Houseplants: 'Plant doctor' warns TikTok hacks like polishing leaves with mayonnaise do more harm than good

Watering your plants with leftover pasta water or using coffee grounds as fertiliser might sound like an eco-friendly and sustainable choice, but experts warn it could do more harm than good.

TikTok's top trending 'plant hack' videos have now amassed more than 1.5 billion views between them. But 'plant doctor' Kelly Dyer, horticultural lead for British houseplant and garden supply company Patch Plants, has warned that many of the so-called hacks have little to no scientific backing - while others could bring with them mould or disease.

Others still are good ideas for some plants, but won't necessarily work well for all of them unless you take special care. And some, Ms Dyer says, border on the outlandish - including polishing your houseplant's leaves with mayonnaise to give them some extra shine.

Here are some of the most risky plant hacks she's seen on TikTok, and her advice on what you can do instead to get the desired effect without putting your plants at risk:

Alternative water sources

Giving your houseplants a nutrient boost by spicing up their water somewhat has proven quite popular on TikTok. Videos on using aquarium water have garnered nearly 275 million views, while using old pasta water (96.8 million) and even sparkling water (131 million) are also building considerable watch-time.

According to TikTok, if you have a freshwater fish tank, the water you remove during a water change contains valuable nutrients like nitrogen that plants can benefit from. But Ms Dyer warned it may actually be too high in minerals. “This depends on a variety of factors. It shouldn't be used if you have added any chemicals to the tank and it definitely shouldn’t be used if the water isn't changed frequently, because it may be too high in minerals and over-fertilise your plants.”

If you want to give this a try, she suggested only applying aquarium water to your plants once a month max - and only in the growing season - to avoid inadvertently poisoning them. Pasta water, however, is a whole other kettle of fish.

“There is no scientific research into this and I would take the balanced view that this hack is no substitute for watering with clean water and maintaining a regular feeding schedule with a balanced fertiliser," she said. "Also, pasta water is often starchy and would leave a residue on the soil surface of houseplants, which will look gross and lead to unwanted mould."

Some TikTok creators also suggest that sparkling water can help liven up houseplants by stimulating them to take up more nutrients from the soil, but Ms Dyer said there was no consistent or up-to-date scientific evidence to confirm this. Instead, she said providing fresh compost through re-potting and regular feeds was a much more tried-and-tested method.

Food for me, but not for thee

A number of popular plant hacks on TikTok seem to involve using common foodstuffs to enrich or enhance your houseplants. These include using sugar to promote growth (3.3 million views), using coffee grounds as fertiliser (4.8 million views), and even polishing leaves with mayonnaise (605,000 views).

However, Ms Dyer said using sugar could actually reduce your plant's ability to absorb water. “Plants don't absorb sugar through their roots - they make sugar themselves." she said. "They make as much as they need, using nutrients that they absorb from the soil, together with light and water. Adding sugary water to your contained houseplant's soil could actually reduce the plant's ability to absorb water. It could also lead to the build-up of nasty bacteria and attract pests.”

While coffee grounds can be a helpful addition to outdoor garden plants, inside they also risk creating a water barrier because of their fine particle size - and could change the pH of your soil. “I would actively advise not applying coffee grounds to your houseplants, and if you do, only do this occasionally. While it's true that coffee grounds contain some of the macro-nutrients that plants need, this should not be used as a substitute for a balanced houseplant feed," she said.

The grounds are also likely to sit on top of the soil surface, and unlike in your garden, there are no living organisms like worms to integrate the grounds into the soil. Coffee grounds in a contained vessel like a pot also carry the risk of changing the pH of your soil, making it too acidic for the neutral pH most houseplants prefer.

Ms Dyer said the "magic" in mayo was the vinegar, a common household cleaning agent, as well as the fatty oil and egg. But the mixture risks clogging your plant's stomata - little holes that open and close to release moisture and gases into the air. This could negatively affect your plant's health in the long term, she said.

"It is not worth it. Your plant's leaves need to be dust-free to photosynthesis and transpire effectively, but they do not need to shine. This is purely an aesthetic preference. Any houseplants that have naturally shiny leaves are best kept this way through ensuring their robust health through watering, potting and feeding.” There were many safer options available, including dusting, wiping with a damp cloth, and natural leaf shine sprays.

The sound of silence

Unlike other potentially harmful hacks, talking (56 million views) and playing music (171 million views) to your houseplants is harmless fun. But some of the claims about their alleged benefits being made on TikTok are wildly inaccurate.

Ms Dyer said if you try to promote growth via music, you may be waiting for a long time. “There is no consistent or robust enough research to suggest one way or the other. What scientific theory does suggest is that if there is a benefit, it is caused by the vibration of the sound waves stimulating cytoplasmic streaming [the movement of fluids that contain nutrients] through the plants, encouraging growth.”

There was no scientific backing for talking to your plants either, and she said claims online that it could increase the amount of carbon dioxide in the air available for your plants were a total myth. "The CO2 benefit of this theory is completely unscientific and illogical, you release no more CO2 from talking than you do from breathing," she continued.

However, she added that you should keep talking to them if it's something you enjoy. “Any benefit in talking to your plants is psychosomatic, and the powers of this should not be underestimated. Chat away, whisper your encouragement and glee at their growth alongside your acts of care like watering, feeding, training, pruning, and repotting.”

Soggy bottoms

Bottom watering is a useful care practice for some houseplant species, and it's really racking up the views on TikTok - more than 335 million of them. But although bottom watering is a good option for some plants, others could end up over-fertilised and with rotting roots if you don't take care.

“Because minerals aren't being flushed through the soil when bottom watering, if your plant doesn't have many roots or is not drawing up the water for active growth, it could end up sitting in wet soil, leading to root rot," Ms Dyer said. To avoid this happening, you should regularly check whether the soil just below the surface is wet.

However, if your plant is small and can be easily lifted, bottom watering can help make sure the soil is wetted evenly throughout the pot - and for the likes of spider plants, philodendrons, and peace lilies, it can actually help grow a stronger, more stable plant.

Ms Dyer warned you should always check your plant has lots of roots before attempting this.