13 Jun 2022 21:11

The amazing world of water


Text & photo: Christine Wiemann


Water is amazing, but also more expensive nowadays. Every year there is a recurring task in our greenhouse – irrigation. This article is about water and the properties of water.


Water theory for the nerds

The water molecule H2O contains three atoms held together by chemical bonds called ‘covalent bonds’.

The water molecule is a dipole, which means that the molecule at one end has a positive charge, and at the other end it has a negative charge. The small water molecules attract each other due to the positive and negative ends just like a magnet. The attraction gives the water molecule a strong bond to the other water molecules, and they are connected like beads on a string.

It is easy to move one’s hand through water in a tub. The strong bond between water molecules can best be illustrated by pouring water between two glass plates. The bond will be so strong that it can be difficult to separate the plates from each other. And this strong bond also exists in the greenhouse.


Drip irrigation

Drip irrigation systems for the greenhouse are also making use of the unique magnetic properties of water. Here the capillary action is used. The capillary action is a natural phenomenon that works against gravity. This happens almost automatically since the properties of water molecules are quite unique with their positive and negative ends. First, the water is pulled upwards to the water tank. Then the water continues up through tubes with a large diameter. Like beads on a string, the water is on its own pulled through small drippers. Since the water molecules cannot be pulled apart due to the positive and negative charges, there is only one way for the water molecule which is up. Again, the water is pulled like beads on a string through the drippers and then into the soil.



  • It is an advantage to use drip irrigation for pots and plant hangers. These containers will dry out quickly on warm summer days.
  • Drip irrigation is ideal for tomatoes and cucumbers. But try to avoid placing the dripper too close to the main roots of the cucumber plant, since this part should be kept dry.
  • Peppers and chillis are better off if they are let to be dried out in between irrigations. When the plant experiences water stress, it will automatically start producing more fruits. Therefore, it is wise to remove the drippers so the plant can dry out in between irrigations.


Water is not just water

There is a great difference between tap water based on groundwater and rainwater. It is important to know the difference. The main difference between rainwater and tap water is the pH-value of the water and the content of calcium.

In Denmark and in several places in Germany and England the tap water contains calcium if it is collected through the groundwater. It is the same with the coffee machine when it becomes calcified. Due to calcium the tap water is slightly alkaline with a pH-value of 7 - 8,5.

Rainwater does not contain calcium. When it rains, the raindrops collect impurities from the atmosphere when falling. One of these impurities are for instance atmospheric carbon dioxide also known as CO2. At last, the rainwater will end in our water butts. Because of the collected CO2, the pH-value is less than 7, making the rainwater slightly acidic. With a low pH-value, the water is poor in nutrients.


But does it matter whether the water is acidic or alkaline?

Yes, it will affect your plant cultivation. The liquid and solid forms of fertiliser for the greenhouse are made specifically for slightly alkaline tap water and not for rainwater.

If only rainwater is used

If you only use rainwater, the plants will start looking like they need fertiliser, despite that fertiliser is already in the irrigation water. The reason for this is that liquid fertiliser cannot work effectively in rainwater that is slightly acidic. In fact, many nutrients cannot be absorbed by the roots when the water has a low pH-value.

You can for example observe the tomatoes by the end of summer. You may have seen that the tomatoes have gotten black spots at the bottom of the fruit – this is called blossom rot. The plant indicates that the plant roots struggle to absorb calcium. The reason for this is that the irrigation water has a low pH-value. The problem becomes more visible if rainwater is the only kind of water that you use in your capillary boxes.



  • A solution could be to add more calcium directly to the soil. This way the pH-value will increase.
  • If you use drip irrigation and you want to irrigate with rainwater, I recommend using a mix of rainwater and tap water. By doing this the water will have a pH-value that makes it possible for your plants to make use of the fertiliser that you add.


Collecting water and shading

Save time and make the warm greenhouse days easy with an irrigation system

Shading in the greenhouse helps to keep a healthy climate in the greenhouse. Shading blinds prevent blossom rot when cultivating tomatoes and the plants will need less water.