Like most things Bonsai is governed by the weather, and each season brings about a wonderful new phase in the trees development.
As with any new hobby there is a tendency, to take as gospel, everything you read about Bonsai, but as you gain knowledge and experience you will realise that there are no hard and fast rules, only guide lines, and if it works for you, stick with it.
Apart from the weather many things can influence the timing of certain processes, the town or country location, is the Bonsai in and exposed or in a protected garden, at the top of a hill or in a valley. Even in the smallest garden conditions will differ creating micro-climates, so when using this guide do be flexible, we can still get snow in June.
To enable bonsai trees to flourish they must be repotted and March is the traditional time for this to be carried out, but again the weather can have a great influence on this, moving the optimum time as far back as mid February or even forward to early April. If we divide the trees into evergreen and deciduous some useful identification of when to repot can be gained. Firstly, evergreen trees are never fully dormant and they are reasonably safe to repot in March. However, deciduous trees do go dormant in the autumn and break dormancy in early spring, they must only be repotted when they are dormant. A good indication that a deciduous tree is coming out of dormancy is when the buds start to swell and that is the best time to repot. If the leaves have started to open it is too late and it is best left for another year.
Why do we need to repot? A bonsai needs to grow continuously in the same way that a tree growing in the ground does, consequently when the pot becomes full of roots the tree will exhaust the nourishment in the compost and will eventually become pot bound and die. Therefore, when the pot is full of roots we repot into a larger pot to allow the tree to continue its growth.
Generally, with an established Bonsai, it is usual to repot back into the old pot, to do this root pruning must be carried out and, once again, this can only be done when the tree is dormant. To root prune, lift the tree out of the pot and carefully tease out the outer roots, remove as much soil as possible without disturbing the inner ball, and then, with a sharp pair of scissors, cut off enough root all the way round to leave about two thirds, this will allow enough space for a further season’s growth. Finally, repot the bonsai using a free draining compost.
Young trees need to be repotted and root pruned annually whilst for old established trees, every two or three years is sufficient, and root pruned annually whilst for older trees, every two or three years is sufficient. After repotting, trees need to be kept out of strong sunlight and wind for three to four weeks. Be careful not to over water after root pruning has taken place until new growth is established approximately three to four weeks. Over watering at this time can lead to root rot.