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Watering your Bonsai

The single most import skill in growing a healthy Bonsai is Watering. Bonsai Master Nobuyuki Kajiwara often says:

"Watering Bonsai is 50% of the Art of Bonsai, it is also the most difficult to learn."

This statement at first may seem a rather bold and difficult to grasp, but if you step back and look at the importance of water in a Bonsai's health and vitality you will see how true it is.

Bonsai need water to survive, without water it will die; this is a plain and irrefutable fact.

Beginners to Bonsai often have no difficulty understanding this. However, one often forgets or ignores the fact that excess water will just as easily kill a bonsai as no water at all. It is these seemingly contradictory "capabilities" of water that needs to be understood if one is to be able to success fully grow Bonsai.

Plant Water Relations

Perhaps the best place to start is with understanding the role that water plays in a plants physiological well being. We will take a little science detour to help us better understand this finely balanced relationship.

In terrestrial rooted plants (as bonsai are) practically all of the water which enters a plant is absorbed from the soil by the roots. The water absorbed is to all parts of the plant by osmosis. The water also carries nutrients dissolved in the water, this concoction, or Sap, distributes vital nutrients around the trees. The water while carrying raw nutrients up the tree also helps distributes nutrients that are made by the leaves and photosynthesis.

Water Flow in TreeThe make up of a plant is approximately 55-85% water by weight. Water not only helps with translocation of nutrients also helps with its turgidity. In stem and leafs the percentage of water is between 70-90%. Often the first signs of water scarcity is flaccidity occurs in stems and leafs leading to them droop. Prolonged water scarcity can affect photosynthesis.

Water enter the plant through the younger parts of the roots, the area just behind the root tip, by hundreds of root hairs which are just projections of the epidermal. The fine root hairs greatly increase the surface area exposed to the water in the soil, making it easier for the water to enter the roots through the fine membrane by osmosis. Osmosis which is just water moving from areas of higher concentration to areas of lower concentration through fine membranes. These root hairs are short lived and are being constantly replaced.

Once inside the root tips the water is moved through a series of supporting cell into the water transporting vessel the xylem. The xylem are long, narrow, hollow tubes containing no living material and joined end to end to provide a continuous pathway from the roots through the stem and to the leaves. Water moves up the xylem from root tips to leaf tips. Excess water is then lost to the atmosphere through transpiration. The rate of transpiration is regulated by stomata little window like cells which closes and opens in response to temperature and carbon build up in the leaf. In most plants about 98% of the water taken in by the roots is transpired from the leaves' surfaces. It has been proved that the transpiration of water from the leaves greatly aids in the pressure required to move the water up the xylem, this study of this is beyond the scope of this article but is commonly referred to as transpiration-pull theory and sometimes also called the cohesion theory.

The leaves use some of the water and nutrients along with Carbon Dioxide (CO2) and from the atmosphere and sun energy to photosynthesis a host of Organic molecules such as sugars mainly sucrose and some amino acids and certain hormones. The concoction of organic molecules and even some even messenger RNAs are transported by the phloem through a series of sieve tube elements from leaf tips back to stems, branch, trunk and roots which use the sap concoction to build mass and grow.

Watering and Bonsai

Now that we have a better understanding of how plants use and need water we should better be able to water it.

The most important thing to remember is that a Bonsai is Pot bound it is entirely dependent on you for it water needs. Without you watering it, it will definitely not do well and will soon die.

Primarily how you water a Bonsai is essentially defined by the species of the Bonsai, and to some extent the area that your Bonsai is being grown in. Despite these differences there are a few practical practices that will ensure that you are watering your Bonsai properly.

One caveat to mention is that proper watering as always starts with having an appropriate potting/growing medium for you Bonsai. This is discussed in more detail elsewhere on the website in detail but suffice it to say that in practice the best bonsai growing mediums even the ones that hold on to water for longer are well drained and free flowing. This ensures that while water is held in the soil in required quantities there is also ample amount of air (oxygen) in the soil.

Does my Bonsai need watering?

Recognising when your Bonsai needs water is a key skill in growing healthy Bonsai. To determine if your bonsai need water there are a few simple check you can undertake:

When to water?

The idea of watering is to ensure that it there is enough water for its transpiration process. It stands to reason that the best time to water a tree is in the morning when the tree starts the days' photosynthesis. Providing water early morning ensures that the tree has fresh supply of water and that the water that is supplied also has a fresh supply of oxygen dissolved in it. The flow of excess water from the top of the soil out through the bottom of the pot helps draw fresh air into the root ball; this expels the stale air from the soil and helps keep it healthy and vibrant.

The trouble with watering the plants in the evening is that as the water is not immediately used by the plant; the roots invariably tend to stand in water overnight.  This can sometimes lead to the fine roots drowning and even rotting and eventually root rot setting in. So as a practice it is advisable to avoid watering regularly in the evening.

During summer periods you may find that you need to water your pots twice a day; I find it better to water the plants late afternoon early evening.  Watering around this time ensure that during the long summer days when there will still be a lot of photosynthesis and transpiration happing during the long day-light hours which ensures that excess water in the pots are used and as such there is less chance of water logging and root rot.

Water Flow in Pot

How to Water

While to the novice this question seems quite trivial, one soon begins to appreciate that it is critical skill and can in-fact can make the difference between an exhibition grade bonsai and a dead bonsai.

An old Japanese Bonsai saying teaches us to water three times; "Once for the pot, once for the soil and once for the roots."

In practice; the first pass of the fine spray shower is used to saturate the soil surface, the second pass ensures that there is enough to saturate the soil in the pot, and finally the third pass ensuring that there is enough there for the plants it self to use.

Figure A depicts a pot that needs watering. Most "bonsai" soils when dry are lighter in shade than when they are wet.

You use a first pass of watering to saturate the surface soils (Figure B). If you do not use a "first pass" to saturate the soil surface, the water just flows of the surface, and out of the pot.

The "second pass" ensures that the soils in the pot is properly saturated (Figure C). Here depicted by the soil colour becoming darker.

The "third pass" ensures that there is some addition water in the pot for the bonsai use (Figure D).

With practice you will see that you can vary the passes required. I find that I can move between 1 and 5 passes, depending on weather conditions, season of the years and even dryness of the pot.

If you have a collection of bonsai them you will do well to remember that each bonsai has it own requirement so best to water each pot individually, as per its requirements, resist the temptation to water all your bonsai as just one mass of plants.

Watering problems

Being able to recognise watering problems early can help you take corrective action and ensure that lasting damage to your bonsai is avoided.

Over-watering

More often than not over watering is more a symptom of poor soil drainage. Look out for growth of liverwort and algae developing on the soil surface. Remove liverwort as soon as you notice it and regulate your watering to compensate. You may need to re-pot and use a more free draining soil.

It is advisable to follow a routine while watering – but remember only water if the bonsai pot needs it.

Under-watering

Under watering is often easier to notice as there are more dramatic and visible very physical signs. Dried leafs, crisp leafs dead stems and more. The trouble is that these signs invariably mean that significant damage has already been done. You can however, train yourself to start looking for earlier sign, like flaccid and limp stems and leafs. On some species this is very visible on others species you may need to feel/brush you fingers against the leaves looking for turgidity.

Here the tree, which are usually the first signs of lack of water. In the summer months if you notice your pot drying out before the evening, make sure you use extra watering passes to saturate the soil in the pot.

If the soil is clogged or the pot root bound you may find it beneficial to occasionally soak the bonsai pot in a bucket of water. Soak the pot till you see no more air bubbles rise from the soil in the pot.

Salt imbalances

Most of us water our bonsai with tap water. Tap water in most systems is treated with a mixture of Salts (Sodium carbonates) solutions to make the water to remove the acidity of treated water. This alkalinity of the water helps to protect the water distribution network. However, prolonged watering with the tap water increases the total dissolved salt value (TDS), which in turn affects the roots effectiveness in absorbing water as osmotic imbalances occur. Regular applications of use of Gypsum (calcium sulphate) can help with washing away excess salts and return a balance to the pots (use as advised by manufacture).

In summary:

  • Water your bonsai thoroughly, use the 3 pass method to ensure that the water reaches the roots - use a fine shower so as not to disturb or wash away the surface soil
  • Ensure that the surface soils of your bonsai is allowed to dry out between each watering
  • It is preferable to water your bonsai in the morning - during the longer summer days water in the evening too if required
  • Follow a timetable while watering - but always remember - water the bonsai only if it needs it