Many bonsai enthusiasts have experienced difficulties keeping junipers alive and healthy over a longer time span. Foliage seems over time to weaken and eventually the tree risks dying.
Please read the article “The wrong bonsai answer” too…
A common belief in the western bonsai world, is the way scale leafed junipers (Shimpaku and varieties) are controlled. Often we have learned how pinching new growth is the way to go keeping a nice dense canopy of the tree, and keeping it small. A habit of also trying to keep a bonsai controlled and exhibition ready all year round, adds to the mistreatment of the tree. Bonsai needs growth to be healthy, and will therefore benefit from growing a little out of shape in periods. Afterwards they can be trained into shape again up to exhibitions i.e.
Despite common belief new growth on junipers is not forced by dense pinching of the soft needles. In contrary. Said before but worth a repeat. As also american Michael Hagedorn has described earlier, pinching is a no go at junipers, but still many do.
What happens when pinching the foliage is that junipers are weakening, growth is hold back, and new growth will either grow very slowly or not at all. The tree is stressed and will sometimes push unwanted juvenile needle like growth to survive. Inside growth that can take over from older elongated growth will not happen, because light is not allowed to reach the inners parts. In the end risking the life of the tree is a possibility.
The method that japanese shohin bonsai expert Tomohiro Masumi (Koju-en), refers to for especially shimpaku juniper is described here; just cut long new growth with scissors (it is important to leave some new growth parts). Remove old weak leaves/needles in order to let light reach into the foliage. This will encourage new growth that takes over from the old. The new growth inside is mostly coming out on parts of branch separations.
Sometimes cut the long foliage (branches) and let younger foliage take over. New growth will appear inside (mostly coming out on parts of branch separation). With this technique it is possible to keep the outline small.