Deadwood is a prized feature on many conifer bonsai. For the most part, shari and jin on the upper part of the tree rots slowly, as it dries out faster. If in contact with the soil, however, rotting is accelerated.
As a general comment, any collected juniper’s wood will outlast a cutting-grown juniper’s wood.
This is likely related to how tight the rings are. Many collected junipers have a ring count in excess of 100 rings to the inch.
In contrast, it’s possible to grow a 2-inch Itoigawa juniper from cutting in 10 years. The growth rings are wide on these. And the rot resistance appears to be lower.
There may also be species variations in juniper deadwood rot resistance. If seen, it may simply be the slower growth rate due to a shorter growing season for some collected junipers, like Rocky Mountain Juniper, resulting in closer rings, compared with California Juniper. Or, more resin sequestered in the wood in some species than in others.
A cutting-grown Itoigawa Juniper, about 40 years old, with a familiar situation, a rotted base. Cutting-grown junipers have wider growth rings than collected.
A collected Rocky Mountain Juniper, with no rotting near the base. This is old, dense wood. But eventually even jin on collected trees will rot.
What can we do to slow down rot?
Lime-sulfur helps, as it kills the microorganisms that rot the wood.
Even better is an application of wood preservative that penetrates into the wood. If you try this, consider thinning with isopropyl alcohol to give better penetration, and then paint many coats on. If shininess is apparent don’t be too concerned, as this will dull in several months.