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Repotting and root pruning: why, when, how?

Richard Kernick
6 March 2012

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:

Why:

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

When:

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.

How:

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.

Nobuyuki Kajiwara carrying out the initial spring pruning of a bonsai trident maple

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.

Nobuyuki Kajiwara using a root sickle to cut a bonsai trident maple out of a pot

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

Nobuyuki Kajiwara thumps the base of the pot to release any roots that may be holding the bonsai in place

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 Trident Maple bonsai can now be eased out of the pot.

The tree and root ball can now safely be removed from the pot.

Nobuyuki Kajiwara uses a pointed metal tool to reveal a little more of the surface rootage of this Trident maple Bonsai tree

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.

Nobuyuki Kajiwara uses a roothook to comb and rake out the roots in the periphery of the bonsaiÕs rootball

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).

Nobyuki Kajiwara again uses the roothook to loosen roots from the base of the bonsai treeÕs rootball

Once the roots around the edge have been loosened up, the same process is applied to the base of the rootball.

Using root shears, Nobuyuki Kajiwara removes excess fine roots from the base of the bonsai Trident MapleÕs 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.

Nobuyuki Kajiwara uses a large branch cutter to prune away some of the thicker, woody roots of this Trident maple bonsai

Remedial work is now carried out on thicker, woody roots. A branch-splitter is being used to sever the roots.

Nobuyuki Kajiwara continues the root pruning process to include the sides of the bonsaiÕs rootball

The root pruning process is now carried out around the edge of the rootball.

The bonsai tree undergoes a Òtest-fittingÓ to see if more of the rootball needs removing.

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..!

Nobuyuki Kajiwara combs out more roots from one side of the bonsaiÕs rootball to better fit the tree in the pot

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.

Nobuyuki Kajiwara pours akadama soil into the bonsai 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 bonsaiÕs root ball and carefully worked in with a chopstick to prevent any air spaces.

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 rootballÕs surface

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.

Nobuyuki Kajiwara now thoroughly waters the bonsai Trident maple

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.

Nobuyuki Kajiwara uses a small trowel to gently press the wetted sphagnum moss into the surface of the bonsai potÕs soil

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 -

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Comments

10 April 2013 I have a much loved bonsai which I bought about 10 months ago (I've been keeping it in its original ceramic bonsai pot indoors near the window.It is a satsuki azalea and it flowered absolutely beautifully throughout last summer however since the cold winter I believe it has been struggling. All the leaves have withered and died and even thouhgh the description states it is evergreen it has no leaves so I think something has gone wrong. The branches/twigs are also looking quite dry despite regular watering.Is it dead?Or is there anything I can do to save my bonsai?
Abuse Reported. Comment will be reviewed and removed if necessary.
16 March 2012 Hi Richard, just found your reply to my query, thank you! Very useful.The Bonsai House is my favourite part of Kew and I've often wondered who looks after them. Would be lovely to have more posts on this blog. I found this one through the Kew Twitter feed, so make sure they put up an alert, or maybe you should get your own Twitter account!
15 March 2012 Hi Jane, I would recommend never removing all the old soil from a rootball; you’re looking to preserve and encourage the fine feeder roots. Bending them and exposing them to fresh air will damage and kill them off, so it’s best to leave them undisturbed if at all possible. As most of the bonsai in the collection here at Kew are mature specimens (most are aged over 40 years) there is now little need for overly invasive re-potting. It would be virtually impossible to totally remove the soil without using a pressure washer and causing some pretty major damage! Sometimes most of the soil will come away when re-potting younger trees (up to about ten years old) as they have not yet developed many of the finer feeder roots that hold the rootball together. I will add some pictures soon of a stage that was not carried out in the previous blog as it was deemed unnecessary. Nobu removed several thick areas of root from beneath the root ball so we decided it was unnecessary to remove any more. What usually happens is that once the edges and base of the rootball have been pruned, a mental note is made of where the most woody roots are concentrated. A wedge can then be cut into the soil of the rootball from the outside edge in towards the trunk, and will usually locate a thick root that can be entirely removed. These thick roots can be a drain on the trees resources, and if removed can produce many smaller roots which will increase the trees efficiency. I’ll endeavour to add another blog soon showing the cutting of wedges and methods of securely wiring bonsai back into their pots. Regards, - Richard -
7 March 2012 Do you only ever remove old soil from underneath and around the edge, or do you sometimes remove all the old soil from the entire rootball?
6 March 2012 Fantastic blog entry. Descriptive and explanatory, flowing and visually documented. Not only helpful in explaining the effort and expertise employed to maintain Kew's collection, but helpful to home horticulturists as well.