A Gardening Journal
Must Have: 'Gold Standard' Scholar Tree
- Published: January 07 2017
Gardening would be intolerable without the certainties: Plant this, and it will thrive. Prune that, and it will become bushier. But a garden of only certainties would be a bore—while a garden of only surprises would be a nightmare. It's the balance of the two that rings the bells the best.
Hooray for plants, such as this gold-stemmed scholar tree, that provide both certainty and surprise. One certainty is that a scholar tree is fairly easy to establish. (The species can be used as a street tree, which is a tough environment by definition.) And, once established, it can live for generations. (One specimen at Kew was planted in 1762 and is—just barely—still alive).
The surprise of this cultivar, Styphnolobium japonicum 'Gold Standard', is that the bark of the young stems is bright orange-yellow. Conveniently, the foliage drops cleanly in the fall, allowing maximum display of the bare branches month after month into early spring.
Even when the tree is allowed to grow free-range, like this young specimen at The National Arboretum, the effect can be electric. Backing the tree with evergreens highlights the show.
The bark on the trunk and the older branches is comparatively boring. Plus, you'll notice that, in the detail picture above, there are some dead twigs near the colorful live ones; their bark isn't showy, either.
Scholar trees grow quickly—which is another way of saying that they form new stems that lengthen impressively. Those new stems, of course, are also the ones with the colorful bark, whereas stems that are older or dead are a distraction. Is there a technique that, in one fell swoop, encourages formation of new stems, clears away dead twigs, and reduces the number of stems so old that their bark has matured from sensational orange-yellow to boring brown? Even for this plant geek, any strategy necessitating branch-by-branch grooming would be a nightmare.
But of course, pruning is the solution. Scholar trees are rarely pruned in North America, but take a look, below, at a Chinese nursery's production field of Gold Standards. Partly to save space so that more nursery stock can be grown in a smaller plot—but also, I suspect, to enhance the display of yellow twigs—these young trees seem to have been pruned regularly. At any rate, there are no mature (i.e., boring) stems in their young canopies, only the vividly-colored juveniles.
Here's another shot from that nursery's on-line listing, showing scores of a purple-leaved poplar cultivar whose canopies are striking in their density, compactness, and vivid dark foliage. Have they been pruned back to achieve that look? And what about the comparatively thick trunks for such small canopies? Those suggest that, yes, the canopies have been pruned regularly, or that the purple-leaved form has been grafted atop trunks of the straight species, or both.
The nursery often doesn't label cultivars accurately, but this form might be Populus deltoides 'Red Caudina' or, below, 'Purple Tower'.
The young growth of purple-leaved poplars is colorful in both leaf and stem. This Czech grower of Purple Tower says that the tree would be a great candidate for pollarding, which is the annual cutting of all year-old stems of a tree's canopy back to stubs so that they can't mature to boring, too-bulky old growth. Conveniently, such removal also eliminates dead twigs as well as stimulates emergence of the stubs' side stems that will yield a fresh crop of colorful young growth. My hunch is that, while the poplars in the Chinese nursery may well have been formed by grafting, the small sizes of their canopies are the result of pollarding.
That same Czech nursery stocks Gold Stem scholar tree, which is likely the same as Gold Standard. The nursery also recommends pruning it way back, which means that, like the purple-leaved poplar, the tree is also a good candidate for pollarding. Here's another surprise: the straight species of Styphnolobium japonicum would make a great pollard, too, even though the bark of its young stems isn't orange-yellow. But it isn't dull brown or gray either: it's distinctly green, and can make a noticeable display even on free-range trees.
In the cool season, any colors besides brown and gray are a joy, and a pollard of the straight species of scholar tree would flaunt a canopy that isn't just more compact, and without brown-barked branches that are green only at their slender tips. Rather, the canopy would be comprised entirely of stems that are green their full length, so would be solid green though also completely leafless. Although all forms of Styphnolobium are deciduous, pollarding the straight species therefore transforms the tree into a dense evergreen, with green twigs fall to spring, and green leaves spring to fall.
Scholar trees are most comfortable in cold climates—zones 4 to 6 in eastern North America—where there's always a shortage of green in the winter. With fewer broadleaved evergreens available than in milder climates, and with a given space able to handle only so many conifers, other options for increasing the proportion of green in a cold-climate garden would be welcome. No problem: pollards of the straight species of Styphnolobium are evergreen thanks to their green bark. Hardy orange and tree caragana are two more deciduous woody plants with green-barked young twigs and a deep tolerance for pruning. They can also also be transformed into striking leafless evergreens.
With such a bounty of certainties and surprises, I'm certain that both the gold and green-barked forms of Styphnolobium will be formed into pollards in my garden over the years. And maybe Populus deltoides 'Purple Tower' too, if I can locate a North American supplier. Populus deltoides itself is too disease-prone to be sold by most growers; the species is only favored where a severe climate precludes more desirable trees. (You're likely to see these poplars in frigid Quebec, say, or along rivers in arid, alkaline-soil Nevada.) Pollarding might help a purple-leaved poplar escape its usual canker and the witch's broom disfigurement it produces, so how can I resist experimenting?
Yet one more certainty: after I've established a pollard or two of scholar trees, then I can respond with smooth and casual accuracy, "Oh that colorful thing? Just my Styphnolobium." I'll be pronouncing the genus "stiff-no-LOBE-ee-uhm." So far, the name isn't getting any less humorous to hear.
Styphnolobium japonicum is widely available. Its Gold Standard cultivar is available at this peerless nursery in Connecticut. I'll be establishing both forms of Styphnolobium in 2017. After my pollards are under way, I'll provide the full How to Grow It post.