Watering is an essential part of bonsai care. You might think watering the soil will suffice because plants absorb water through their roots. But what about watering the bonsai leaves? Is it necessary?
Watering bonsai leaves should be done to help leaves (1) cool and prevent losing water, (2) absorb water, and (3) wash away dust/dirt. This will ensure that your bonsai tree grows healthy.
Why you should water bonsai leaves
It is true that the roots are the main organ for plants to absorb water but bonsai leaves should also be watered because watering leaves will;
- cool the tree and prevent losing water
- absorb water, and
- clean the leaves.
Cool and prevent losing water
Water is constantly evaporated during the daytime through the leaves in a process called transpiration, which keeps plants from overheating. By transpiration, the heat in the leaves is transferred to the vapor to the air which enables the plant to cool down when the weather is very hot, followed by water pulling from the stem and roots into the leaves.
Plants need to cool themselves to efficiently create and store energy. With high temperatures, their functions slow down. Watering the leaves will help cool the bonsai tree without losing water through transpiration especially when it is hot.
In addition, water deposited on leaves reduces transpiration because watering the leaves forms a protective barrier. Transpiration will not occur until the water on the leaf surfaces evaporates.
Despite the common myth, plants can actually absorb water directly through their leaves. When water condenses on the leaf, bonsai leaves can take in some of the surface water and keep them hydrated.
Leaves will be withered, like a balloon that lost air, even if they lack a bit of water. When leaf’s cell wall loses water, the entire leaf becomes “soft” because the leaf’s cells cannot maintain elasticity. By watering the leaves, the bonsai trees can absorb water through leaves and restore the tension of the leaf.
Watering the leaves will be their source of hydration, especially for the bonsai trees whose roots are recently pruned and the root systems are not fully developed.
Avoid watering leaves in a hot daytime
Though bonsai trees can absorb water directly through their leaves, watering leaves in a hot summer daytime should be avoided because it might cause leaf scorch.
Leaf scorch occurs when water is lost from the leaves more quickly than the roots can take it up. It often happens during the hot summer day when water is evaporated through leaf surfaces but the water absorption by the roots is less than it should be.
If water supply is deficient, leaves turn brown, particularly around the tips and margins. It is followed by the leaves curling and shriveling if the stress is hard enough.
Watering leaves during a hot summer daytime can actually cause leaf scorch, especially to deciduous bonsai trees such as Japanese maples, not prevent it. If leaves are watered on a hot daytime, the leaf stomata (leaf pores) that were closed to conserve water will open to transpire because the leaves are cooled and the tree thinks it is evening.
If this happens, water will be lost very quickly from the tree leaving leaves to scorch. Watering the bonsai leaves in summer should be done in the morning or the evening to prevent leaf scorch to happen.
Clean the leaves
Watering the leaves will clean small debris on the leaves and can make more energy for bonsai trees’ healthy growth.
Dust and dirt from the air pollution or the soil bounced with watering are accumulated on the surface of the leaves over time. These particles inhibit capturing the sunlight and inhaling/exhaling the gas the plants need to create energy through photosynthesis.
As plants do not eat food as animals do, they need to create energy themselves by absorbing sunlight with chlorophyll, a green pigment in leaves. This process, photosynthesis, enables plants to convert water and carbon dioxide absorbed from the air into glucose (sugar) they need to grow and survive.
A scientific study revealed that plant leaves that were exposed to particulate matter such as dust and dirt showed less chlorophyll (green pigment) because of the shading effect from particulate matter on the leaves. These pollutants are capable of damaging plants leaves, reducing plant growth and reduce the photsynthetic efficiencies of the plants.
An added bonus for cleaning the leaves is to wash away insects (their eggs) and fungi. Not all of the infestation could be wiped with watering, of course, but keeping the leaves clean is definitely good for the trees’ sound growth.
“The ultimate guide to watering your mini bonsai” (Link here)
Neelima Meravi, Prasant Kumar Singh, Santosh Kumar Prajapati, “Seasonal variation of dust deposition on plant leaves and its impact on various photochemical yields of plants“, Environmental Challenges, Volume 4, 2021. (Link here)