Colouring flowers – opening up the world of capillary action

Lately we have looked at how water behaves and even approached the topic of capillary action here. However, I really like this experiment explained here because the results are so clearly visible. I’ve only gotten it to work with blue colour. I am a bit unsure as to why, but I think it is most likely due to the size of the colour molecules. I will research this. But blue works.

Basically for this experiment you need white flowers, water and some blue food colouring. Add the blue colour into the water and place the flowers in it and let the capillary action colour the flowers blue (see image below). This happens within a few hours, but in the image below I’ve let the flower stay in the coloured water for a few days to get that deep blue colour.

Colouring white flowers blue using capillary action

Experiment colouring flowers using capillary action. Top left: original flower with white flowers and green leaves. Bottom left: both leaves and flower are starting to be blue. Top right: leaves are clearly coloured. Bottom right: flower is more blue than white.

So what happens? Plants have very small veins running through them through which water flows up from the roots to the flowers and leaves. It flows because the plant vaporises water from the leaves and flowers. Because some water leaves the plant and water molecules like to hold on to other water molecules, every time water evaporates from the leaves, more water flows in its place from below. This is how trees get water from the roots up against gravity to the leaves. As water is carried upwards to the leaves and flower, it takes some blue colouring with it. The water then evaporates, but the blue colour does not, colouring the flower and leaves blue.

Wikipedia explains capillary action (also known as capillarity, capillary effect and capillary motion) as the ability of a liquid to flow in narrow spaces  sometimes in opposition to gravity. It happens if the diameter of the tubes are sufficiently small as it is in plants. Also the walls of the tubes need to be such that the liquid can actually flow upward. Too much friction can prevent this from happening. But if the walls of the tubes are suitable, then the surface tension of water (water molecules hold tightly onto other water molecules) lifts the water upwards. This same effect can be found in porous materials like paper, hand towels and the hairs of a paint-brush.

My daughter sometimes asks for blue roses. Previously I have replied that roses do not come in blue. However, I think this is the time to try and make her wish come true as this is the way it can be done.


If you try these experiments at home and your child asks questions, please let me know what types of questions they ask in the comment section so I can further develop these instructions. If you have questions about these experiments or instructions, leave me a comment and I will answer and also improve these instructions. Also, please remember like and share if you find this useful.

Copyright text and images: Satu Korhonen. You are free to try these experiments out, use them in your teaching. But instead of copying the text or images, link back to this page.

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