Gardening is full of received wisdom that is treated as gospel and handed down across the generations – from putting a layer of crocks at the bottom of pots for drainage, to the back-breaking work of Victorian “double-digging” to improve soil structure. But when tested scientifically much of this old-school advice turns out not to be supported by evidence. In fact, in the above two examples, they are actually likely to give you worse results than if you simply hadn’t bothered at all.
Even scientists aren’t immune to repeating received wisdom, or potentially extrapolating more from the data than it actually shows, particularly if the claim supports our existing views. However, the wonderful thing about science, unlike gardening dogma, is that it is forever changing as new evidence comes to light. In fact, as a botanist, I think the freedom to change one’s mind, to hold your hands up to getting it wrong, is science’s greatest strength – particularly in 2020. So I am starting, in my own small way, right here.
Five years ago I wrote a column in this very magazine about how houseplants can purify the air, based on research carried out by Nasa. Since then, there has been a slew of online articles, not to mention industry campaigns and even new gadgets, centred on this claim. The only problem with it is that more recent and better quality research has found this to be extremely unlikely.
So what was wrong with the initial study? Well, nothing really. Being one of the first studies to test this hypothesis, it measured the ability of a range of houseplants to remove three common indoor pollutants from small, sealed chambers, and it did indeed turn up some positive results. Plants do clear these toxic compounds from the air.
But living rooms and offices are not tiny, sealed chambers in labs. They are subject to constant inflow and outflow of air. So how does the ability of plants to clear these compounds from the atmosphere compare with, say, just opening a window – especially when living rooms are typically nowhere near as crammed full of greenery as a tiny experimental chamber filled almost entirely with houseplants?
A 2019 review at Drexel University, Pennsylvania, of the dozens of subsequent trials including the Nasa study, attempted to tackle this question. It found that you would need between 10 and 1,000 plants per square metre to remove the same amount of these pollutants as occurs with natural air exchange. In the Palm House at Kew Gardens, plants may well be doing an excellent job at mopping up toxic gases, but in your living room it is very unlikely.
However, other research shows that having plants indoors has a range of other benefits. They can boost productivity. They can improve mood. They can regulate humidity – all on top of looking beautiful. If you want fresh air, open a window. If you want to witness the joy of nature and feel a daily sense of wonder, get some houseplants.