MISTING the air around houseplants such as maidenhair ferns and pitcher plants has long been seen as essential to protect these species, which evolved in steamy rainforest understoreys, from the draughts and dry air of the average home. It is a practice that is so widespread, echoed in almost every indoor gardening book of the past century, it seems no one has stopped to ask: is there any evidence this actually works?
Standard advice is to give your plants a quick spritz between one and three times a week. However, given the average spray with a home mister contains just 1 millilitre of water, I have always been sceptical that 1 to 3ml of moisture a week can make a meaningful difference. The fact that not a single study on this in the context of a home appears to exist in the published scientific literature inspired me to take matters into my own hands. I bought a handful of humidity meters and got spritzing, stopwatching and spreadsheeting.
Advertisement
As I suspected, a generous spritz was effective at raising ambient humidity, but only for a short period. It immediately boosted the levels around a selection of plants in my warm living room from 50 to 70 per cent 鈥� the ideal level for some of my plants. However, within 30 minutes or so, this had returned to baseline. Just 10 minutes after I spritzed, almost half of the benefit had been obliterated. In my drier bedroom (35 per cent humidity), a repeat of the test showed similar results, but the return to baseline happened in just under 20 minutes.
Now, although I repeated both trials three times to ensure they were consistent, the results would vary enormously depending on the size of the room, its ventilation, its temperature and so on. But my experiment suggests you would have to mist not three times a week, but closer to three times an hour to make any meaningful difference. Plants that need high humidity are therefore best grown in terrariums, whose glass walls are designed to seal moisture in, or sited in groups on trays of pebbles filled with water, which continually evaporate moisture over a few days.
But do my colleagues in horticultural science agree? , the Royal Horticultural Society鈥檚 director of science and collections, says: 鈥淭here are many external environmental influences created by growing a plant in an indoor room, which are highly likely to override any improvements in the humidity surrounding your houseplants by misting them a couple times a week.鈥� , head of horticulture at the University of Leiden Botanic Gardens in the Netherlands, adds that indoor gardeners 鈥渟hould just pick plants that require lower humidity rather than permanently risking ruining their books and wallpaper鈥�. Horticulturists aren鈥檛 known for sugarcoating their facts.
James Wong is a botanist and science writer, with a particular interest in food crops, conservation and the environment. Trained at the Royal Botanic Gardens, Kew, in London, he shares his tiny flat with more than 500 houseplants. You can follow him on Twitter and Instagram @botanygeek
For other projects visit newscientist.com/maker
