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Eastern White Cedar - Pinching Techniques
By Greg Cloyd, with Bruce Baker
At a recent advanced study group meeting, eastern white cedar, or arborvitae, (Thuja occidentalis) pruning, foliage plane maintenance, and pinching practices were discussed.
Thuja occidentalis is a fragrant, evergreen, false cedar of northern North America. It is differentiated from its western cousin (Thuja orientalis) by its smaller size, prominent resin glands, slight differences in cone morphology, and its un-winged seeds. It grows as a telephone-pole-straight tree of moderate height in its southern range. Farther north, in Canada, it is a twisted and contorted, wind- and snow-tortured, deer-browsed, small tree of bog and limestone lakeshore. Along the Niagara escarpment and in Quebec it is a cliff-dwelling ancient dwarf growing less than an inch in radius per century.
Arborvitae is very amenable to bonsai culture, readily tolerating a wide range of soils, fertilizers, and climates. It is impervious to poor water, soggy conditions, and shallow pots. It can be twisted in a knot, air-layered, repotted out of season, and generally manipulated with impunity. It has a pleasantly shaggy bark. The species is widely available in the wild and in landscape nurseries.
One physiological feature of the plant is worth noting. Each hydraulic pathway, cambial lifeline, or xylem and phloem conduit system, is sectored-completely divided from the rest of the plant. From a practical viewpoint, this means that each major root supplies one side of the tree or one major branch. If the root or branch is killed, that entire portion of the plant will die. This phenomenon is seen in other trees, but is strikingly dramatic with white cedars. For this reason, collected white cedars should be allowed to establish in a growing pot for at least two growing seasons prior to styling. Otherwise, you risk building a design around a branch that is condemned to wither due to lack of a lifeline.
Illustration 1-(10x magnified frond tip) This shows
the scale structure with prominent resin glands of a
magnified cedar frond tip and where it is traditionally pinched. The most pressing challenge of creating effective cedar bonsai is foliage management. Cedar is possessed of a coarse, medium-green foliage frond, which emerges alternately at varying angles above and below the horizontal. This gives the foliage fans a somewhat unkempt appearance. Because of this, many people find cedar foliage unattractive. The foliage is, additionally, very shade intolerant. The shade intolerance of cedar is somewhat similar to Hinoki cypress. Cedar demands attention to annual cleaning and thinning chores, as well as frequent pinching to maintain "tight" contours. Full sun exposure, good air circulation, and pot rotation, along with moderate fertilization and proper pinching technique must be observed. Beyond using the interesting collected trunk forms to full advantage, artistic handling of the foliage is the challenge of the plant. Some artists tightly pinch their branches into solid pads, especially on larger collected trees. The solid pad de-emphasizes the coarse frond, and the collected trunk thus steals the show.
Illustration 2-(Whole cedar frond) The outer tips of the frond are
pinched using the traditional technique. The arrows show the rounded
interior leader, which will grow into the new leader of the bra rich let.
Another approach to foliage management I've observed is to use much more open and airier foliage planes, leaving each frond fully visible. This technique has been particularly well used by Paul Chong of Toronto. Paul does not wire branchlets into flat planes. Instead, he emphasizes the unique fronds of cedar, rather than obscuring them in a densely pinched pad or overpowering their character by flattening the planes with wire. In leaving fewer fronds, the individual character of the plant and the complex nature of the frond seem to be more beautifully revealed and the individuality of the species preserved. I get the sense that I'm looking at an ancient cedar rather than a generic bonsai when I view Paul's plants.
A final note about white cedar foliage is that once a frond is fully differentiated, one side faces the sun and the other side is shaded, just like the leaves of a deciduous tree. If, as a result of bonsai training, the shady side is exposed to the sun and the sunny side is shaded, that frond is almost certain to die.
Bruce Baker and I have observed the growth pattern of this plant and commented on the compound nature of the frond. Bruce has observed that the branch leader, or apical meristem, of the branchlet is interior to the last fully exposed frond (see drawings). The apical leader is round in profile as opposed to the flat profile of a frond. The leader also has a more angular and less rounded scale pattern. Simple pinching of the tips of differentiated fronds misses this branch leader and does not fully promote interior budding.
In our Great Lakes environment, early spring pinching of soft frond tips seems to temporarily retard growth. This growth suppression is especially noticeable after repotting. This type of pinching can even kill a marginal specimen. Greater success has been achieved, in our hands, by waiting until the first spring flush of growth is hardened. Removing or pinching the aforementioned "leaders" after hardening off, as well as pinching the frond tips, promotes a profusion of interior budding (back-budding). This causes the emergence of multiple new interior "leaders" as well as complex "staghorn" growth of the remaining fronds. The new fronds and leaders may emerge anywhere frond scars are visible (young, smooth, tan bark with abscission pits, rather than shaggy old bark) as well as on new green wood. The buds may also emerge at branch crotches. As with any bonsai, the new leaders may be allowed to grow and lengthen the branch, or pinched to maintain the current silhouette.
The rounded apical meristems of the new leaders can be removed at any time during the growing season to promote interior budding and to induce new growth of the existing frond tips. It is particularly beneficial to do this to very old collected specimens that have grown in harsh environments and are already dwarfed by their environments. These older specimens have become habituated to growing very slowly and will continue to do so in a pot unless steps are taken to induce more rapid growth. Removing the apical meristems on ancient specimens can force them into a more rapid growth phase that will also induce more vigorous root growth and tend to ensure success of the plants as bonsai.
The remaining challenge of arborvitae is mid-summer thinning to avoid shading out of interior branches. Any branch significantly shaded will quickly weaken. The shaded fronds will turn yellow and drop in the fall. Caution is necessary when thinning the plant after midsummer. Weak interior shoots may have already been physiologically programmed to fade to yellow, abscise, and drop. If you have removed the more vigorous shoots and left only shoots programmed for fall shedding, you will be left with a jin candidate.
The branches to be "rounded up" for thinning should be selected from "the usual suspects": up and down branches, deformed or diseased branches, inward growing branches, branches emerging from the inside of curves, overly strong or weak branches, etc.
Even if you forget to pinch the foliage and dislike cedars, don't despair. Your local deer population will appreciate your keeping the plant in your collection and do the pruning work for you.
Whether your artistic aims are for more open and airy fronds or for solid pads, modifying your pinching technique to include the interior "leaders" after spring hardening of new growth should produce more rapid ramification and shorter development times for your branches.
Bonsai: The Journal of the American Bonsai Society/Spring 2.000