Why
Repot in Spring?
As temperatures
slowly rise in early Spring, the roots of a bonsai slowly become
active. The energy resources of the tree that have been stored
in the roots over winter begin to move back up into the tree.
As they do, the dormant buds on the branches (produced in late
Summer and Autumn of the previous year) begin to swell.
This is the
signal that the tree is beginning to come out of dormancy.
As ambient temperatures continue to rise through the next weeks,
move of the tree's stored energy moves from the roots, up into
the tree in readiness for the explosion of new growth as the tree
comes into leaf. At this point, the roots stop being an energy
store and their primary function is to supply the newly opening
leaves with moisture from the ground.
This process
can be followed from start to finish by observing the state of
the buds visible on the branches on the bonsai. This process can
be broken down into 4 stages;
The
4 Stages of Spring Buds
Stage One,
Winter Buds: The Winter Buds on most trees are tiny and are typically
colourless. At this point the roots are also dormant. The majority
of the trees resources are contained within the roots.
Stage Two,
Swelling Buds/Bud Swell: A period of time after the roots begin
to wake in early Spring, the buds on the branches will begin to
swell. In many (but not all) species, the swelling buds will have
a stronger colour than those that are still dormant (Winter buds).
It is at this
stage that new adventitious buds can appear on the trunk and existing
branches of the tree.
Stage Three,
Extending Buds/Bud Extension: The buds will lengthen, take a stronger
colour and in many cases appear to be on the verge of opening.
By this point the majority of the tree's resources are no longer
in the roots but in the above ground parts of the tree.
This is the
best time to repot and root prune as the roots are very active
and can repair themselves rapidly, and the majority of the trees
resources are contained in the tree. In other words, pruning and
removal of the roots will not deplete the tree of energy.
Stage Four,
Bud Burst/Bud Opening: The buds open and the tiny new leaves begin
to appear. Repotting must be finished by now.
Identifying
the 4 Stages
This
image of part of a Hawthorn (Crataegus) was taken mid-February
in the UK. The swelling buds have brighter, more colourful tips
than the dormant buds.
Not
all the buds on any one tree will all change from one stage to
another uniformly. The tendency of most trees is that the apical
buds (those on the tips of branches and in the higher areas of
the tree) will always progress more quickly than buds on the inner
and lower branches. This Hawthorn branch has 2 swelling buds at
the tip and one bud lower down that is still dormant.
This
Hawthorn now has swelling buds all over the branches and a few
extending buds. These will begin to open soon and repotting must
be carried out.
So
During Which Bud-Stages Can a Bonsai be Repotted or Root pruned
and When is the Best Time?
Stage
One: Dormant Buds.
When
the buds are dormant, the roots are also dormant. During this
time the tree can be lifted from the ground or its current pot,
bare rooted if necessary and repotted.
However,
during this time, rootpruning will remove some of the stored energy
of the tree and this will affect the growth rate of the tree during
the forthcoming growing season. Any damage to the roots (either
accidental tears, wounds or pruning cuts) will not be repaired
until the roots are more active, leaving the possibility of dieback,
loss of the root or even the possibility of infection to the site.
Trees
can, and are, collected and root pruned at this time with no obvious
ill-effect but there are risks to the health of the tree (that
increases according to the amount of the root system that is removed).
Stage
Two: Swelling Buds.
During
this time the roots are coming out of dormancy but do not have
to support any foliage and so can still be disturbed.
Repotting
and rootpruning can be carried out safely. The closer the tree
gets to Stage Three the better as repotting early will deprive
the tree of some energy. However, those enthusiasts with large
collections and with many tree trees to repot in a short period
of time, will typically start early and repot trees that have
many Swelling Buds.
Stage
Three: Extending Buds
The
very best time to repot and rootprune is as soon as extending
buds are visible, typically in the upper branches of the bonsai.
The roots are active and will repair any damage or pruning wounds
quickly, the majority of the tree resources are above ground and
the roots are not yet required to support any foliage.
Be
warned; the extending buds stage can last a relatively short period
of time!
Stage
Four: Opening Buds
The
majority of tree species must be repotted and rootpruned
by this time or they should be left until the following year.
As the first opening buds appear, the roots are required to supply
moisture to the new foliage and should not be disturbed!
These
Acer palmatum Kiyohime buds are in the Swelling Buds- Stage Two
phase though the buds at the tips will begin to extend soon.
Elsewhere
on the same tree, the first two opening buds are found, the tree
must be repotted today or left until next year. Here it is very
clear that the apical buds at the tip of the branch have progressed
quicker than the other buds. It is when these apical buds begin
to open that repotting should finish. Some weaker buds on the
insides of the branches will remain at Stage One or Two and not
reach the stage of Extending or Opening.
Further
Notes on Spring Buds and Repotting Times
It
is not easy to immediately identify the different stages of Spring
Buds until you are familiar with a particular species, as their
appearance differs from species to species. However, the basic
descriptions of a winter bud, a slightly swollen bud, a lengthening
bud and an opening bud does hold throughout all species. If you
observe all of the buds on any particular tree, you should be
able find and compare examples of at least one or two of the 4
stages, sometimes with 3 or 4 of the stages occurring on just
one branch (like that of the Kiyohime pictured above).
Repotting
can be safely carried out when the buds are swollen and extending,
repotting is best carried out when the buds are extending.
Repotting
can be carried out when the buds are dormant but will result in
a loss of vigour in the tree until it recovers the 'energy' lost
from the roots. For this reason, winter repotting (repotting when
the buds are still dormant) is not advised.
There are exceptions to these rules but as a fail-safe and general
indicator, repotting should not be carried when the buds begin
to open.
