Watering is the foundation of health in bonsai. To adequately understand how often and why we must water it is vital to visualize the physiology of a plant. First, we must be able to differentiate between vascular tissues which consist of the phloem and xylem. The phloem and xylem are created by the cambial layer which acts as the regenerative layer. Below is a photo of the phloem and appears to look green. The phloem is responsible for moving sugars and metabolites. Below is a photograph of the green phloem.
There is a very thin layer under the green phloem that is called the cambial layer. The cambial layer creates cells for the phloem and the xylem. In the photo below I drew a thin band where the cambial layer should reside. It can only be properly seen with a microscope.
Adjacent and to the left of the cambial layer (red line above) is the xylem which is responsible for the movement of water and minerals.
How Does water Transpire?
Now that we understand some basic anatomy, we can move on to how plants move water. Water moves from the root tips through the trees xylem and transpires out mainly via the stomas. The stomas are very small, microscopic opening on the underside of a leaf. The stoma is also responsible for gas exchange which is paramount for photosynthesis. Below is a picture of several stoma on the underside of a leaf. The stomas look like jelly beans with a mouth at the center.
Factors that affect Transpiration
There are a multitude of factors that either increase or decrease transpiration via the stoma including: relative humidity, sunlight, and wind.
Relative humidity has a dramatic effect on transpiration. As humidity rises transpiration slows and as humidity lowers transpiration increases. This is due to differences in vapor pressure between the stoma and the air. Sunlight will also affect vapor pressure as higher levels will increase transpiration as the plant reacts to the sun’s intensity. Lastly, wind increases transpiration rates as it desiccates the plant. Plants will eventually close their stomas in conditions of high wind and sun to prevent drying out by halting water loss via the stoma. When the stoma closes photosynthesis also stops because gases can no longer travel into the plant.
Another factor to consider when watering bonsai is pot size. Pots act like water reservoirs for your tree. You can imagine it like a gas tank in a car. If evapotranspiration rates are high and you have a tiny bonsai pot (small gas tank) then you are going to need to water very often.
Quantity of Leaves
The next variable is the quantity of leaves on the plant. More leaves mean more stomas which means higher rates of transpiration. A useful way to visualize this is imagining the leaves like your car’s engine. The more leaves the bigger and more consumptive your plants engine.
Next, we must consider the water holding capacity of your medium (bonsai soil mix). Some commonly used bonsai mediums include akadama, pumice, and scoria. Akadma is a clay derivative which gives it a high water holding capacity. Pumice has average water holding capacity and scoria has poor water holding capacity. Now you can change the ratio of this bonsai medium depending on the species you are working with and the climate. For example, if your condition has high levels of evapotranspiration then you should consider adding more akadma and pumice to your mediums ratio. This will give your plant a larger “water tank” on those hot and dry days.
Understanding these variables will help translate local weather patterns and individual bonsai trees into useful information. For example, areas of Los Angeles have moderate humidity, high winds, and high levels of sunlight so transpiration rates are usually high. Consequently we will be watering far more often. Many areas are quite different to these conditions, so it is always important to be mindful of your local climate and accordingly adapt your watering schedule.