More Thoughts on Thuja
by Dr. Edward Hasselkus

ARTICLE FIRST APPEARED: 1998 Summer

(Also authored by Dr. Mark Zampardo)

Known in northern and eastern Wisconsin as White Cedar or Arborvitae, Thuja occidentalis is a native conifer found growing in "cedar" swamps and on the Niagara escarpment which extends through eastern Wisconsin. Adapted to a wide range of soil pH, the tree tolerates poorly drained soils by developing a shallow, surface root system. In the landscape, small transplants tolerate planting into wet soil conditions whre they adapt through development of surface roots; hoewver, larger b&b (balled & burlapped) or containerized plants would fail unless planted on a well-drained site.

Arborvitae is the ideal screening plant. It retains its lower branches indefinitely and adapts to grouping in mass-plantings better than most conifers. Even if space is limited, an effective year-round screen can be created by planting columna or small-scale cultivars of Thuja. Arborvitae responds well to shearing if a formal hedge is desired. Arborvitae takes sun or part-shade but not full shade.

Cultivar Evaluation

Over 100 cultivars of Thuja occidentalis and Thuja plicata (Western Red Cedar) have been under evaluation in the Longnecker Gardens of the University of WI Arboretum-Madison since 1969 (USDA Zone 5a). At this site, as with other suburban landscapes, deer browse during winter is a serious problem. Other observations have been made at the Chicago Botanic Garden and Morton Arboretum in IL, the Bickelhaupt Arboretum in IA and at Hidden Lake Gardens in Tipton, MI.

Upright Cultivars

Dr. Clark West has commented favorably on Thuja occidentalis ÔDegroot's Spire', ÔHolmstrup', ÔNigra', ÔPyramidalis Compacta', ÔSmaragd' (also known as ÔEmerald'), ÔTechny' and ÔWareana'. Professionals appear to favor T.o. ÔNigra' (also known as ÔDark Green') and T. o. ÔTechny' (also known as ÔMission') for their superior dark green winter foliage. T.o. ÔTechny' has the form and ultimate spread of the species in contrast to the straight-sided columnar form of T.o. ÔNigra'.

Thuja occidentalis ÔHetz Wintergreen' is noted for its fast growth rate, dark green winter color and columnar form with a dominant central trunk. It is an ideal choice for a quick screen where space is limited. In addition, it is less likely to be damaged from ice or snow-load than the previously mentioned cultivars.

Thuja occidentalis ÔSherwood Moss' was introduced around 1975 by Sheridan Nurseries in Ontario. It has a dense spire-like habit and very dark green foliage, becoming dark olive in winter. A 10-year old plant is four feet tall with an 18" spread. If the ultimate height proves significantly less than the similar ÔDegroot's Spire', it would be a valuable addition to ou list of best garden conifers.

Globe Cultivars

In the upper Midwest, all globe forms of Arborvitae may be subject to malformation and branch breakage from accumulated ice or wet snow. Dr. West has suggested that T.o. ÔTiny Tim', among the smallest and slowest-growing globes, is one of the best.

Thuja occidentalis ÔMr. Bowling Ball' is one of Joe Stupka's witches' brooms introduced by Lake County Nursery in Perry, OH. It has a perfect ball shape, grows to 2-3' in diameter and has fine-textured, sage-green juvenile foliage.

Thuja occidentalis ÔRecurva Nana' is not really a globe, but a dense, low-mounded bun shape with interesting recurved foliage. A 22-year old plant measures 2.5' high with a spread of six feet. It has not suffered ice or snow load damage.

Golden Foliage Cultivars

Dr. West gave high marks to T.o. ÔPumila Sudworthii', T.o. ÔSunkist', T.o. ÔWareana', and to T.o. ÔLutescens'.

We agree with this evaluation. At the Longnecker Gardens, T.o. ÔRheingold' has been regularly browsed by rabbits and suffered malformation from wet snow accumulation.

T.o. ÔYellow Ribbon', a 1983 Dutch introduction, has been available in North America during the past decade. This columnar Arborvitae with bright yellow foliage makes a striking hedge at the Missouri Botanical Garden in St. Louis.

Variegated Foliage Cultivars

As pointed out by Dr. West, the overall appearance of variegated cultivars from a distance is light, dull green. When viewed at close range, we find that vigorous growing plants of ÔSherwood Forest', ÔSnow Tip', and ÔWansdyke Silver' are indeed quite attractive.

Thuja plicata's Cultivars

Cultivars of Thuja plicata that have performed well include ÔExcelsa', ÔFastigiata' and ÔVirescens'. ÔExcelsa' has grown well for 16 years. It is a compact, narrow cultivar with glossy, dark green foliage.

ÔFastigiata' has been under observation for 20 years and has done well. The habit is narrowly columnar.

ÔVirescens', under observation for 5 years, has a habit similar to the species of Thuja plicata but has bright, glossy green foliage year round.

Thuja plicata has a high degree of deer resistance to deer browse, is more shade tolerant, and is more likely to develop a dominant trunk than Thuja occidentalis. For these reasons is is likely to become a more common sight in Midwestern landscapes.

Edward R. Hasselkus, Ph.D., is Professor Emeritus at the University of Wisconsin-Madison, and Curator of Longnecker Gardens. Mark Zampardo, Ph.D., is currently the American Conifer Society's Central Region Secretary/Treasurer. He lives in Chicago and has taught "Woody Plants" and "Plant Propagation" for 18 years at Lake County College in Grayslake, IL.

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