Pines, along with Junipers and Maples, are the one of principle subjects for Bonsai. An association with rugged and mountain scenery they evoke the struggle for existance that we seek to characterise in our creations. However, their growth style and consequent treatment and development, which is somewhat different to most other subjects, can make for confusion and apprehension for some.
The key to understanding how to develop pines is to know how their growth pattern happens over a year.
At the start of spring there is usually a single bud at the the end of the previous years growth. At the base of the same shoot is a ring (whorl) of buds which may number from 2 or 3 to 6 or even 8. By late spring, these buds will start to extend to produce a new shoot that is initially without leaves. This is called a candle. Because they are long and thin leaves are more commonly known as needles.
After 2-3 weeks the candle will develop needles in small bundles. Depending on the species there will usually be pairs of needles in each bundle with a couple of species having bundles of 3 needles or 5. The candles continues to extend as summer progresses and needles will be produced as the shoot extends.
Sometimes the lower part of the candle will be devoid of needles which will present future problems. Many pines tend to go dormant over summer, a process called aestivation and will resume growth again in the late summer and autumn.
At the end of summer a new bud forms at the end of the years growth and a whorl of buds form at the base. It is rare for buds to develop in the middle of a years growth.
If left to themselves pines will produce a standard Xmas tree shape with a single straight trunk and rings of branches at wide intervals. Only as a tree matures and ages will some of these braches decline and produce something interesting, by which time it has long gone beyond being useful for Bonsai purposes.
In the UK, most of the pines are of a small subset of species that exist. Imported varieties from Japan are the Black and White pines, rarely the Red pine. Native to the UK is the wonderful Scots pine and from Europe the Mugo and occasionally the European Black and Cembra pines.
All of these have their own characteristics which also affect how they are treated.They principal characteristic is how much they are inclined to produce new shoots if so induced. The Japanese Black pine and to a lesser degreee the Red pine are 'multi-flush' pines in that they will produce more than one candle set per year. The rest are 'single-flush' pines which can only be relied on to produce one candle set per year.
There are many ways of developing these but there are two variations that will cover most situations: Maintenance and Development pruning.
Maintenance pruning is for mature, strong Bonsai that have largely been completed and just need to be kept in shape. Development pruning is for younger trees, that have yet to reach their desired size and structure. Most of our trees are in the development phase!
In Maintenance pruning, all candles are reduced the little more than one or two cm in late spring before the needles have fully developed. It is important that all candles are treated as there will be an imbalance as the uncut shoots will develop at the expense of the uncut ones.This keeps the tree to its desired structure and new buds will appear at the end of summer in the usual way. Bjorn Bjorholm made a great video of this (but beware of initial vulgarity).
In Development pruning we desire to keep the tree vigourous and strong, thickening the trunk and ready to produce lots of new shoots and even back budding on older sub-branches for the coming years. Delay the shoot shortening until mid/late summer (mid August in the UK). This allows the tree to utilise the new growth to feed and build up the resources ready for next year. It also results in multiple buds at the end of each shoot in Autumn. Reduce to 3 buds at the end of each shoot in late Autumn and further reduce to 2 buds in late winter keeping the two most similar in size at each tip.
The process the repeats each year using your method of choice for the desired results.
The halt of growth in mid summer allows an opportunity to wire a tree while sap flow is low. On the other hand, beware of old wire from late summer onwards. You may suddenly find braches thickening far more than you expect and unwanted wire damage.
With multi-flush pines the objective is to get two growth seasons in one and also keep the needle length as short as possible which is important with these species.
In summer (early July in UK), reduce the shorter shoots to just 4 bundles of needles. Ten days to a fortnight later, shorten the longer shoots similarly. This balances the relative strengths of shoots to get an even break of new buds. The very shortest and weakest buds should be left untouched to allow them to build strength.
These are mountain pines accustomed to harsh winters and are much slower growers than the previous mentioned pines. They are generally left to their own development and only if a shoot gets excessivly long for the design should it be shortened.
The Japanese White pine has long been regarded as tempramental and accordingly been kept in shelter over winter. This has not worked and recent thinking is that it will benefit from a good winter and is better left outside.