Arborvitae a Midwestern View
by Clark D. West, M.D.

ARTICLE FIRST APPEARED: 1998 Summer

The four work horses of the ornamental conifer world, as attested by their frequency in garden stoes and suburban landscapes, are Yews [Taxus], Spruces [Picea], Pines [Pinus], and Arborvitaes [Thuja]. Of these, Spruces, Pines and to some extent Yews have cultivars which tickle the fancy of collectors because of unusual color or habit, but rare is the person who collects arborvitaes. Many think arborvitae prosaic and dull. Many of those who champion them are simply tired of babying less hardy genera.

Most Arborvitae cultivars come from three species: Western Red Cedar [Thuja plicata], Oriental Arborvitae [T. orientalis, whose latest, most correct botanical name is Platycladus orientalis], and American Arborvitae or White Cedar [T. occidentalis]. Of the three, the Western Red Cedar, has the fewest cultivars; the names of only 17 have been verified by the World Conifer Data Pool as compared to 36 for Oriental Arborvitae and 105 for American Arborvitae. Other species include Korean Arborvitae [T. koraiensis] and Japanese Arborvitae [T. standishii].

Unfortunately, Western Red Cedar [T. plicata] and its cultivars are rare in the east and Midwest and the species is seldom seen as a large landscape specimen. The cultivar, T. p. ÔAtrovirens' becomes and attractive, medium-sized tree with dark green foliage and should be more widely planted. It is hardy to Zone 5 and is especially good in moist areas. It is said to even grow where the soil is anaerobic. More important, deer shun it. Eastern landscapes would be much improved if T. plicata replaced the old, depressing Norway Spruce [Picea abies]. Their absence is not because they have been considered poorly adapted to eastern climates; many Western Red Cedar are happily growing in Spring Grove Cemetery in Cincinnati, Ohio, planted 50 to 100 years ago. It is difficult to find an American Arborvitae there. Why specimens of the Western Red Cedar are scarce in the east and Midwest is not clear.

Oriental Arborvitae [T. orientalis] is not quite as hardy as the other two species. It does well in Zone 6. In Zone 5, it often comes through the winter unscathed but in some years it may have large patches of winter burn. The burned areas usually recover and fill in during the next one or two growing seasons. For those who wish to take the risk, or who live where winters are less severe, Oriental Arborvitae cultivars provide many attractive forms with interesting foliage. However, one has to like conifers which are stiffly symmetrical and stylized; even those with juvenile foliage are stiffly upright. They are not commonly listed even by speciality nurseries, but many interesting seedlings can be had from a few seeds. The variations are endless: Seeds from T.o. ÔAurea' or ÔAurea Nana', for example, may produce plants which are tall and columnar, conical and shoter or petite globes. They can be bright green, gray green, or yellow. The foliage may be coarse or fine. The seeds are large, like radish seeds, germinate rapidly without pre-treatment and the seedlings are robust. In three or four years you'll have the start of an interesting collection if you pursue this hobby.

In the suburban landscape, the American Arborvitae [T. occidentalis] has come to be the true work horse of the Arborvitae family. Many cultivars of T. occidentalis are nothing like the species. the species is a small tree known as the Northern White Cedar. The Northern White Cedar is a denizen of swampy areas in the north and has been described as an ugly small tree (I, for one, disagree). Those who think so bemoan that the foliage turns yellow to brownish green in winter and that the wood is only good for fence posts. One would never predict that the germplasm of such a nondescript tree could be molded into large numbers of cultivars useful in the landscape.

Details of the history of the development of cultivars are not recorded. According to den Ouden and Boom in the Manual of Cultivated Conifers, the species was introduced into France in 1534 by members of the Cartier expedition, explorers of Canada. By 1855, several cultivars had been selected by Carriere in a book, "Trait General des Coniferes". A variant known as Thuja occidentalis var. sibericum or the Siberian Thuja, presumably more attractive than the species, was found in the wild along the Niagara River and where this variety was used in the slection of cultivars is not clear. The Siberian Thuja resembles, or is the same as, the cultivar T. occidentalis ÔWareana'. Currently, Arborvitaes with the widest landscape use are T. occidentalis ÔSmaragd' (also known as ÔEmerald Green'), ÔTechny', ÔNigra', and, of course, the old standby, ÔPyramidalis Compacta'. All are upright, columnar or conical with the first three having the advantage of maintaining good bright or dark green color in the winter. They are extremely cold hardy and good for hedges. T.o. ÔPyramidalis Compacta' has coarser foliage and has greater ultimate height than the others, reaching 30 feet. Much more attractive and more difficult to obtain is T.o. ÔHolmstrup'. It is also conical in habit but differs from ÔTechny', ÔNigra' and ÔPyramidalis Compacta' in having much finer textured foliage. It too maintains a green color in the winter. It has a more refined look than the others, perhaps because of its fine foliage. Older plants ten to develop a skirt or shelf at the base. Although some recommend it for hedges and screens, it also serves very well as a specimen. It is apparently widely available in Europe but sources are limited in this country.

For a specimen columnar form, the relatively new introduction, T.o. ÔDegroot's Spire' is superb. In a somewhat sheltered location with good sun exposure, it will achieve at least 10 feet in height but may be less than a foot in diameter. It is not only thinner than most columnar arborvitaes, but also its outline is less smooth and summetrical and is therefore more intersing. Where it thrives, it is an excellent plant.

While discussing the large cultivars, mention should be made of Thuja ÔGreen Giant'. Recently named, this cultivar was selected in 1935 and is now becoming more widely available. It is perhaps a cross between T. standishii and T. plicata. It behaves like a hybrid, growing up to three feet a year. It will grow to 60'. It was predicted by the late J.C. Raulston to be the next Leyland Cypress in the mass market of plants suitable for screening. It is very hardy, does not burn and has good green foliage, even in winter. It responds well to pruning.

There are large globes, and small, slow-growing globes. Among the large ones are T.o. ÔUmbraculifera', ÔWoodwardii', and ÔLittle Champion'. The last two are similar, growing to about 3 feet high and wide. ÔUmbraculifera' develops an umbrella shape and has foliage which is more gray than that of other Arborvitae. In central Ohio, ÔUmbraculifera' has grown less than two inches a year but has eventually attained a height of eight feet and a spread of fifteen feet. The other two may have similar potential, given enough time.

Among the smallest globes are T.o. ÔHetz Midget', ÔDirego Dwarf', ÔLittle Gem', and ÔTiny Tim'. One of the best of these is ÔTiny Tim'. It is very slow growing and excellent for the rock garden.

The more striking members of the Arborvitae genus are those with yellow foliage. The premier and most popular yellow Arborvitae is T.o. ÔRheingold'. There is much confusion between this plant and T.o. ÔEllwangeriana Aurea'. The latter was introduced in 1895 and is a sport of T.o. Ellwangeriana', a green form', whereas ÔRheingold' originated as a seedling in 1904. They are very similar, but "Ellwangerianan Aurea' is said to have mainly adult foliage and can become large, whereas ÔRheingold' has mainly juvenile foliage with small amounts of adult foliage developing as it matures. ÔRheingold' may slowly exceed three three feet in height. Both are yellow in summer and orange in winter. In recent years, ÔRheingold' has become widely available in the trade. It is one of our most attractive conifers.

There are a number of other worthy yellow Arborvitaes, all with typical adult folige, but they are rare in suburbia. Three of the old standbys are T.o. ÔLutea' (also known as ÔGeorge Peabody'), ÔPumila Sudworthii', and ÔSunkist'. ÔLutea' was introduced in the late 1800's and the other tow were introduced about 30 years ago. The three differ from ÔRheingold' in that they are mottled yellow and green and, of course, quite uniformly green if grown in the shade,m whereas ÔRheingold' has no green. ÔLutea' is an antique; very large specimens may be found in old arboreta. ÔSunkist', introduced in 1968 from Holland', is a very good yellow. ÔPumila Sudworthii', in my experience, is almost as yellow as ÔSunkist' but is more vigorous and may become treelike.

T.o. ÔWatnong Gold', named in 1972 by Don Smith, should be more widely planted. It is a sport of ÔEllwangeriana Aurea'. It is a brighter yellow than the above three and has the advantage of remaining yellow in the winter without the dull orange overtone. It is columnar and quite vigorous.

The yellow Arborvitae which I find most attractive is the little known ÔWareana Lutescens'. It has adult foliage that overall is yellow or lime green and from a distance is not mottled green as are the other yellow cultivars with adult foliage. However, with a run of dark, dull days it may become for a time quite green but with prolonged sunny weather, it lives up to its name. In fact, on the sun-side, the foliage can los pigment and be almost white in the center of the scales. Because of the color and the layered effect which develops as a result of overlapping leaves as it matures and become conical with age, it can be valuable as a focal point in the large garden. It apparently differs from its parent, the green ÔWareana', in that the latter is said to have foliage oriented vertically whereas ÔLutescens' is overlapping horizontal.

Less attractive to me are three with interspersed cream colored foliage. The most striking of these are T. o. ÔSherwood Frost' and ÔSilveriana' but they are a bit disappointing in that the overall effect from a distance is a light, dull green. Even less intersting is ÔWansdyke Silver', which is green with cream colored mottling.

A couple of Arborvitae cultivars are conversation pieces. One which has difficulty defining itself is T.o. ÔOhlendorfii'. It is upright and has modifed adult foliage for about two-thirds of its height and whip-like folige protruding from the top. At the very base, it retains foliage identifcal to that of arborvitae seedlings. It is not particularly attractive but demonstrates how badly confused the genes controlling morphology can get. Another which is not recognizable as an Arborvitae is T.o. ÔFilicoides'. Thi has a rounded outline as a mature plant and is entirely composed of whip-like foliage, the strands of which can be very long. It tends to be more bronze than green and because of its narrow foliage, has an open and airy appearance. It can become very large.

In addition to ÔRheingold', there are a few other T.occidentalis with juvenile foliage. One is ÔEricoides', said to be identical to one discovered independently and known as ÔHetz Junior'. ÔEricoides' is said to be a dull green in summer and brownish in winter. Supposedly, it is neat in its youth but becomes open and less atrractive with age. One of the recent introductions, which is attractive and worthwhile but does not look like an Arborvitae is ÔLinesville', found as a witches' broom by Joe Stupka. In the summer it is a mound of gray-green juvenile foliage with just a hint of some adult foliage on older plants. It makes a nice contrast in the landscape with ÔRheingold'. In the winter, it is a tan-green color. It does not winter burn, even in exposed locations.

In many parts of the country, T. occidentalis cultivars are the most satisfactory of the evergreens. Hence, the selection of new cultivars has the potenial to contribute to the landscape. Although witches' brooms can develop on Arborvitae, most new cultivars come from seedlings.

So, an ugly tree living in swamps has spawned a large number of cultivars; some are standbys for nurserymen, others are of little interest, and still others are loved only by collectors looking for unusual plants. Perhaps their greatest charm is that the grower or collector has minimum worries about their hardiness. In most areas they come through winter with no special attention at all.

Clark D. West, M.D., lives near Cincinnati, OH, and is the Vice President of the American Conifer Society and a director on the Board. He has been a member of the ACS since 1987 and has a passion for growing conifers from seed.

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