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Potty: line your containers with gravel
They say:听Improve water drainage by covering holes at the bottom of pots with broken pot shards and a layer of gravel
The advice, which may even be on the label your plant came with, makes intuitive sense. But it disobeys the laws of physics. Following it could end up waterlogging the soil and suffocating your plant鈥檚 roots.
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It all comes down to water鈥檚 surface tension. 鈥淲ater is held up in narrow spaces with a strength that is inversely dependent on the width of the space,鈥� says Martin Chaplin of London South Bank University. As a result, it will not move easily from the narrow spaces between dirt particles to the wider ones that separate gravel.
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Only once the soil layer becomes nearly saturated will water flow into the coarser gravel layer, says Stephen Neethling at Imperial College London. At that point, having a gravel layer will indeed offer more routes for water to escape than a single hole at the bottom of the pot. But it is probably best not to waterlog your pots in the first place.
If you have been covering the holes with shards to stop earth falling out, try using pieces of nylon or mesh instead.
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Mad dogs and Englishmen鈥� water at midday
They say:听Water droplets left on leaves can magnify the rays of the midday sun, burning the plant
This advice probably only holds true in very specific circumstances and depends on how hairy your plant鈥檚 leaves are, .
In experiments with maple leaves, Horv谩th and his team found it was impossible to burn them using water and sunlight. In part, this was because water droplets cool a leaf鈥檚 surface on contact, making it harder to scorch. What鈥檚 more, the sun鈥檚 rays have to be at just the right angle to focus through a droplet onto a leaf. The team calculated that the burn risk would be greatest in the early morning or late afternoon. But sunlight at these times is too weak to cause damage. Finally, the odds of a drop being held perfectly still on a smooth leaf for long enough are slim at any time of day.
鈥淚f a plant is starting to wilt, don鈥檛 wait. Terminal wilt could set in鈥�
The situation was different with hairy leaves. Hairs can hold water drops above the leaf surface, so they don鈥檛 touch and cool it. The team found that if a droplet sat at just the right height, the midday sun could sometimes burn a floating fern leaf.
Even though the risk is small, you should still avoid watering in the middle of the day. It鈥檚 a waste: much of the water will evaporate. Watering in the morning is best. It is less likely to encourage fungal infections than watering in the evening, when leaves stay wet for longer. But if a plant is starting to wilt, don鈥檛 wait, regardless of the time of day. Terminal wilt could set in, and then it is too late.
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A lot at stake? Support your saplings
They say:听Stake a newly planted tree or shrub to protect it from gusts of wind that can tear the roots before they become established
Here鈥檚 the rub: wind helps saplings become strong. 鈥淲ind stress encourages root growth and trunk thickening,鈥� says Linda Chalker-Scott at Washington State University. As the trunk flexes, the disturbance triggers a process called thigmomorphogenesis. This has been linked to a suite of chemical changes within plants. They switch on genes that help thicken cell walls and lay down lignin, a strong woody polymer. All this hardens the stem, which then sends signals down to the roots, telling them to boost their growth so the plant can brace against the wind.
The end result is shorter and stockier plants. The process also restricts the growth of branches and leaves, so the trunk and roots can better support their weight. Trees that are staked too tightly or for too long grow taller and thinner, and are more likely to snap or be uprooted once the stake is removed. The same applies to just about any plant 鈥� even veg like broccoli.
鈥淢y suggestion is to wait for a day or two and see how [a sapling] does,鈥� says Chalker-Scott. 鈥淚f it starts to lean, then you can go back and stake it.鈥� There are a couple of exceptions. Providing support makes sense if your garden is exposed to constant strong winds, for instance if you live on a windy coastline. In that case, it is best to use a stake no more than two-thirds of the sapling鈥檚 height and to attach it loosely using a flexible tie so the whole trunk can sway a bit. If you use wire, run it through rubber hosing first to avoid damaging the bark.
Flowers are another exception. We have bred ornamental plants to produce large, flamboyant blooms, often without also selecting for a strong plant structure to support them. If you are a fan of heavy flowers such as peonies, delphiniums or stocks, you will probably want to stake them up while they are in bloom purely for the sake of your display.
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Dodgy grounds: coffee fertiliser
They say:听Dump coffee grounds in the garden to fertilise your plants and cut waste
Spent coffee grounds generally contain a lot of nitrogen, which is why some people believe they make great fertiliser. It鈥檚 a hipster match made in heaven. To test it, Sarah Hardgrove and Stephen Livesley at the University of Melbourne grew broccoli, leeks, radishes, violas and sunflowers in soil that contained fertiliser, spent coffee grounds or both. All their plants . Other studies suggest caffeine and polyphenols in the grounds may be toxic to plants in high concentrations.
If you are dead set on recycling your grounds, it might be a better idea to compost them. This should make the most of the nitrogen they contain, while lowering the concentration of harmful chemicals. That is unproven though, so it may not be worth the risk.
This article appeared in print under the headline 鈥淕rowing skills鈥�
