Many Landscape Grasses Survive Long-term Hardiness Trials in United States Department of Agriculture Plant Hardiness Zone 4b

Abstract

A long-term, landscape grass hardiness study was initiated in Summer 1987 at the University of Minnesota Landscape Arboretum in Chanhassen, MN, USA [United States Department of Agriculture Plant Hardiness Zone (USDA PHZ) 4b, –20 to –25 °F]. This location averages a 158-day growing season (frost free), summer temperatures of 79.9 °F, and winter temperatures of 10.4 °F. Over 35 years, 392 different kinds of plants from the grass (Poaceae) and sedge (Cyperaceae) families were planted to evaluate winter survival, landscape value, flowering, and pest resistance. Most plants (n = 271, 69%) survived at least 4 years, 186 (47%) survived 10 years, 81 (21%) survived 20 years, and 29 (7%) survived 35 years. Sixty-eight plants (17%) were deemed not winter hardy in this location (USDA PHZ 4b), and 53 are listed with insufficient data for a hardiness rating. Changes in maintenance as well as challenges encountered with long-term trials of herbaceous plants are discussed.

Abstract

A long-term, landscape grass hardiness study was initiated in Summer 1987 at the University of Minnesota Landscape Arboretum in Chanhassen, MN, USA [United States Department of Agriculture Plant Hardiness Zone (USDA PHZ) 4b, –20 to –25 °F]. This location averages a 158-day growing season (frost free), summer temperatures of 79.9 °F, and winter temperatures of 10.4 °F. Over 35 years, 392 different kinds of plants from the grass (Poaceae) and sedge (Cyperaceae) families were planted to evaluate winter survival, landscape value, flowering, and pest resistance. Most plants (n = 271, 69%) survived at least 4 years, 186 (47%) survived 10 years, 81 (21%) survived 20 years, and 29 (7%) survived 35 years. Sixty-eight plants (17%) were deemed not winter hardy in this location (USDA PHZ 4b), and 53 are listed with insufficient data for a hardiness rating. Changes in maintenance as well as challenges encountered with long-term trials of herbaceous plants are discussed.

The desire for durable, low-maintenance plants that provide ecosystem services has increased sales of landscape grasses. The 2019 Census of Agriculture reported $178,791,000 in ornamental grass sales, up 13% from $158,061,021 in 2014 [United States Department of Agriculture (USDA), National Agriculture Statistics Service 2020]. New plants are in continual demand in the horticulture industry, especially plants that can perform ecosystem services in challenging man-made environments while still providing beauty. Landscape or ornamental grasses, particularly native grasses, fulfill many ecosystem services such as erosion control and reduction of runoff, and require minimal water or nutrients (Darke 2007). Growing plants in challenging climates, however, such as the upper midwestern United States often limits what will survive, let alone thrive. Without field hardiness trials for USDA Plant Hardiness Zone (PHZ) 4 or colder, few growers were selling, let alone growing, landscape grasses in the upper midwestern United States in the 1980s (Davidson and Gobin 1998; Meyer et al. 1994). A long-term winterhardiness study was initiated at the University of Minnesota Landscape Arboretum (Chanhassen, MN, USA) in Jul 1987. Preliminary results were reported for USDA PHZ 3b and 4b (Meyer et al. 1994). Many plants survived the original project in Minnesota, and results through 2012, including landscaping characteristics, have been published (Meyer 2012) showing some plants were clearly not cold hardy to USDA PHZ 4.

Additional shorter term landscape grass field trials have been conducted: Davidson and Gobin (1998) reported on USDA PHZ 3 for 169 kinds of landscape grasses over 4 years in Morden, Manitoba, Canada, and Perry (1992) examined 22 grasses in USDA PHZ 4b in Vermont, USA. During a 3-year trial in North Dakota, USA (2010–13), only 3 of 15 grasses survived in USDA PHZ 4a (Zuk et al. 2016). The multiyear Chicago Botanic Garden (Glencoe, IL, USA) grass trials resulted in detailed information for USDA PHZ 5b for 109 different grasses, most of which were evaluated for 4 years (Hawke 2018). Shorter term trials for landscape grass performance in the desert southwestern United States (Worku and Umeda 2021), Florida, USA (Thetford et al. 2009), and Georgia, USA (Corley 1975), have been reported.

Some field trials have focused on specific genera such as 10 fountain grass (Pennisetum sp.) cultivars in Florida (Wilson and Knox 2009). Chinese silvergrass (Miscanthus sinensis) has been the focus for several specific trials, including landscape performance and seed set in Florida (Wilson and Knox 2006), seed set across multiple hardiness zones (Meyer and Tchida 1999), and hardiness and chromosome analysis for biomass breeding (Dong et al. 2019). Giant reed (Phragmites australis) and giant miscanthus (Miscanthus ×giganteus) produced high aboveground biomass yield over a 10-year period, with greater production of giant reed than giant miscanthus (Angelini et al. 2009).

The Royal Horticultural Society (London, UK) trialed 105 different kinds of Chinese silvergrass, with 16 selections winning the Award of Garden Merit (Jones 2004). Grass trials at multiple midwestern US locations (1995–99) summarized the top 46 grasses (Kopf et al. 2001). Performance of 17 cultivars of switchgrass (Panicum virgatum) and five of little bluestem (Schizachyrium scoparium) across eight US states found consistently superior cultivars (Meyer et al. 2017). A study of 70 native sedges (Carex sp.) trialed in sun and shade conditions for 4 years at Mt. Cuba Center (Hockessin, DE, USA) ranked 16 as top performers (Hoadley 2023). However, not all of the trials cited here considered plant hardiness as a trait of importance to measure.

Field evaluations are critical in understanding and predicting plant performance across regions with widely variable weather conditions (Meyer et al. 2017; Wilson and Knox 2006). This report provides landscape durability or sustainability within the context of cold hardiness performance for landscape grasses by showing the results of 35 years of data for 392 different grasses and grass-like plants grown in USDA PHZ 4b.

Materials and methods

The planting site was the University of Minnesota Landscape Arboretum in Carver County, MN, USA (lat. 44.5°N, long. 93.3°W; American Horticulture Society Heat Zone 5; USDA PHZ 4b; Hayden loam, pH 7.6). The site was level and in full sun for most plants. Species and cultivars of sedges and rushes (Juncus sp.) were planted under a lath that provided shade conditions.

Weather data were compiled from 1987–95 from the Chaska, MN, USA, National Weather Service location (less than 5 miles from the planting location) and from 1996–2002 from the Chanhassen, MN, USA, National Weather Service location (3.7 miles) (Table 1).

In early Jul 1987, 159 ornamental grasses or grass-like plants were obtained from Bluemount Nurseries (Monkton, MD, USA) and Kurt Bluemel Nurseries (Baldwin, MD, USA) and planted for the original collection and trials. An additional 233 kinds have since been added, from additional commercial sources—primarily, Emerald Coast (Pensicola, FL, USA), Hoffman Nursery (Rougemount, NC, USA), Intrinsic Perennials (Hebron, IL, USA), Walters Gardens (Zeeland, MI, USA), and Northcreek Nursery (Landenberg, PA, USA) (Fig. 1). Plants were obtained in 4-inch to 1-gal containers or as plugs or bare root plants. Plugs and bare root plants were potted and maintained in a greenhouse or under shadecloth until a sufficient size was reached to be transplanted into the field. Except as noted, a minimum of four plants of each different kind were trialed in rows, with spacing equal to the mature height of each plant. Plants were placed within 10 in-ground full-sun beds or two shade beds: each ∼1200 ft². Supplemental water was used only in the first year during establishment. Hand weeding and wood chip mulch were used to control weeds; occasionally, a pre-emergent weed control was applied if high weed pressure was observed. Supplemental fertilizer and pesticides were not used. No winter protection was provided for any of the plants.

In early spring (usually late April or early May), grass tops were burned or cut back to remove the growth residue from the previous year. Plants were divided as needed as a result of rodent damage or loss of vigor. For some plants, division was never needed, whereas for others, it was necessary as often as every 5 years. Nomenclature follows Plants of the World Online (Kew Science 2023) and Plants Database (USDA 2023).

Winter survival data were collected each spring, usually in early June. A rating scale of 0 = dead, 1 = some dieback, and 2 = no winter injury was implemented. Additional data on spring green-up, flowering time, overall appearance, height, width, and pest damage was collected monthly throughout the growing season by the primary author and has been reported elsewhere (Meyer 2012).

Results and discussion

Weather data show the coldest temperatures of –31, –32, –35, and –37 °F experienced during 2019, 1994, 1998, and 1997, respectively (Table 1). Over the 35 years of this trial, the average minimum winter temperature was –20.5 °F, consistent with the –20 to –25 °F typical for USDA PHZ 4b. Although indications of warmer winters now occur in the upper midwestern United States, these data show little variation in average minimum temperatures over the past 10 (–20.3 °F) or 20 years (–19.6 °F) from the overall average. Average annual rainfall from 1987 to 2022 was 32 inches, and the average growing season or number of frost-free days was 158 d (Table 1).

Survival data are divided into 68 kinds that were marginally hardy or not hardy (Table 2), and 271 that have survived from 4 to 35 years and are categorized as hardy in USDA PHZ 4b (Table 3). Fifty-three additional kinds are reported with insufficient data. For example, plants that died before winter, or died after only one winter and were not replanted, and plants with less than 4 years of winter survival data are listed in Table 4. Plants with origins in milder climates were often injured or completely dead after the winter, such as ‘Blonde Ambition’ blue grama [Bouteloua gracilis (New Mexico, USA origin)], ‘Dallas Blues’ switchgrass (Dallas, TX, USA origin), and ‘Thin Man’ yellow prairie grass [Sorghastrum nutans (New Mexico, USA origin)].

Although winterhardiness is not the only factor involved in plant survival in northern climates, cold temperatures tend to dominate over other factors, including soil type and moisture availability (Bélanger et al. 2002; Ouellet 1976). Native range and habitat can be helpful predictors of survival but are less reliable than the confidence derived from long-term field trials. However, exceptions might include June grass (Koeleria macrantha), needle grass (Stipa sp.), and blue fescue cultivars (Festuca sp.)—species that are known for being short-lived, cool-season grasses that require well-drained sites (Darke 2007). These genera also rely on heavy seed set for regeneration, rather than perennial crowns and roots. Thus, it is less likely that winter conditions are fatal to these species, but rather their own inherent genetics likely lead to reduced longevity in the landscape.

Over the 35 years of this trial, ∼30 Chinese silvergrass seedlings were found in the adjacent woods and open fields near the original plantings. These seedlings were removed annually. Scouting for any of the grasses in adjacent areas was part of ongoing management. Without this annual scouting and removal, Chinese silvergrass could have increased and become naturalized and perhaps invasive, as it has in the eastern United States (USDA 2023). No other plants in this trial were found self-seeding outside of the original planting location.

Chinese fountain grass (Pennisetum alopecuroides) was one of the most problematic grasses. Plants often showed large dead sections of winter damage, with lopsided growth in the spring. Even so, by fall these plants were filled with inflorescences that disarticulated and self-sowed in nearby turfgrass and planting beds, thus becoming weeds. Lawn mower blades shredded Chinese fountain grass into white fluff, making it quite conspicuous after mowing (Fig. 2). Tedious hand weeding was required to remove these plants in the turf. Reduced seed set in new cultivars of Chinese fountain grass (Hanna and Schwartz 2020b) and Chinese silvergrass (Hanna and Schwartz 2020a; Ranney and Touchell 2016) can reduce weed and invasiveness problems. These reduced-seed set cultivars have lived 4 years in our trials.

Maintaining genetic integrity of unique cultivars can be challenging in a long-term herbaceous trial because of seedling establishment from nearby plants and variation in vigor among cultivars. This proved especially difficult in purple moorgrass (Molinia arundinacea and Molinia caerulea) and switchgrass. Annual inspection of each cultivar throughout the growing season and especially during flowering was essential to remove errant seedlings.

Originally, the plant residue or growth from the previous year was burned annually before new growth began each spring, usually in April (Fig. 3). Burning rejuvenates perennial grasses and reduces crown and foliage diseases, but can kill overwintering larvae. In recent years, the native grasses have been cut back and the tops removed in early spring, with burning only occurring every third year. This allows overwintering larvae to survive in the base of the grass crown, thus supporting butterflies and moths (Lepidoptera) that have been reported to overwinter at the base of these grasses (Narem and Meyer 2017).

Variegated cultivars such as Avalanche, Overdam, and El Dorado feather reedgrass (Calmagrostis ×acutiflora) required removal of all-green sections, as did variegated cultivars of other grasses. No fungal pests required management; however, field ants (Formica sp.) did considerable damage in some grasses, such as tufted hairgrass (Deschampsia caespitosa) and fall reedgrass (Calamagrostis brachytricha), by building their nests directly in the grass crowns. We used ant baits (Terro Ant Killer; Woodstream, Lancaster, PA, USA) and often resorted to dividing the entire plant and disrupting the ant colony. Voles (Microtus sp.) required management effort, especially during the fall and winter, when bunch grasses provided the perfect habitat and food source during the winter. Baits (Molmax granules; Bonide, Oriskany, NY, USA) and repellents (Molmax; Liquid Fence Co., Bridgeton, MO, USA) were used as annual management efforts against voles.

International collaborations and plant trial conferences have helped to advance plant trial objectives, organization, and results (Cockshull et al. 2013). Kingsbury (2011) lamented the lack of long-term information on herbaceous perennials and proposed five key indicators for measuring their long-term performance: longevity, vegetative spread, competitiveness, speed of establishment, and spread by self-seeding. Using these factors, he categorized many perennials, including three grasses, of which only Chinese silvergrass was in our trials. Without field trial data, growers and retail garden centers understandably are reluctant to risk offering plants with unknown hardiness to consumers. Proper hardiness labeling is enforced in some states that often rely on field trials to determine their ratings (Minnesota Department of Agriculture 2023).

We were fortunate to have planted these trials in a public arboretum that had space and resources to dedicate to a long-term research project. This enabled gardeners, as well as growers and retailers, the opportunity to view the plants throughout the seasons and years. Additional detailed features of specific grasses grown in this study and their use in the landscape has been published (Meyer 2012). This winterhardiness information demonstrates the survival likelihood of many landscape grasses in the upper midwestern United States and will help inform growers, retailers, and gardeners when selecting grasses to grow in cold climates.

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