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Blue, a trick of light

Updated: May 13, 2021

As bluebells start to carpet the woodland floor, did you know that less than ten percent of flowering plants are blue? With blue such a relatively rare colour in the plant kingdom, you’d be forgiven for thinking it was of no importance to plants. But it turns out that the colour is actually key to their survival.

We all get that plants absorb energy from sunlight and turn it into food to grow. But they don’t absorb all light, only the colours in the spectrum that help them the most, the visible spectrum to us is red, blue and green. Red and blue light contain more energy, so plants absorb these colours. The colour which isn’t absorbed is reflected back, and this is what we see when we look at plants. So leaves are green because green is the colour the plant doesn’t take up from sunlight, whereas blue light is vital.

This is the opposite of the animal kingdom as animals can absorb colour from their environment and reflect the colour of the food they take in (rather than reflect the colour they don’t absorb, as plants do). Flamingos, for instance, are born grey but turn pink from the shrimp they eat, and who can forget the kid that turned orange after drinking too much Sunny D. (or maybe I’m showing my age)

So when we look at plants we’re seeing the light they are reflecting, rather than the light they take in. And blue flowers? Turns out, that plants can’t actually produce blue flowers. Instead the ‘blue’ we see in bluebells, forget-me-knots and borage is actually red. 'Blue' is achieved through tweaking this red with pH, pigments, molecules and ions. This is why there is lots of variations of colour when we see a carpet of bluebells, and why we could argue that blue-bells are more purple-bells.

Given that blue is not a straightforward colour for plants to produce, why do they bother at all? Well, plants which aren’t reliant on pollinating insects don’t bother, none of them have blue flowers. But it turns out that bees and other pollinating insects have a preference for blue because of how they see light. So if the plant has figured out the trick of turning blue, then it may give them an advantage.

So blue light is key to a plant's survival and blue flowers help them attract pollinators.

Birds, in case you're wondering, prefer red.

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