Why
Repotting is Essential to Bonsai
As
plants of all kinds grow, their root systems become larger and
more extensive in order that they can supply their ever-expanding
canopy of foliage with the necessary quantities of water and nutrients.
Trees and shrubs grown in the ground can have root systems that
extend beyond the shadow of their own foliage canopy in a search
for water and nutrients.
On
the other hand, containerised plants are limited by the size of
their pot as to how far they can extend. They need to be constantly
supplied with food and water on which to survive. Their root systems
however, continue to grow in tandem with their leaves and branches
above the surface of the soil.
After
a period of time that varies between different plants and plant
species, the root systems of all pot-grown plants fill their containers,
and become 'pot-bound'. Under these conditions, new fine feeder
roots that are so essential to the uptake of water and nutrients
in a plant have little room to grow, the soil structure deteriorates
and the plant starts to suffer.
With
an ordinary pot-plant the solution is to pot the plant into a
larger container which allows room for new, fresh compost around
the rootball. With a bonsai, the aim of repotting is the same,
to allow fresh compost in and around the root system so that it
can continue to form fine feeder roots and so that fresh soil
can be introduced around the root system. However, with Bonsai,
the container, and more specifically, the size of the container
is not only part of the design but its size is specially selected
to suit the tree. For this reason, Bonsai are root-pruned.
A
side effect of root-pruning is that it increases the density
of the root ball. From every root that is trimmed, a number
of new roots will emerge from the root-tip that was removed. As
the rootball is repeatedly pruned over the years, the rootsystem
becomes denser and denser. Within a well-developed rootball, dozens
of fine feeder roots can occupy the same volume of soil that one
unpruned root may ordinarily use. So though the size of the rootball
is regularly reduced, the actual volume of root within a certain
amount of soil increases, and sufficient to support the canopy
of the tree.
Root
pruning does not dwarf or stunt the tree in any way . The tree
may lose a little vigour for around 6 weeks after rootpruning,
as it regenerates its root system (this is more noticeable with
evergreen tropicals such as Figs), but after this short period
of adjustment, the tree becomes more vigorous than before as new
feeder roots are able to develop in the new soil.
How Often Should Bonsai Be Repotted?
Bonsai
need repotting when they become pot bound. A Bonsai is considered
to be rootbound when its roots entirely fill the pot and there
are long roots circling the rootball or inside of the pot. In
some cases, roots will also be seen to be growing out of the drainage
holes at the base of the pot.
The
time a tree takes to become rootbound varies from one year to
five. A number of factors affect the amount of time a tree takes
to become rootbound in its pot; different species of tree have
different levels of vigour. Fast growing species and individual
plants (Figs and Larch for instance) tend to require more frequent
repotting and rootpruning. Other factors that contribute to regular
root pruning include size of container, development of rootball
(a dense established rootball will not require as frequent repotting),
and the age of the tree, older trees are not as vigorous and require
less frequent repotting.
The
rootball should be checked for its condition annually in Spring;
gently ease the tree out of its pot and examine the rootball.
If the roots of the tree are still contained within the soil,
the tree can be returned to the pot and repotting can be left
for another year.
When Should A Bonsai Be Repotted?
Bonsai
are repotted whilst the tree is dormant, this reduces the stress
on the tree as it is not having to supply water and nutrients
to its leaves, or in the case of evergreens, the supply of nutrients
from the roots is minimal. To repot and rootprune when a tree
is supporting a full canopy of leaves and is actively in growth
would result in loss of foliage through desiccation, as the roots
are unable to supply sufficient water and nutrients to its leaves.
Repotting during the growing season is highly stressful to almost
all tree species and can often result in death or dramatic loss
of vigour.
Therefore,
Bonsai require repotting when they are dormant or when there is
a minimal requirement for the roots to supply the rest of the
tree with nutrients. Another factor that influences the exact
timing of repotting a tree is the length of time that injured/trimmed
roots will remain exposed to moisture and frosts. The rootsystem
will not repair itself fully until the tree starts into growth
during the Spring; this means a tree repotted early on in Winter
may not repair its roots for a number of months, leaving the root
tips open to infection, root rot and frost. By repotting closer
to the time that the tree starts back into growth, the less time
that the cut roots are left exposed to soil moisture and freezing
conditions.
The
best time to repot a tree is therefore right at the very end of
its dormant period, just as it is about to start back into growth.
The tree is still dormant, but, as the tree is about to start
back into growth, the repair of trimmed roots is fairly immediate.
The exact timing of this varies between tree species, and more
importantly, varies according to local climate and individual
plants. It is not realistic to expect to find out when to repot
a particular species on a certain date as there is so much variation
between different climates; as weather conditions subtly change
year on year in the same geographical area, it is also not true
that a tree is repotted on the same date each year.
Observe
each tree to see when it is ready to be repotted. Deciduous species
during the Winter, if inspected closely, will be seen to have
small leaf buds that are held closely against the branch. As the
tree starts to come out of dormancy, these leaf buds will start
to extend, ready to open out and reveal their emerging leaves.
