People like to believe some really mad things about gardening. Here’s three recent howlers

A s a scientist who is fascinated by horticulture, I am often deeply torn by gardening advice. Sometimes, I feel we are incredibly constrained by established rules that lack any real basis in evidence and which can not only put people off growing but actively set them up for failure. Breaking free of these, and embracing the fact there are as many gardening techniques as there are gardeners, is therefore essential to progress the art form.

However, just when I start putting myself firmly in the “horses for courses” camp, something inevitably comes up – usually starting on social media, then swiftly recycled in slots on daytime telly – that gets the geeky pedant in me exploding in Hulk-like fury. Perhaps nowhere is this more the case than in the world of houseplants. So, in order to protect my screens at home from my urge to throw objects at them, here are the top three howlers I have seen this season.

First up: watering your plants with the cold water left over from cooking rice will get rid of fungus gnats. In fact, as their name suggests, fungus gnats live on the mould that can grow on the surface of compost. Adding a bunch of easily digestible carbs in the form of rice starch will actually cause a boom of fungal growth and also the gnats you are trying to get rid of. Simply adding a layer of gravel to the surface to prevent this growth, or using a soil-based (rather than compost-based) growing medium would be just as easy – and far more effective.

There is also a claim that wiping the inside of banana skins over the surface of your plant leaves makes a great substitute for leaf shine. Let’s examine why anyone thinks they need to use leaf shine in the first place. This lacquer spray of waxes (and sometimes silicone) was hugely popular in the 1970s and 80s as a way to give the foliage of large-leaved plants a patent-leather shine. At best, it doesn’t have any effect on plant health, at worst it can clog the tiny pores they use to breath and really hamper their growth. Banana skins contain water, sugar and starch grains, so the thin layer of moisture they add can induce a temporary shine for a few seconds, but the second this dries it creates a sticky surface that actually traps dust and reduces shine. Total waste of time.

Finally, there is a lot of talk about which plants are appropriate for use in the bedroom, based on the idea that some can suck the oxygen out of a room at night and harm the health of those sleeping within. Now, if this was true, camping in a forest would be deadly. And given the tiny amount that houseplants respire, and the fact that bedrooms aren’t hermetically sealed chambers, the negative effect they have would be less than 1/1000th that of sharing your bed with someone. So have whichever ones you like and as many as you want.

Follow James on Twitter @Botanygeek