Horticultural Processes and Bonsai: Transpiration

Have you ever wondered how water travels from the soil into the roots and then up the stem to the leaves? Well, here we go. The process is called transpiration and starts in the roots through a process called osmosis. This is the movement of water across a membrane from an area of higher pressure to an area of lower pressure.

Now I am going to confuse you completely. I already said that it starts in the roots, but the condition for the process to happen actually starts in the leaves. As the water accumulates in the leaves, the internal volume of the leaves now have a higher pressure internally compared to the atmospheric air. That means that the water leaves the leaves through a structure called a stoma. There are hundreds of these on the leaves and they can open and close depending on the plant’s water needs and the climate on the outside. Almost like little valves.

Transpiration

Osmosis

Now we have the picture on both ends of the tree. The water flows in through the roots due to water pressure in the soil, it moves up a bit in the tubes (xylem) as these are very thin and act like thin straws. Have you ever noticed how when you place a straw in water how the water level in the thinner straw is higher than the level in the water. That is called capillary pressure.

What now happens is all of these work together. The water is pushed into the roots, the narrowness of the tubes give it a bit of a head start and then the pull from the leaves draws it further up the stem or trunk and the flow happens. This can happen as fast or as slow as the conditions dictate. If everything is perfect, a continuous flow will happen, but as soon as something changes, the tree will adapt. Let’s say there is not enough water in the soil for the osmosis to take place. In other words the pressure is not high enough for the water to enter the roots through the different membranes, well, water will not be taken up and the tree will dry out.

At the other end things can go horribly wrong as well. Let’s say it is a very hot day and a dry warm wind is blowing. That means the pressure on the outside of the leaf is very low and if there is any water inside the leaf, this steep difference will cause water to evaporate or transpire through the stomata and if this happens rapidly and the cells lose their turgidity (pressure), it will wilt and lose structure. If this happens for too long, it is possible that the plant will dry out and leave this earth for Bonsai tree heaven.

Before we get to the practical things to do and look out for, it is important to remember that water travelling through a plant will contain minerals and other chemicals that it absorbs from the soil. It therefore plays a very important role in the distribution of these minerals throughout the tree.

The practical things:

  • Watering is a very important part of keeping Bonsai and comes down to a balancing act to ensure that optimal conditions exist for the processes as described above to take place.
  • Make sure that your trees are placed in an environment that suits the watering needs of the tree. Plants with thin leaves that can dry out easily i.e., Maples and should be kept out of harsh sun and dry windy conditions.
  • Trees with smaller leaves or even needles (Pines) can withstand this a bit more as they have less stomata and a thick waxy cuticle that covers the leaf or needle to minimise the area exposed to sun and wind and therefore slow transpiration down.
  • Get to know the water requirements of your trees and if possible group trees with similar water requirements together.
  • Check the drainage of your trees regularly. This starts with ensuring that the potting medium / soil is correct for the type of plant. Plants that thrive in drier conditions will have to be planted in a coarser and free draining medium.
  • When watering, water from the top over the whole tree as it washes dirt off the leaves to enhance transpiration as well as photosynthesis. It also cools the plant down so not to lose too much water.
  • If the soil is too wet, root rot can occur. This is not necessarily due to something going wrong with transpiration, but more a case of the roots not being able to breathe. Yes, they have to breathe as roots consists of living cells and through the process of respiration, need to take in oxygen. More on this in the next article in this series. It is also a perfect living condition for fungus that causes root rot.
  • Other techniques to ensure the soil does not get too wet is to tilt the pot during long periods of rain for water to run off. You can also push a rod through the drainage holes through to the top to create a channel for water to run through. These are drastic measures only to be used in circumstances where you need to get rid of water quickly.
  • Soil can dry out very quickly in a small Bonsai pot. When you get to a situation where the soil is so dry that water on top just runs off the surface, break the surface up a bit and dunk the whole pot in a container of water. Wait until all air bubbles disappear and place it back on the bench.
  • Another thing to look out for is when the roots grow so fast that it basically replaces the soil in the pot and all you have in the pot is a ball of roots. It is difficult for the plant to absorb water under these circumstances. Prevention is better than the cure. Repot with fresh soil when necessary and treat your trees as if they are pets.
  • When pruning, cover the cuts with cut paste or something similar as you create a wound and in the context of this article, a leak. Plug the plumbing to support the flow of water through the tree.

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