Repotting and root pruning: why, when, how?
Kew's Bonsai Specialist Richard Kernick explains the 'why, when and how' of the bonsai year's first major job: repotting.
Welcome back. With spring fast approaching it’s time to start on the first major job of the bonsai year; repotting. It’s going to be a rather image heavy blog so I’ll start with a why, when and how:
Bonsai trees do not require the large, woody, anchoring roots that trees in the wild need to prevent them from being blown over. It is necessary to develop a mass of fine ‘feeder roots’ close to the bottom of the trunk which will help the bonsai to take up water and nutrients more efficiently.
- Repotting provides the tree with fresh soil, including fresh minerals and nutrients which may have been leached out by the watering process or successfully used by the plant
- Repotting removes inefficient woody roots that have become too long and are taking up essential space in the pot
- Repotting improves the air circulation around the roots by removing unnecessary roots and old compacted soil
- When repotting we can work on and improve visible surface rootage
Repotting should be carried out every three to five years, dependant on the age and vigour of the individual tree; younger trees can be repotted more often. NEVER carry out unnecessary repotting of trees, as it is stressful to the plant, and could kill it.
You should consider repotting a tree when its vigour starts to drop off i.e. if the tree sets less buds in the autumn, or if twigs or branches start to die off unexpectedly. Other signs that repotting is required are difficulty in watering a tree; water fails to penetrate the root ball due to the air spaces between the soil particles filling up with old root material and broken down soil grains. Root pruning can be carried out in the late autumn before a tree enters dormancy (usually for conifers and other evergreens), but is best carried out in early spring, just as it is beginning to break dormancy.
Try to dry out the soil in the pot slightly before repotting. It is not important to add 'crocks' or other drainage material in the bottom of the pot – this simply uses up valuable space – there is little enough soil in the pot to start with! Prepare an ample supply of soil or compost before you start; this will minimise the stress to the tree (and yourself) should you have to stop halfway through the repotting process to prepare more. Soil should be sieved to remove dust, and it is often a good idea to prepare different grades (grain sizes) to use in different areas of the pot; larger grains towards the bottom of the pot, smaller towards the surface. If you are looking to change pots, or it is the first time you have repotted your particular tree, it may be an idea to prepare an extra pot (or two, if available) just in case the root ball does not fit in your new pot.
Refrain from fertilizing for up to a month after repotting. Appyling fertilizer can 'burn' weak new roots.
So that’s it for the whys, whens, hows, dos and don’ts. Below is a step-by-step guide to how you should go about repotting a bonsai tree. Kind thanks go to Mr Nobuyuki Kajiwara (Nobu) for granting permission to use the images I took of him repotting one of the Kew bonsai, a trident maple (Acer buergerianum) several years ago.
Carrying out the initial spring pruning. Pruning away any excess branches and buds at this point will reduce the 'workload' on the pruned roots when the tree comes out of dormancy.
Many bonsai pots feature an in-turned lip which can make it almost impossible to easily remove the tree for repotting. A 'root sickle' is used to cut the roots and soil around the edge of the pot to release the tree easily (for more on tools and their use
After using the root sickle, the base of the pot is thumped to loosen up any roots that may still be gripping the sides of the pot.
The tree and root ball can now safely be removed from the pot.
A 'scribe' type tool is now used to tidy up the surface of the soil (a chopstick will do). This reveals any large roots that may require remedial work. The surface of the soil can be brushed down with a hemp brush to remove any old moss and debris.
he roots around the edge of the rootball are combed or raked out using a root hook (for more on tools and their use, see my previous blog Tools of the Trade).
Once the roots around the edge have been loosened up, the same process is applied to the base of the rootball.
Now, the excess roots can be removed. Roots should be cut at a 90 degree angle, leaving as small a cut surface as possible. Where possible, the cut surface of the root should point downwards.
Remedial work is now carried out on thicker, woody roots. A branch-splitter is being used to sever the roots.
The root pruning process is now carried out around the edge of the rootball.
The tree now undergoes a “test-fitting” in the pot to see how much more of the root ball still needs to be removed.
At this point, any changes to the trees position in the pot need to be carefully considered; a little to the left, further towards the front or back, or rotation to better show the front of the tree? Stand back and consider the planting position carefully, as you’ll have to look at it for the next few years..!
More roots are combed out from one edge of the root ball with the root hook and trimmed off to better fit the tree in the pot.
A thin layer of akadama soil is poured into the bonsai pot
A thin layer, less than a centimetre, of dust-free (sieved) akadama compost is now added to the bottom of the pot (I will write a future blog about various different bonsai composts and their uses). A small mound of compost is built up in the centre of the pot to fill the concave area under the root ball that has resulted from the pruning off of the large roots.
Retaining wires have been added through the drainage holes to securely wire the root ball in place (short wires to the back of the pot, longer ones to the front)
More akadama compost is now added round the edges of the bonsai’s root ball and carefully worked in with a chopstick to prevent any air spaces.
The periphery of the bonsai pot has been filled with soil, and more has been added to thinly cover the rootball’s surface. The retaining wires now hold the bonsai in place.
The tree is thoroughly watered
The tree is now thoroughly watered in. Water should be applied until it runs clear through the drainage holes, washing away any akadama dust that sieving failed to remove.
The surface of the soil has been coated with chopped, sieved, sphagnum moss; this helps to draw the moisture within the pot up to the surface, the area which is most likely to dry out quickest. Roots are more likely to be active in this top layer of soil, where there is a ready supply of water, air and warmth. The top-dressing of sphagnum moss will also help natural-looking green mosses to colonise.
A small trowel is used to gently press the wetted moss into the surface of the soil.
Well, that's all from me (and Nobu) for now. I hope it proves helpful during your repotting endeavours!
- Richard -