It’s considered good advice not to water a plant’s leaves. If you have to water a plant from standing position, you’ll pour water from the top of a plant, and it will spill down the leaves and into the soil. Sadly, this is probably the worst way to get water to a plant, and it’s analogous to waterboarding in some ways. That’s an assertive analogy, but as we’ll see here it’s actually pretty accurate. In short, you always want to water plants ONLY at the roots directly.
You probably already know that plants get their water by absorbing at the roots. Water drains into the soil and the roots take it into the plant’s stem, and it moves upwards through the plant. You might notice a small problem called gravity here — how could a plant with no muscles move water upwards along its stem when the force of gravity is so much stronger in comparison? Plants actually have a few nifty tricks to get around their lack of muscles. The magic starts at the plant’s leaves, so we’ll check in there first.
Leaves are exposed to the sun and the air, and as a result they’re constantly losing water from dehydration. They need a constant supply of water from the roots, moving upwards, to keep them alive and well. When leaves lose water to the air, it’s called transpiration. It’s sort of like sweating (perspiration) in humans.
If you’ve ever seen water bend upwards at the very top of a cup, it’s because water molecules are attracted to other water molecules. They have a certain amount of cohesion to each other, and that’s why rain tends to form drops – each water molecule on the outside of the drop is more attracted to the next inner molecule than it is to the surrounding air, and so they huddle inwards and stick together.
The same thing happens in a plant’s “veins” – As water is evaporating from the leaves, the water molecules attracted to those about to leave are attracted upwards, replacing the water just lost. Not only this, but just water will climb the sides of a glass, the same happens in a plant’s vessels — water bows out at the sides of the vessel wall and moves upwards against gravity!
When you get water all over a plant’s leaves, the water molecules in that leaf can no longer evaporate away, and water from the roots will cease to flow. You’ve blocked the water from exiting the leaf, which causes a chain reaction farther down the plant that prevents upward flow. Repeat this experiment enough times and you’ll deprive your beloved plants of the water they need to stay alive even though you’re drenching them from head to toe!
It’s quite easy to forget this concept, but it makes all the difference. Feed your plants by watering the soil directly, and let the plant transport its own water. It’ll reward you by growing bigger and stronger, and if you’re lucky, some more delicious fruit!
Your Brain on Sci 