Watering plants: Leave the watering can dormant during the day

Science flower care

Leave the watering can dormant during the day

You often have more time to water during the day - but is that good for the leaves?

You often have more time to water during the day – but is that good for the leaves?

Source: Getty Images/Westend61

Drops of water bundle the midday sun’s rays – and bang, the leaf is sunburned. But it’s not that easy with the burning glass effect. A little fact check for all hobby gardeners and plant lovers.

EIt means that you shouldn’t water at midday, because that damages the leaves of the plants. But is there anything to it at all?

Claim: Plant leaves can burn if watered in the midday sun.

Evaluation: Not universally tenable.

Facts: When rays of light hit a lens made of glass, for example, they are bundled and converge at a point behind the lens, called the focal point. It can get very hot there, which is why you can use a magnifying glass to set fire to paper and other things under sunlight.

Can water droplets on leaves in the midday sun also have such an effect? Not really. The water droplets almost never have the right shape that would be necessary to act like a magnifying glass, explains the physicist Gernot Münster on a website of the University of Münster, where he deals with popular misconceptions in physics. Also, the focal point would have to be on the underside of the drop in order for it to rest on the leaf. And the sunlight would have to hit the drop at the right angle of incidence.

Everything for balcony and garden friends

Summarized: A drop would have to bundle the light falling onto the surface of the leaf in an ideal way – which even a drop that is exactly hemispherical cannot quite do. When water evaporates on the leaf in the sun, there is even a cooling effect. However, if the water contains salts that remain after evaporation, these can damage the cells of the leaf.

More than ten years ago, researchers at the University of Budapest addressed the question of the magnifying glass effect: They let sunlight shine on various leaves on which they placed small glass beads or water droplets of various shapes and sizes. In addition, they simulated these conditions on the computer. Their result: it is unlikely that water droplets will damage the leaf tissue on smooth, hairless leaves in the sunshine. In the case of leaves with fine hairs, however, drops held by these hairs could actually cause one sunburn cause – if their focal points fell on the flat part of the leaf.

The climate-changed garden

An Australian biologist reacted to her article, which appeared in the specialist magazine “New Phytologist” in 2010, with a scientific comment. He comes to the conclusion that further research is needed – for example on the temperature threshold that must be exceeded for leaf burns to occur. There are also initial indications in research that increased radiation intensity could damage part of the system involved in photosynthesis, writes the biologist.

However, you should generally water in the morning or evening: If you water plants in the blazing midday sun, the water evaporates relatively quickly, so some of it is wasted. Some experts also point out that large temperature differences can cause a kind of cold shock in plants.

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