Oak Bonsai – how to make them drought tolerant

I have just been reading the latest scientific report on the development of cells in oak, and it is clear that there is some useful information here for Bonsai.

The research* suggests that trees which have abundant water when laying down earlywood (formed during the springtime flushing) causes the trees to create larger vessel diameter in the conductive tissue of the stem. Conversely less available water at this time causes a smaller diameter of vessel to form.

Later in the season the larger vessels need more water to fill them and to allow the natural transpiration process to ‘pull’ water up from the roots to the leaves, if the volume of water is not high enough them the leaves do not receive the water and will suffer scorch in hot sun. Smaller earlywood vessels need less water to active the ‘pump’ and can more efficiently provide water to the leaves, making them more drought tolerant in high summer.

So water less in spring and you will need to water less in summer.

However, the side effect of this is that for a tree in development we want the largest diameter growth on the stem and so can directly effect the annual increment by watering heavily in spring – more wood is created at this time than any other time of year as the late-wood (laid down in summer) is shown not to be effected by climatic conditions but by overall tree health.

Now the research paper was specifically on oak so I will have to find out if the same applies to other species, but it does seem to be a clever evolutionary trick, and I imagine similar mechanisms exist in other species.

Interesting stuff.

* Price (2015) Shake in Oak: an evidence review; Forestry Commission Research report


Bonsai Forest from nursery stock

I was fortunate enough to be given a bundle of nursery planting stock. These are healthy young trees that have been grown to sold and planted out in the thousands so don’t have that degree of attention that bonsai seedlings would have lavished on them.These are cell grown Alder.

The trees are grown and delivered from the nursery in little plugs of soil so the roots are undisturbed during the transport and planting out, they come in wraps of 25 with a variety of degrees of sizes and shapes. As these are intended for the forest these small differences do not matter, however this difference can be exploited for a forest group planting.

Let’s see what we have as our Raw Bonsai material:


The first step is to sort out the sizes. I arrange them into Large (5 in number), Medium (7)  Small (10) and dead (3). Obviously the dead ones would not form part of the arrangement!!!

The large trees will form the central focal points and have already started branching. Medium trees will form a ring around these and the small trees, which seem to have already developed bends and twists in the trunks will be spread around the outer ring

The soil of the cells was loosened and any upward growing roots were removed, leaving any surface roots that will develop over time and be an attractive feature.

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Trying hard to hold the trees with one hand and take a photo with the other, it became impossible after this stage to take any more during photo’s so you will just have to fill in the gaps. The trees are sitting together in the pot but the roots are all separate and very easy to move about, also because the trees are cell grown then they can be turned individually to ensure that the natural shape of the stem is consistent with the overall image. Once the trees were in the correct placing the next step was to infill with bonsai soil, as this was pushed down it gripped the trees more and more firmly until they did not need to be held and the arrangement could take shape.

The trees are planted quite deeply at this stage. At the next repot, which could be as early as next year, the bottom layer 2cm of soil and roots will be cut off and the group can then go into a shallower wider pot more conventional for a forest planting, this pot is going to give the trees the best possible growth without allowing free running space for the roots

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And the finished group planting after a good amount of tweaking to give an outline of a forest grove. I will report back after the growing season to see the development.The photo tries to give a good silhouette of the final grouping. Some of the trees were pruned back to fit the outline, I have left the clippings which can be seen at the front of the photo.

I have tried to achieve a calm, balanced arrangement of stems, which will form a rounded dome very quickly. The growth habit of alder is to be tall thin trees, so the shape of the dome will ultimately reflect this. I have purposefully included some stems that cross low down, and some small stems at the back which run slightly against the natural flow in order to give it that little touch of imperfection, wabi-sabi, understanding of which is so important in appreciating bonsai.


Deciding to remove a literati pine branch

Pine beforeI was recently tidying up a Scots Pine literati that I have a great fondness for and have been working on for many years on and off. The trunk has some lovely bark on the lower section and just enough movement to make it of interest. When I was given this tree it had already grown a little tall and leggy and would never be any good as a normal informal upright – the height suggested literati. The image on the left is how the tree looked after tidying up the old needles and a little light wiring to keep the shape of the pads.

Pine potential

To make this work the foliage has to be very light and after a very long hard look at this tree I wanted to see what it might be like without the lower branch. This seemed to be an anchor

to what should be a tall elegant tree reaching up for the heavens. Rather than commit it is always sensible to try to understand what the tree would look like without the branch. A white cloth against the white background gives an impression:

Pine after removal

The remaining pads have a consistency to them, the apex needing a little more development but clearly having the dome in the right place to link the other three. This gave me enoughconfidence that my initial hunch was correct and so the branch was removed. However it was kept on as a jin to suggest age and to point to the empty space emphasizing the floating feeling of the retained foliage.


One feature of note during this process of bark stripping was on one of the small branch stubs where the wire had clearly been bitten in the bark over the last year of growth, however when stripping the underlying wood as part of the jin it was revealed that the wood had not been effected. The soft tissue damage had not translated to the wood. When you think about it the wire can never actual cut into the wood itself but it is the growth around the wire which causes it to appear to bite in. If this is left for more than one growth season the new layer of cells added annually to the wood will eventually not have enough room and so the wire will affect the underlying wood. It is therefore perfectly possible that once the wire is removed the growth of the bark in the following year will start to normalise and the wire marks would slowly grow out. This is why wire marks are not as serious as they appear on thick barked species.


Watering an evergreen bonsai – wartering blog part 2

Now for evergreen trees the myth of a growing season is even more dangerous as they do not give such clear clues as deciduous trees do when the buds burst in spring or drop their leaves in winter. Also it is possible that they come out of ‘hibernation’ and grow in the depths of winter if the days are warm enough (or if we stick them in a warm greenhouse).

The phenomenon that causes a tree to start growing is known as ‘day degrees’. It is simply the temperature each hour of the day added together to get a total figure for the day. When this figure is high enough the tree will start growing. So a hot day in winter can trigger a higher day degree figure than a longer colder day in spring when you might expect a tree to be growing. Whenever a tree is growing it needs water, so you must always be alert with evergreen trees that the soil remains moist through the winter. I also think it is worth repeating a sentence from the last blog: it is always prudent to ensure the soil is slightly moist as a desiccated soil can draw moisture reserves out of roots and could lead to ‘freeze drying’ (the same way food in the deep freeze can dry out if the packet is open).

Evergreen are also more able to be active at lower temperatures because they quickly incorporate water with their resinous sap, a substance able to withstand freezing at normal temperatures. The plant can pump this sap around the system causing growth to occur even at incredibly low temperatures. For evidence of this look at the growth rings of a felled tree – you will see wide summer rings where growth is at its most vigorous but material is added to stem diameter throughout the winter period.

Therefore it is safe to assume that the Growing Season never really stops regardless of the species and winter watering is needed. Now having said all of that of course trees are clearly more active in the spring and summer and need much greater volumes of water during hotter months.

It might seem like a stupid question but ‘Why do trees need water?’

It might seem like a stupid question but ‘Why do trees need water?’

Trees draw water from the soil and transpire this moisture through the leaves, this moisture carries with it minerals from the soil that has been absorbed by the roots. The leaf uses these minerals to help build new leaves and twigs, buds and flowers.

Just a quick note here that we need to differentiate between evergreens and conifers because of deciduous conifers (larch, taxodium, etc) and evergreen broadleaves (holly, winter oak, etc)

Some water is retained within the tree and absorbs the sugary, energy rich products of photosynthesis and the oxygen from respiration, returning both to the roots to help them grow, depositing some to all the parts of the tree as it passes, and collecting some in the heart of the trunk as storage for future use. Basically water performs the same function in trees as it does for human blood.

Now when humans get hot they sweat, emitting water on the skin surface that gets evaporated off taking heat energy away from the core of the entity. This exact same process happens in trees, the ‘sweat pores’ are found in highest concentration on the underside of leaves, but also in lesser numbers on the woody areas such as trunk and twigs. If this process is prevented by lack of water the organism, human or plant, will overheat at a cellular level and die. An adequate supply of water is therefore critical. This dangerous overheating is worse in some species and you will see reference to scorch in some text books. Of all my trees it is the beech that are most prone to scorch. The easiest way to deal with this is to keep trees prone out of the full summer sun and drying winds so they only have to deal with ambient air temperatures.

So let’s focus for a moment on the actual practise of watering. Once the soil in a pot has dried out the ‘standard’ watering from a hose or can will simply run off – you must quickly learn to recognise this sign as you tree will be in peril. Rectify by dunking the whole pot in water until all the air bubbles are gone, leave for another 5 minutes just to be sure, and then return to its home.