I would like to achieve a Cascade looking shape for my bonsai and would appreciate advice as per how to trim my current tree.
Here are the pictures:
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What is cascade style and what are the basic steps involved?
Cascade style is where your tree trails down over the side of its pot. The overall most basic shape of a bonsai is a triangle, in order for your bonsai to maintain a nice shape you should be able to shape the tree around the idea of a triangle from the viewing side. The viewing side is the side of the plant that is on display.
Looking at the image below you can see the pot represented by the blue rectangle and the shape of the tree by the green triangle. This triangle is just representative of one type of cascade, but based upon the tree you have to work on, the three points would shift to suit. However, the idea with cascade style is that the lower most corner is placed below the top of the container and this point being the furthest away from the center.
Also notice that there should be three parts to our triangle in order for the tree to have a triangle shape. This is usually achieved by having two side branches to be the back and top and the main trunk of the tree to produce the cascade. This can also be achieved by using three branches if the main trunk is already too rigid or old by selecting a long branch to cascade and removing the main trunk or significantly shortening it show that new growth coming from the top can be the top of our triangle.
See the image below for an example of the main branch structure underlying the cascade shape.
When creating the cascade shape if there are more main branches than necessary these should be removed to encourage growth to the other branches. Other main branches would be any branch that is coming off the main trunk which do not fit well within our triangle shape. If you are unsure about which branches to remove then wait until your tree has been wired and you can see exactly where everything sits and then remove anything that looks wrong.
Because we have a trailing branch / trunk going over the side of the pot you will need to have a pot which is a vertical rectangle rather than horizontal rectangle. Some large cascades can cascade for over a metre from the top of the pot. In this case a plant is sometimes displayed on a stand (kind of like a small bar stool).
So, in summary we need to have a vertical pot, a tree which has one long branch or a soft enough trunk and some side branches to make the top and back.
What equipment should I have before I begin shaping my tree?
Begin by repotting the tree
In most cases if you are transforming an existing tree into cascade style or if you just bought a new tree from a nursery you will need to move it into a new vertical pot. At the base of the pot place the crushed tile (or similar material) and fill about 1/5 to 1/4 of the pot. This step is not necessary if your tree is small, the idea is to assist with draining but for small pots the water should easily drain freely.
On top of the crushed tile fill the rest of the pot with a good soil mix (search on the internet for what mix you should use or buy something suitable from a nursery) making sure you leave enough room for the soil attached to the tree.
Remove the tree from its existing pot keeping as much of the existing soil as possible. Because we are already going to disturb the plant by shaping it, it is best not to replace the soil and do significant pruning / wiring / shaping at the same time. You may be able to tease some of the roots out at the base of the root ball depending on the size of the plant.
Place the tree on top of the new pot at a suitable angle (refer to the picture below). Firmly press down so the base of the trunk is near level with the top of the pot. The back side of the plant that is now sit on the angled up side of the root ball needs to be push down so that it secures the tree firmly inside the pot (refer to picture below). If you leave some of these roots exposed they will die off if they are not getting enough moisture. If you feel necessary you can cover these roots with extra soil to maintain plant health until more deeper roots have grown. If you have a sufficient number of roots you can trim off some extra roots at the back to get the right aesthetic.
For the picture below the green triangle is only shown in the first part, but this doesn't mean that you need to remove leaves, it is just easier to show branch structure rather than the shape of the tree.
Wiring the tree
I'm not going to go into detail on the general techniques for wiring a tree. Again, that is another topic and you can either ask another question here on general wiring practices or search the net for how to wire a bonsai tree (someone may have already added this info to this site or asked since I wrote this). Beside some main techniques it mainly comes down to trial and error and practice.
The idea with wiring our tree is that we want to achieve the main strucutre mentioned above, one long trailing branch, two smaller branches for the top and back. Once you have selected the three branches you are going to wire prune any larger branches that will not be needed as part of our structure prior to wiring to create space for yourself.
My preference is to wiring the long, main, cascading branch first because we know roughly the direction this is going to go and it will allow you to consider the structure of the rest of the tree in relation to the cascade once it is done.
Once you have wired the main cascade, wire the top and back branches to suit and you are nearly done.
Next wire side branches and trim any other side branches which you will no longer need.
Shaping and trimming
I suggest you leave shaping and trimming the tree until the following season after the plant has been allowed to recover and go through another spring/summer growth cycle. This will allow your tree a greater chance of survival and it's not going to look its best if you try and trim it now.
In the following winter when the plant is dormant make some adjustments to the overall shape and rewire any branches as needed. Then during the summer after shape and trim your tree to make it look its best.