With so many variables in weather, pots, soils, water qualities and species its difficult to give a cover all explanation into the deeper complexities of watering so we need to break it down for further understanding.
In terms of watering, it is very important to understand each species and its specific water needs. Typically the best way to arrange your bonsai is to group each species together so that when you water your bonsai you can group the watering practices together, this will simplify your watering regime. If you would like to think of watering needs by the area in which each species in nature is found. That way it’s easy to understand each species needs. Take species that are found closer the the equator and at low altitude; these species will usually consume the most amount of water, the further a species is from the equator and the higher the elevation it is found at the less demand it has for water. Take a Japanese white (pinus parviflora) pine for example; this species is found in its natural habitat quite high in elevation and further from the equator than a Taiwanese box (buxus harlandii) and each species has huge discrepancies in their watering needs. If we were to make a scale of each species and plot the distance from the equator and elevation over watering needs it would be simpler to visualise however check out the species you grow in your collection to find out more on each species and their needs.
Soil composition is a huge factor into the bonsai watering needs. In Australia, the most common soil used is a pine bark, sand and aggregate mixture. Typically the pine bark is a larger format (4mm screenings) which allows for great drainage, the aggregate is typically the same, and sand is less than 1mm. This all works very well for drainage, however pine bark is an organic material and therefore will break down to humus over time, which will hold more water. This can lead to water logging, especially closer to the trunk and under the trunk within the root mass.
Alternatively, within the Hakuju-En nursery we use the same potting medium blend used in Japan which consists of Akadama, Pumice and Scoria. Which has its own properties that include and breaking down over time. Akadama is a type of hard clay, which has been baked, which has a rock like appearance and has a rather good, yet limited water holding capability, pumice and scoria are covered with pores which are able to capture water however no where near as well, however do an excellent job of storing a coating of fertiliser within their pores to be held and consumed by the tree over a long period of time (typically 4 weeks). Let’s discuss the qualities of Akadama to understand why it works for bonsai throughout its life time in the pot. Initially Akadama is quite hard, and does not hold much water, it drains well, and being inorganic it won’t bark down to humus like pine park, however that doesn’t mean it’ll break down. Over time Akadama will break down from a well formed stone like appearance to to a homogeneous binder for the pumice and scoria. At this point the Akadama has a lot better water holding capability, yet is harder to fully saturate, and as a result lessens its drainage capability. This is why many bonsai watering educational videos and texts coming out of Japan, will discuss watering a tress many times in a single session, which allows for the quality of Akadama, which takes more water to become fully saturated.
This evolving nature of Akadama suits the life cycle of a freshly potted tree quite well: let me explain. When a tree is freshly repotted it’ll need to have a free draining soil. A freshly repotted tree typically has had some root pruning, and therefore has less of a capability to consume the water in the pot, and so without a free draining soil the roots can become waterlogged. So we know now that a free draining soil works well for trees that have been freshly repotted, but how do we look at trees which have been in a potting medium for some time. Well the speed at which Akadama breaks down and holds more water, is roughly the same rate at which the roots grow and extend into the fresh potting medium. With more roots there is more demand for water, which is now supplied via the better water retention.
While this Does sound very similar to the nature of a pine bark medium, a few factors have to be explained. Firstly, Akadama has a wicking property, which will draw the water evenly through out the pot, and therefore its very uncommon to find wet patches in the roots mass where water logging can occur. Secondly Akadama’s mass will stay consistent where as pine bark as it breaks down will reduce in mass, and the smaller particles will wash out and create cavities under the surface, combines with the humus means that the drainage will be severely hindered and water logging will occur.
How pots will effect your watering tree to tree. Most of us follow a common logic when we first get interested in bonsai when considering the water holding capability of a given pot. When discussing a pot’s ability to hold water we tend to liken pots to vessels, and I’ll explain why this thinking is wrong, and help you to discover why. For this example Ill discuss two opposite pot types, and allow you to figure out how similar pot shapes can vary given this series of explanations so that you are able to make informed decisions on how to approach each pot and its watering needs. So we will take pot #1; a wide and shallow group planting pot. When I first started practicing bonsai I thought that this pot would need the most amount of watering, because its shallow and can’t hold much, id like that the water would evaporate the quickest from this pot. Pot #2 is a tall cascade pot, which is shapes loosely like a funnel, I would think that this pot would hold a large amount of water, but I soon realised that my logic behind this water holding was a polar opposite to the reality.
All pots have a depth at which they will stop draining efficiently, in a shallow pot thats a larger percentage of the depth of this pot compared to our deeper cascade pot to contrast. And surprisingly evaporation from heat isn’t a large factor of drying, however wind is, so we now know wind is the biggest threat to our trees drying out.
One of the most obvious points about weather would be that when it rains you don’t need to water your trees, and while this is almost always correct, let’s discuss some of the times you might bamboozle your family and be outside watering when its raining. Despite rain seeming like a great source of watering your bonsai, it must be raining significantly hard and for a long period of time for the water to A, soak in and B, be enough to wet all your soil. So when it rains, please check your soil to ensure the rain is sufficiently wetting your soil before passing it off as a non watering day. What is more significant is the hot and dry days, and when its’ really important to watch and act on your watering. Hot days can be detrimental to your bonsai, not because you are so worried about the soil drying out, but because some species cannot transport the water from their roots to their leaves quick enough to keep everything well hydrated. The sun is very good at burning the leaves and for this reason its important to check on whether your collection needs sufficient shading during the scorching days of summer. Wind is one if the most important factors when considering when to water. Wind will dry the soil much quicker than sun alone, and will even dry your bonsai when its cold.
Try keep on top of you watering, and begin to build a profile of each of your trees to ensure you know each of their watering needs. As time goes on you will begin to know your trees well and which need more water than others.