A shift in thinking
Watering is the most fundamental and crucial technique to master in cultivating bonsai. Unfortunately, it is also one of the most elusive and misunderstood, with a copious amount of confounding advice out there about when and why we water. At Mirai, we always seek to ask ourselves the ‘Why’ behind every interaction we have with our trees to ensure we address their individual needs rather than the hearsay of commonly accepted practices.
While watering, there are a few fundamental horticultural ‘nuts and bolts’ that we always keep in mind when approaching the practice from season to season. First, water’s primary function in the tree is as an air conditioner. As stationary living beings, trees must find a way to move heat energy out of their system so their cellular structure and integrity are maintained. Water has a high capacity to bind and hold heat. And although water’s function and impact are multifold, when it is moved through and transpired out of the tree, it conveniently takes heat energy back into the atmosphere.
The next horticultural ‘nut’ we keep at the forefront of our mind is where the tree is at in terms of the state and production of growth. Throughout the year, trees’ priorities shift, so our watering should reflect that.
We have an archived live stream on our YouTube channel, Spring Watering, which goes into great depth about a tree's purpose and needs during this season's intense growth. But in a nutshell, trees are focused on producing foliar mass in the spring. They want to increase their solar panels for energy generation to maximize their resource consumption during the expanding length of days and rising temperatures which both catalyze more rapid metabolic activity.
When trees are focused on producing foliar mass, their watering needs increase – they are losing excess water to transpiration from the new, fleshy growth while also transporting stored nutrients from cellular loading and fertilization to the canopy areas where growth is abundant. This double-duty, if you will, dramatically increases the necessity for water during this active growth.
In the summer, the tree experiences a reduction in the addition of growth; therefore, the purpose of water shifts from facilitating new growth to simply cooling. Reducing resource allocation towards producing new growth allows the tree to focus on restocking its energy stores. The tree is still ‘growing,’ but the growth is in the form of metabolic recuperation towards an energy-positive and replenished sugar store instead of exploding with visible growth.
In addition, trees form protective mechanisms to reduce their summer water consumption. Namely, as new growth ‘hardens,’ it develops a waxy cuticle or protective layer over its leaves. This layer dramatically reduces transpiration and excess water loss and helps cut down on UV degradation in intense heat.
Considering cooling and season is to underestimate the driving forces of water movement in our trees. The third ‘bolt’ of info any practitioner should consider is the impact of fertilization.
In the spring, increased fertilization increases water consumption based on the concentration gradient formed with added ionic content in the soil and the plant’s cellular structure. Combining this need with the rapid loss of water to create new growth and keep it cool makes it easy to see how water consumption in the spring can be immense.
However, during the summer, since temps are rising and the tree adds little to no fleshy growth, we cut back on fertilizer and allow the tree to focus on moving water to cool itself until the summer heat subsides. This reduction decreases ionic gradients to allow water to move more freely through the plant and lessens the competition for water uptake into the roots from the container.
Additionally, achieving the correct balance of water and oxygen in the container is paramount to establishing ultimate health in the plant. With watering in the summer, we start to become wary of decomposition in the root mass if the container becomes too hot and there is an overabundance of water. So, while we are trying to water the tree to support its cooling system, we don’t want to overwater it to prevent hot, stagnant water in the container that can contribute to root damage.
All of these factors result in a significant decrease in water uptake by the tree. And while it may be tempting to rely on a hard and fast rule of ‘water more in the spring, water less in the summer,’ practicing bonsai well means taking our understanding further. Before we make those assumptions let’s arm ourselves with one more major ‘nut and bolt.’
We must acknowledge the different reasons trees use water throughout the various seasons. In the spring, we water our bonsai to facilitate the production of foliar mass; in the summer, we encourage the tree’s cooling process with a different, hardened foliar mass. And while it might be counterintuitive to us – that a tree would need more water at 80 degrees in June than at 90 degrees in July – this is all a part of unpacking the myths of bonsai with a horticultural approach.
With all that understanding, we are still left to navigate the unclear nuances of knowing when your tree needs water. How do we tell that our tree has a good balance of water and oxygen in the container; how do we walk that line between watering too much or too little?
Unfortunately, the answer is not so simple. It comes down to the myriad elements of your environment that vary from acre to acre – the intensity of the sun, wind, temperature, relative humidity, size of the container, size of the foliar mass, work recently performed, and tree species. When we come to water at Mirai, we approach each season, each day, and each tree anew, with a confounding set of variables to dance around and work with.
To water your bonsai with proficiency, you must have the horticultural foundation to understand where your tree is in the growing season and the different reasons it uses water. On top of that, you must have an immense awareness of the surrounding environment the tree exists in to tailor your approach and maximize the resources at hand.