I am often asked when it is time to repot a bonsai and whether root pruning is carried out to restrict growth.
Firstly, any containerised plant, not just bonsai, will suffer if allowed to grow continuously within a container. In its natural habitat a plant?s roots will extend outward in search of moisture and nutrient whilst in a pot it does not have this opportunity. Instead a plant depends entirely on the care given by the owner. So, periodically most containerised plants will need some adjustment to their roots.
Is root pruning part of growth restriction?
Consider this. In a natural environment the finer roots extend in order to absorb moisture and nutrients. These fine ?feeder? roots are the most efficient to the plant and as they increase in number the supporting roots become enlarged and woody to act as a conduit and anchor. When this occurs in a container the woody roots begin to take up more space and the root ball becomes compact and less efficient. Therefore, contrary to conventional wisdom removing a proportion of woody roots and creating air pockets in the root mass actually helps improve top growth as the root system returns to efficiency.
When is it time to repot?
Although there are seasons to bonsai work in the same way as sowing and harvesting, maintenance work on pot plants should not be carried out systematically to a set schedule. Instead the decision to carry out work should be judged on the health and vigour of a plant. Lift the plant out of its container if the roots are growing around the edges and the root ball feels firm and compact when squeezed then it is time to act. If you had been observant you might also have noticed a reduction in vigour since the first year you bought the plant. The time of year to carry out root work for deciduous trees is when they are dormant. Usually, this is just as the buds begin to swell in late winter but before the new leaves emerge.
The process.
It is difficult to cover every eventuality here, so I can only generalise.
Prepare before hand so that roots are not exposed for too long.
You will need: Potting soil, in bonsai we used a very free draining mix consisting of loam, grit and bark, but this is a whole new topic.
Have a root hook, bent screwdriver works well. A pair of old, strong scissors or old secateurs.
A container. Some garden wire to secure the plant in and some coarse mesh to cover the drain holes.
Comb out the roots from around the sides, bottom and surface. Be careful on the surface not to spoil the visual effect of the root flare. [another topic]
Inspect the root ball. If there are bare areas then inspect closer as this usually means a problem with the soil or the presence of a pest such as vine weevil. Clean out these areas.
As you work do not remove more than half of the overall root mass.
Choose a couple of dense areas on which to work bearing in mind the above.
Look for the thick woody roots and cut these as close to the trunk as possible without spoiling the cosmetics.
Push a screwdriver or similar into the unworked compact areas to create some air pockets. Try not to use too much force to avoid root damage.
Lightly trim off the straggly roots around the root ball.
Prepare the pot with the mesh and securing wires.
Do not put stones into the bottom of small pots this only uses up valuable growing space.
Place some soil into the bottom of the pot and place the tree on top.
Take care to position the tree properly. [another topic]
Tie the tree into the pot with wire to reduce wind rock.
Fill with soil making sure that soil fills the cavities by using a chopstick or similar to push the soil down.
Top dress the soil surface with fine material or grated sphagnum moss.
Water well, until water runs out of the drain holes.
Give some protection for a few weeks.
