Potential of Alternative Tsuga spp. as Landscape Replacements for T. canadensis: Longer-Term Evaluation and Propagation of T. chinensis.

Research Site

Located in Katonah, New York, U.S. (USDA Hardiness Zone 6b), Lasdon Park and Arboretum is a 94 ha public park that hosts various horticulture research and teaching interests (Anonymous 2013). On 30 September 2003, three experimental plots were established within the deer-fenced portion of the property, designated Front Gate, Magnolia Garden, and Hemlock Hedge.

Plant Material

Tsuga selection consisted of the following seven species based on commercial availability:

Tsuga canadensis (L.) Carrière (Canadian/eastern hemlock)—Although a highly valued ornamental tree in the northeastern U.S., its range extends south to Georgia and Alabama and west to Minnesota. Hardy to USDA Zone 3, it is also found growing at its northern range that extends into the central-eastern portion of southern Canada. Its response to shearing and tolerance to shade have made it an ideal hedge species, however. Its graceful form and size—the national champion is 47.5 m in height—also make this tree valued as an individual specimen tree or grouped planting (Farrar 1995; Dirr 2011). As HWA has reduced the salability of T. canadensis and its numerous cultivars throughout significant portions of the infestation range, horticulture specialists have continued to answer questions from arborists and other horticulture professionals, as well as the general public, about suitable woody ornamentals that may supplant the landscape niche that this tree has successfully occupied. Specifically, this includes the natural screening benefits from a maintained T. canadensis hedge, serving to divide shaded suburban properties (J. Giordano, Cornell University Cooperative Extension, pers. comm.). Numerous published reports have affirmed host susceptibility to HWA (Table 1) and 28 of the original 36 research specimens have persisted at Lasdon Arboretum (Table 2).

Tsuga caroliniana Engelm. (Carolina hemlock)—Although not common in the horticulture trade with no formally recognized subspecies, forms, or varieties, several references to the ornamental application and value of this tree have been made (Dirr 2011). Tsuga caroliniana can be found in a limited range that includes southern Virginia, North Carolina, South Carolina, and Georgia, in the U.S. Its limited growing range often includes elevations between 640 m and 1220 m (Anonymous 2008), which seems to have limited both its exploitation as a significant contributor to either the forest products or ornamental horticulture industries (Coladonato 1993). Though it has been noted successfully growing well outside its range of origin as far north as Amherst, Massachusetts, U.S. (RWH, pers. observation), concerns related to climatic challenges from more northerly sites do not appear to have yet been formally researched; this information would be important as only one of the 18 research specimens have persisted at Lasdon Arboretum (Table 2). Still, this tree is shade tolerant, may grow to a height of up to 21 m and may offer some degree of promise in the horticulture trade (Coladonato 1993; Dirr 2011), although its susceptibility to HWA and EHS may inevitably be limiting. Numerous manuscripts detail host susceptibility to HWA (Table 1).

Tsuga heterophylla (Raf.) Sarg. (western hemlock)—Reputed as an elegant giant, reaching heights in excess of 60 m, western hemlock is known as a versatile tree that can be found growing throughout moist/humid coastal regions of the pacific northwest region of the U.S., as well as inland among the various mountain ranges into Canada and as far north as Alaska, U.S. (Packee 1990). Its graceful appearance and tolerance to shearing and shade have endeared T. heterophylla as a landscape planting typically within its host range, and a number of varieties (cultivars) are commercially available (Anonymous 2014). It is described as being hardy to USDA Zone 6, and only four of the original 11 research specimens have survived at Lasdon Arboretum (Table 2). Numerous manuscripts affirm host susceptibility to HWA (Table 1).

Tsuga mertensiana (Bong.) Carrière (mountain hemlock)—A significantly smaller tree than its native western counterpart (max. ht. about 30 m), T. mertensiana is rarely found growing in close association with T. heterophylla, as it typically occupies the less temperate, more elevated mountainous sites of the pacific northwest U.S., Canada, and Alaska (Means 1990). Several varieties of this tree are available commercially (Anonymous 2014), presumably for landscape purposes. Although it is described as being hardy to USDA Zone 5, and though numerous manuscripts describe host resistance to HWA (Table 1), only two of the original 18 research specimens have persisted at Lasdon Arboretum (Table 2).

Tsuga chinensis (Franch.) E. Pritz (Chinese hemlock)—Described as attaining heights in excess of 30 m in China (Dirr 2011), T. chinensis has been growing successfully in Jamaica Plain, Massachusetts, U.S. (USDA Zone 6b) since a specimen was established at the Arnold Arboretum in 1910 by E.H. Wilson (Havill and Montgomery 2008). Thirty-eight T. chinensis seedlings that had been planted in 1999 were inventoried 09 March 2004, identifying no post-winter mortality despite exposure to a low temperature of −22.5°C, which occurred on 16 January 2004 (Del Tredici and Kitajima 2004). Dirr (2011) describes T. chinensis hardiness to Zone 5, and 17 of the 18 specimens at Lasdon Arboretum have persisted (Table 2).

Tsuga diversifolia (Maximowicz) Masters (northern Japanese hemlock)—Occurring in northern Japan at higher elevations, this tree, records indicate, has been in commercial cultivation and trade since 1861 (Anonymous 2014). Although only three of the original 18 research specimens still persist at Lasdon Arboretum (Table 2), Dirr (2011) describes survivorship of this tree as far north as the state of Maine, U.S. (Zone 4) and reports observing specimens at heights of over 12 m in Congreve Gardens, Ireland. Although many manuscripts report host resistance to HWA (Table 1), findings among all references are not entirely consistent.

Tsuga sieboldii Carrière (Japanese hemlock)—Found naturally growing in southern Japan at lower elevations, this tree, records indicate, has been in cultivation and trade, successfully, since 1914 (Havill and Montgomery 2008). One of the original 18 research specimens remains living at Lasdon Arboretum (Table 2).

Tsuga canadensis, T. chinensis, and T. heterophylla were sourced from Eby Nursery, Wilsonville, Oregon, U.S., and T. caroliniana, T. diversifolia, T. mertensiana, and T. sieboldii were obtained from Forestfarm Nursery, Williams, Oregon, U.S.

Plot Design

Trial plots were approximately 12 m × 14 m, each, including six rows of the research trees spaced at approximately 2 m intervals. Species were planted in each row in random order and were intended to include at least one of each of the aforementioned seven Tsuga species as well as a second representative of T. canadensis per row. Because seven specimens of T. heterophylla and one T. caroliniana perished before time of planting (30 September 2003), some rows were lacking a complete complement of trees (the remaining specimens of T. heterophylla were distributed as evenly as possible among plots, Table 2). Daylight penetration on the plots ranged from full to partial sun throughout the day (Front Gate and Magnolia Garden plots), to nearly full shade provided by deciduous trees (Hemlock Hedge plot). Plots had been mulched annually, commencing in autumn 2006, with 5–10 cm depth of whole-tree mulch (i.e., assorted wood chips) from a municipal composting facility for the purposes of weed management, moisture retention, and soil temperature regulation. Prior to annual mulching, the area between the trees was mowed as needed during the summer to reduce vegetative competition from grasses and weeds. Soil samples within the plots were submitted to the University of Massachusetts (UMass) Soil Diagnostic Laboratory on 31 May 2013. Soil compaction readings were also taken in each plot at 12 random locations with a soil penetrometer (soil compaction tester, DICKEY-john®, Auburn, Illinois, U.S.) on 29 May 2013.

Trees were assessed for performance and pest occurrence on 19 and 24 July 2012. The height of trees was measured to the nearest 0.03 m using a telescopic measuring rod (Jameson, LLC, Clover, South Carolina, U.S.), and overall health was assessed visually, where each tree was assigned a rating from 0 (dead) to 5 (lush, dark green needles). The presence or absence of HWA and EHS was noted after inspecting several branches from each tree using a simple binary (presence/absence) classification.

Propagation of T. chinensis

Hardwood cuttings of the most recent year’s growth were taken from T. chinensis growing in the research plots on 20 January 2014, and averaged 12.5–15 cm in length. Cuttings were kept in closed plastic bags, transported to the UMass College of Natural Sciences greenhouse, and potted on 22 January 2014 (Figure 1). The cut ends were dipped in a liquid rooting concentrate (Dip’N Grow, Inc., Clackamas, Oregon, U.S.) at a concentration of 5,000 ppm IBA and 2,500 ppm NAA before potting in sand:perlite (1:1) in standard 50-plug, 25.4 cm × 50.8 cm grow trays. Two growing conditions were compared: 1) under a polyethylene (poly) moisture tent with bottom heat (21°C), and 2) on a mist bench with no bottom heat (air temperature 24°C). Four flats of 50 cuttings each (200 cuttings total) were reared in the poly tent and six flats (300 cuttings total) were reared on the mist bench. Rooting could not be verified until cuttings remained for a “3–4 month” period under the aforementioned conditions (J. Alexander, Arnold Arboretum at Harvard University, pers. comm.). Establishment and vigor were assessed at various times over the following ten months. Vigor was assessed visually on a whole-tray basis using a 0–5 visual rating system, where 0 = 100% mortality of grow tray specimens and 5 = 100% viability and lush, green needles, similar to the vigor rating used to assess tree health in the field.

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Figure 1.

Early-stage Tsuga chinensis cuttings under poly tent in greenhouse.

Data Analysis

Tree height was analyzed using randomized complete block analysis of variance (ANOVA) and vigor was analyzed with Kruskal-Wallis one-way non-parametric ANOVA because the data did not meet the assumptions of randomized complete block ANOVA. Because of the low number of trees surviving for many of the test species, these analyses were performed only for T. canadensis and T. chinensis. Survivorship was analyzed for all species, using χ² for the total number of each of the test species surviving, regardless of plot (replicate), and propagation results were analyzed with a two-sample t-test. Analyses were performed with Statistix 9 (Analytical Software, Tallahassee, Florida, U.S.).