Spider plant at East Finchley Station

Dear Readers, there was a very interesting article in New Scientist last week, addressing something that attained the status of holy writ when I was growing up, and I wondered if any of you remembered something similar. I was told that it was very bad to have plants, whether in pots or vases, in bedrooms at night, because of all the carbon dioxide that they emitted. Furthermore, flowers taken to hospital patients were said to be removed from the wards at night for the same reason.

At the time, I didn’t give it much thought – my O-Level biology taught me that plants take in CO2 during the day through photosynthesis, but at night they breathed it out via respiration, just like any living thing, so it all seemed reasonable. But my science degree this year got me thinking that there was something wrong with the argument, and James Wong’s article explained exactly why.

I occasionally indulge in cut flowers, but should they ever be in the bedroom?

First up, how much carbon dioxide do houseplants emit at night, compared to what they absorb during the day? Some scientists in Turkey popped some ficus and yucca into sealed boxes, and discovered that they absorb 6 to 8 times more CO2 during the day than they release at night. At night, a large ficus plant increased the CO2 content in the box by only 351 parts per million, well within the healthy range for any humans who happened to be locked in a sealed box with a houseplant.

Secondly, for those of us who share our bedrooms with other humans (or indeed other animals such as dogs or cats), we should maybe consider how much carbon dioxide they’re emitting. A single human breath emits 40,000 parts per million of carbon dioxide, which is more than ten times more than a ficus emits in eight hours overnight. As Wong says, if we’re concerned about CO2 maybe we should turf out our partners rather than our plants.

And finally, we don’t live in hermetically-sealed boxes (unless you live in a Passivehaus, which is about the closest that we come to such a thing, though these do have mechanical ventilation so it’s not a very close comparison). Indoor air is moved about by draughts, through leaky windows and opened doors, and even by the movement of humans around the house. The carbon dioxide produced by a plant overnight is not going to stay in the bedroom, but will dissipate under the doors, through the windows, up the chimney and even through the brickwork or floorboards if you live in a Victorian end of terrace like I do.

So, this is another tale that has little basis in actual scientific fact. Hospitals in the UK ban plants on the wards these days for a very different reason – the nursing staff are too hard-pressed to look after a pot plant, the water that cut plants stand in can become foul very quickly, and frankly there’s very little room for such things in the average NHS ward. But for me, there’s another reason for not having houseplants in the bedroom – pretty much every horizontal surface is covered in books. It was a tough call, but us bibliophiles have to make tough choices sometimes.