Old Wives Tales


Gardening abounds in wisdom and advice handed down from generation to generation. But how much of it really works?

“Coffee grounds keep slugs at bay”

The theoryCaffeine is toxic to slugs and snails, and mulching with coffee grounds therefore deters these garden pests.

The evidence Researchers have shown that spraying plants with a caffeine solution equivalent to a strong cup of coffee does kill slugs, but there is currently no direct evidence that coffee grounds have the same effect. Most of the caffeine in coffee grounds is removed when it’s brewed, so the amount of caffeine in spent coffee grounds is much lower than the amount needed to kill slugs. The granular texture of coffee grounds may deter slugs, but in a trial of granular slug barriers, they weren’t found to be particularly effective, not least because they are time-consuming to apply and have to be topped up every time it rains.

The verdict False: coffee grounds may not deter slugs, but they are high in nitrogen, so put them on your compost heap instead.

“Pea and bean roots left in the ground improve the soil”

The theoryNodules on the roots of peas and beans (legumes) harness nitrogen from the air and make it available to plants.

The evidence Research has shown that almost all the nitrogen gathered by bacteria in the root nodules of legumes is passed straight into the plant. By the time legumes are in flower, most of the nitrogen is in the leaves and developing pods. So the only way nitrogen will benefit future crops is if the whole plant – including seeds, pods, leaves, stems and roots – is allowed to rot back into the soil.

The verdict False: picking peas and beans removes most of the nitrogen that was gathered by bacteria in root nodules. Put spent plants on your compost heap to harness any nitrogen left in the leaves.

“Tea is a good fertiliser for pot plants”

The theoryTea contains nutrients and trace minerals that promote healthy growth in plants.

The evidence Dried tea leaves contain about 4.4% nitrogen, 0.24% phosphorus and 0.25% potassium. There is significantly more nitrogen in tea leaves than in most liquid  pot plant fertilisers that are formulated for healthy, balanced growth. While nitrogen promotes leafy growth, it is unlikely that much of the nitrogen in tea is actually available to plants. Other elements in tea could be harmful. It contains aluminium, fluorine and manganese, which are harmless to people but high concentrations in very strong or stewed tea may retard plant growth.

The verdict False: regular watering and an occasional liquid feed are better for plant health than relying on tea. Used tea leaves are best added to your compost heap.

“Sing to your plants to help them grow”

The theory – Singing, talking or playing music to you plants will make them happy & help them to grow.

The evidence Dorothy Retallack conducted some fascinating trials on the effects of music on plants, and then published a book called The Sound of Music and Plants in 1973. “Easy listening” appeared to promote growth in plants whilst rock, such as Led Zeppelin, appeared to hurt them. Strangely it also increased their water demand.In a repeated controlled experiment, she left out the plants altogether and simply played music to beakers filled with water. She found that rock music increased the evaporation, presumably due to the rippling caused by the vibrations.

The verdict Maybe true!: There is no firm proof, but if you enjoy talking or singing to plants it seems possible that it might benefit your plants too. Consider avoiding heavy rock!

“Gravel in the base of pots will help drainage”

 The theory – Placing a layer of gravel over the drainage hole at the bottom of the pot before adding potting mix will keep the hole clear and prevent waterlogging.

The evidence American research has found that when you put gravel in the base of a container, you simply make the smaller volume of compost more waterlogged. In fact, the water in the compost does not move easily from the finely textured compost to the coarser textured gravel, compounding the drainage problem.

The verdict False: The conclusion is that drainage is better if you use compost throughout the container. If you want better drainage, just mix perlite or grit throughout your compost to open it up.

“Ash from your fireplace will encourage flowering & fruiting”

The theory – Adding ash from the combustion heater or fireplace to your garden will produce lots of flowers and fruit.

The evidence Wood ash contains potash or potassium, which help plants to fruit and flower. It is alkaline, and is easily and quickly taken up by the plant. Tomatoes & some fruit trees respond especially well to the addition of woodash to the soil.

 The verdict True: Woodash is well worth adding to your soil as a boost to flowering and fruiting.

“Plant your passionfruit vine on top of a lambs liver & it will thrive”

The theory In the olden days every passionfruit vine was planted on top of a lamb or sheep’s liver, ox heart, or some other piece of offal, to provide iron.

The evidence Liver is nature’s most potent superfood. It (& other organ meats) act as storage for many important nutrients. Liver does contain iron. It also contains good levels of calcium, and high levels of both phosphorus & potassium.

The verdict True: Passionfruit vines are gross feeders. Stick the liver at the bottom of the planting hole, cover it up a bit, then plant the passionfruit as normal. The Liver breaks down over time to provide a rich source of nutrient for the growing vine. If you don’t want to use liver or offal, pelletised chicken manure (Organic Xtra) is a great substitute.

“Bury a rusty nail in the ground near a hydrangea & it will turn blue”

The theory – According to folklore, all it takes to change a hydrangea’s colour to blue is to bury a rusty nail in among the roots.

The evidence Hydrangeas actually do not change colours due to the presence or absence of a rusty nail or other rusting metal in the soil. Many hydrangeas may change their colour from pink to blue without having rusty nails buried in the roots of the plant.

The verdict Kinda True! The colour of a hydrangea is a factor of the soil’s pH and the amount of aluminium in the soil. A rusty nail changes the pH of soil so it is more acidic. This is why rusty nails, saw blades, tin cans or other forms of tin buried among the roots of the hydrangea shrub seem to change the colour of the hydrangea to blue. Planting a hydrangea bush near a cement walkway will have the opposite effect. The limestone leeching from the cement will make the soil more alkaline and can turn a hydrangea pink.