How St Patrick’s Day became green

Why are plants (and St Patrick’s Day) green?

St Patrick’s Day — a celebration of Irishness or, in other words, the traditional excuse to duck down to the pub for a pint of plain. It’s a day of green – a nod to Ireland’s association with the colour which goes back hundreds of years. Ireland’s national colour stems from our close ties to the green landscape back home on the Auld Sod.

But why is that landscape green? Come to think of it, why are plants green in general and not any other colour?

You’ve probably heard that plants are packed with chlorophyll — a molecule which acts like an antenna for absorbing light and generating energy. Chlorophyll absorbs blue and red but reflects green — so green is the colour we see on leaves and plant stems.

The funny thing is that green light carries a lot more energy than red light. Plus the sun emits more photons of green than any other colour. So if plants absorbed green instead of reflecting it, they’d be able to harvest a lot more energy from sunlight.

It might seem inefficient for plants to miss out on the rub of the green, but this is actually a kind of protection. Too much energy can destroy the plants’ pigments, or even the cells directly — kind of like plant sunburn. So plants reflect the intense green light rather than absorbing it.

Inspired by nature, at ACES we are developing new solar technologies which absorb as much light as possible, at all colours. We’re using this solar energy for lots of sustainable technologies, such as converting the greenhouse gas carbon dioxide into useful methanol fuel.

So let’s all lift a glass to a green future. (And coming from an Irishman, just please for goodness sake don’t spike it in your beer).