Juniper Procumbens, aka Japanese Juniper, are what most people picture when they think of bonsai. This document is meant to describe them.
Native to the mountains of Southern Japan, these plants are well adapted to life on the edge. Their long branches, often reaching 6 feet, sometimes as long as 12, can contour to the surrounding rocks. They are not tall growers, topping out at around 12 inches. Their natural growth habit is mounding, with new layers growing over each other. These plants are fully adapted to sun. Too much shade, including from its own branches, will cause the Juniper Procumbens to abort the needles that are not photosynthesizing. What’s left is then dead needles in the center of the plant, with a living, vibrant-green outer shell. Although their native climate is warm, these Junipers must have retained the cold tolerance of their relatives for they are cold-hardy and commonly used as a ground-cover landscape plant in the US.
Types Of Juniper Procumbens Foliage
Most people are familiar with the spiky, immature foliage of Juniper Procumbens. This is the growth that forces many people to wear gloves when working with their bonsai. This growth is also the most commonly seen in landscape plants, where they never seem to develop the other two stages, probably due to the constant shading from its own branches. The immature growth is a stress response for the plant in addition to being the default form on new branches. Stress your Procumbens too much, yet short of killing it, and it will revert back to this growth.
This growth looks like a hybrid between the immature and mature foliage. Its needles are far more compact than that of the immature foliage but still has the appearance of spikes. The transition from immature foliage to mature is not all-or-nothing and you will often see all three types on the same plant. However, it is far more common to see the semi-mature growth next to mature than it is to see immature next to mature. You will usually only see this type of growth on older branches.
This growth is characterized by very tight needles that are not spiky at all. They are so compact that they resemble scales of a reptile. This growth looks far more like other Junipers within bonsai and most people do not even recognize them as Juniper Procumbens anymore. In addition to the growth being aesthetically pleasing, the bark is mature, and the tree can start producing pollen cones at this point.
Veins And Pathways
Junipers grow with living veins, and the Juniper Procumbens is no exception. The plants have pathways connecting certain branches with the roots. These pathways can be strengthened or weakened based on how much light the foliage on these branches receive, as well as the roots ability to find and pump nutrients to that foliage. A branch that is receiving lots of light will produce lots of growth, requiring more nutrients and water, and a larger vein. If a pathway is closed because the branch is removed or is no-longer needed due to weakening, the pathway will die and become dead-wood. This effect is often seen with shari (dead-wood on a trunk) and jin (dead branches), and is common in nature. The living vein, assuming it is strong, will continue to expand out in all directions, wrapping itself around the dead-wood. Use it or lose it.
Do not prune all foliage off of a Juniper Procumbens branch unless you do not need the branch to survive. They will back-bud on a strong, living vein. However, unless the vein is absolutely critical to the plant’s survival, and the vein is very strong, the plant will see no reason to put its energy into back-budding when that same energy can be used on the perfectly good living growth that remains on stronger pathways. If you do leave some foliage on the branches, then the plant will have far more incentive to keep the branch, although it may still abort the weak ones.
Where To Keep Juniper Procumbens Bonsai
Juniper Procumbens are not a tropical plant. However, they do grow in some warm climates and can be grown indoors for the winter in some situations but typically should be grown outside in NJ and allowed to go dormant during winter. We recommend to people who want to grow one indoors for the winter to try it. If the plant does not respond well by limping along, send it dormant next season to give it a rest. We do keep some in a heated greenhouse during the winter but only after given a dormancy by leaving them outside through Thanksgiving.
Juniper Procumbens change color when exposed to the cold. That can range from a slight red or blue tint to a dramatic bronze tone. This does not harm the plant, and is quite the opposite in regards to dormancy. A dormancy gives the plant a chance to move energy back down to the roots, accumulate more energy, and send it back up to the foliage to resume photosynthesis.
Pests And Diseases
Juniper Procumbens can be subject to spider mites. These tiny, red insects suck the energy from the juniper’s needles, weakening the plant. They commonly attack in situations of low humidity (mid-summer and indoors winter). They are easily controlled with pesticide.
For more information on how to care for Juniper Procumbens, as well as any other bonsai, please see the bonsai care sheets.