Food, Agriculture, and Fisheries, there is no problem if the water runs over aluminium that is coated or lacquered. (Source: the Danish Ministry of Food, Agriculture, and Fisheries: Fødevarestyrelsen)

Keeping the greenhouse roof clean is easy. Make it a habit to wash the roof a few times a year.

Always place your rainwater tank where it will receive the least amount of sun exposure.

Never overspray your vegetables with rainwater. Instead, water close to the roots to minimize the potential spread of infections to the vegetables.