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Debunking the most common gardening myths
You've likely heard that coffee grounds deter slugs, vinegar is a good weed killer, raised beds are best for vegetables, and houseplants can purify the air, but are these long-held beliefs actually true? To mark National Gardening Week (29th April - 5th May), the Royal Horticultural Society (RHS), the UK's leading gardening charity, is on a mission to demystify the world of gardening, starting with the most common myths – and there are some surprising truths.
Guy Barter, chief horticulturist at the RHS, along with the RHS advisory team (who offer gardening advice weekly to its members), unpick some of the biggest misconceptions in the gardening world.
Gardening myth 1: Digging is necessary
Many gardeners believe you need to dig to improve soil fertility. But this is false, say the RHS advisory team, as digging releases soil nitrogen, damaging soil bio. 'Plants in nature don't need digging, nor do garden ones,' explains Guy. 'However, digging effectively reduces weeds and occasionally relieves compact [soil] and incorporating amendments.'
Gardening myth 2: Deeper soil is better
Another myth embraced by gardeners is the deeper the soil, the better. In reality, 'around 15cm is enough, 25cm at the very most,' Guy advises, adding: 'Those planting should use sharp sand rather than costly topsoil for deep beds.'
Gardening myth 3: You need to fertilise every spring
Fertilisers (concentrated sources of plant nutrients) are often used to improve plant growth. 'It is believed you must add fertiliser every spring. However, most plants don't need feeding,' Guy reveals.
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Gardening myth 4: Crocks in pots improve drainage
Gardeners have long believed that putting shards of pottery at the bottom of plant pots will increase drainage. It turns out this is false. The RHS advisory team explain: 'Disrupted capillary action and reduced soil volume mean they don't help. Instead, gardeners can use a crock to prevent the compost from dropping through the hole at the bottom of the pot.'
Gardening myth 5: Vinegar is a good weedkiller
Vinegar has many uses, but it turns out killing weeds is not one of them. 'Contrary to popular opinion, vinegar is not a good weedkiller as it won't touch grasses and only harms broad-leaved seedlings, not larger plants,' Guy reveals.
Gardening myth 6: Houseplants purify the air
Calling all plant parents! The mention of air-purifying houseplants will nearly always be referenced alongside the influential NASA study from 1989, which found that indoor plants could remove formaldehyde and other volatile organic compounds (VOCs) from the air. However, the study reflected a closed chamber, rather than our real-world settings.
'According to our research, houseplants do not purify the air,' says Guy. 'Unlike ventilated homes with sparse plants, the original study was carried out in sealed growth chambers crammed with plants. There are plenty of other good reasons to grow houseplants, though.'
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Gardening myth 7: Raised beds are best for growing vegetables
We all want our vegetables to have the best growing conditions, but while raised beds can help in wet regions and clay soils, 'they can lead to over-draining, are expensive, and use environmentally significant resources elsewhere', says Guy.
Gardening myth 8: Coffee grounds deter slugs
The warm and damp climate during spring creates the perfect environment for slugs in your garden. There's lots of advice online about the best natural methods to deter slugs, but very few actually work. 'The evidence for coffee grounds' ability to repel ants and slugs is poor, reveals Guy. 'The same applies to myths about eggshells, grit, pumice, and many other products.'
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Gardening myth 9: Tomato leaves need to be removed
It has long been believed that bottom leaves should be removed from tomato plants but this isn't necessarily the case. 'Even lower leaves contribute to the plant until they go yellow (when they can be removed),' says Guy.
Gardening myth 10: There is no such thing as over manuring
Guy explains: 'Although many believe there is no such thing as too much manure, organic matter, or compost, this is wrong. Over manuring leads to excess heavy metals, phosphorus, and pollution.'
Gardening myth 11: Avoid watering in the sun
A long-held misconception is that watering in the sun scorches leaves. 'Though it is best to water at ground level, scorching does not happen,' explain the RHS advisory team. 'Scorching to new plants from lack of hardening off is far more common.'
Gardening myth 12: Tree roots go as deep as the tree is tall
Tree roots form a shallow but broad 'plate' of roots and go wide instead of deep, explains Guy.
Gardening myth 13: You should plant holes in clay soil
A common myth among those planting in clay soil is that it is best to make a large planting hole to fill with grit. The RHS advisory team say that this actually creates a sump, so the roots get waterlogged and rot, therefore harming the plants.
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