How to care for Juniper as bonsai
Posted on February 08 2019
Junipers are the most easily grown, adaptable outdoor narrowleaf evergreen plant used as outdoor bonsai. Fairly fast growing, they will become open and ratty in heavy shade. They also might suffer with excessively humidity and constant high nighttime temperatures. Junipers consume water even in the winter when roots are steadily growing but will languish with water-logging. When we grow our bonsai in training a training pot we never wire the roots down to the pot so we can slip the plant out of the pot to check for overwatering, underwatering, soil pests, etc. If you can squeeze off the outer layer of a slimy brown root, you've got root rot. Some younger junipers may become infected with twig blight, a fungal disease that could lead to tip dieback. You'll notice the newer foliage turning from green to yellow and suddenly brown on the entire branch. Both blight and root rot are secondary to overwatering. After fixing the overwatering problem, you can use a 3 in1 systemic fungicide, insecticide, miticide as a last resort. Horticultural oil addresses overwintering and summer pests.
All take pruning well and have flexible branches. Selectively remove unneeded shoots during the development stage. Always leave some strong new tip growth during the time part of the older foliage is replacing itself. In other words, thin (prune, not pinch) congested foliage mass periodically while leaving strong foliage closer to the trunk and a few other strong branches temporarily to support the rest of the plant. This technique applies to all narrowleaf evergreens. Use wiring and directional pruning. Root system thrives in a cool, moist substrate high in organic matter and in a deep training pot. They don't require much fertilizer but respond to additional Ca and Fe.
- At maturity, most Junipers are largely conical. Some become columnar or wide spreading.
- Natural growth characteristics cannot be stereotyped but Junipers always take on a rugged character as they age. They can be enhanced with deadwood features. Remove a large section of bark and create a few dead branches for that.
- Younger plants may have needle-like (awl-shaped) prickly foliage. Older plants might both needle-like and scale-like foliage.
- A few varieties tend to temporarily acquire bronze colored foliage after the first hare frost.
- Berry-like fruit cone.
- Bark is thin, shedding. Rarely scaly.
- For propagation, stick Juniper cuttings Nov-Apr. when they've stopped the first top growth. Strip and wound lower third. A rooting hormone and additional light are beneficial. Keep moist but not wet. Use soluble fertilizer after rooting and allow to go thru the first winter prior to first transplanting.
Shimpaku is sometimes listed as J. chinensis var sargentii 'Shimpaku'. Discovered by Sargent on coast of Hokkaido Japan. Low, irregular vase shaped form with soft gray green foliage. One of the best junipers for bonsai. Highly sought after.
Procumbens Garden Juniper normally grows as a spreading ground cover. Can be staked and developed as a mounded tree form or wired to a cascade form.
Sea Green is similar in form to Shimpaku but is faster growing and with mint green foliage.
San Jose is a fast growing, irregular spreader with sage green foliage and multiple prominent live veins.
Robusta Green is an upright form with tufted brilliant green foliage year round.
Perfecta is an upright columnar selection with darker green foliage. Retains that bright green color all year.
Foemina is usually trained upright.
Echiniformis is a spherical, slow growing native.
Rocky Mountain Juniper, a native, is found on high dry rocky ridges and grows into a more narrow conical multi trunk tree with an open irregular crown.
Grey Owl, a native juniper, has small soft silver gray foliage and is wide spreading vase shaped.
Hillspire has a more symmetrical upright narrow conical habit with desirable year-round bright green colored foliage.