Water conservation is becoming critically important throughout the United States, especially in Utah and the Intermountain West, one of the driest and fastest growing regions in the United States. The green industry in this arid to semiarid region would earnestly consider alternative water sources such as treated and reclaimed sewage effluent (reclaimed water) for water conservation and improved environmental stewardship. Reclaimed water has been a viable alternative source for landscape irrigation in Utah, with an established use record on golf courses. Reclaimed water is also used by a handful of large corporate and municipal parks and landscapes in arid and semiarid urban areas across the Desert Southwest (Grieve, 2011; Tanji et al., 2008). As Utah and the West work toward improved water conservation, the chances are good that water will be restricted until only minimums are applied. This has the potential to reduce the leaching fraction of irrigation to the point that soil salinity will gradually increase, especially in salt-prone areas using reclaimed water. It is essential to select salt-tolerant plants for landscape use in those salt-prone areas.
Reclaimed water has a relatively high salinity level and undesirable specific ions (Grieve, 2011) that could impose salt stress on a large number of landscape plants (Niu and Cabrera, 2010; Sun et al., 2015a; Wu and Dodge, 2005). Landscape plants have been evaluated for salt tolerance. Since 2005, researchers at the Texas A&M AgriLife Research Center at El Paso have screened more than 150 landscape plant species and/or cultivars in greenhouse conditions (Niu and Cabrera, 2010; Niu et al., 2011; Sun et al., 2015a). All these studies showed consistently that the salt tolerance of landscape plants varies highly with species and/or cultivars (Chen et al., 2017; Liu et al., 2017; Sun et al., 2015a). However, limited research-based information on the salt tolerance of ornamental grasses is available.
Ornamental grasses have drawn considerable attention in the U.S. green industry as a result of their high drought tolerance, low maintenance input, and the unique textures and patterns they contribute to the landscape (Gunnell et al., 2015). Nursery production and landscape use have expanded significantly, with an estimated $158 million worth of ornamental grasses sold annually in the United States (U.S. Department of Agriculture, 2015). Ornamental grasses are popular in urban landscapes in Utah and the Intermountain West of the United States. Eragrostis spectabilis (purple love grass) is a warm-season bunchgrass with flat, coarse green leaves and soft reddish purple flowers in a loose and open inflorescence (Missouri Botanical Garden, 2018). The inflorescence of purple love grass is good for dried flower arrangements. Miscanthus sinensis ‘Gracillimus’, commonly called chinese silver grass or maiden grass, is a clump-forming warm-season grass that features narrow green leaves with a silver midrib and tiny reddish copper flowers in tassel-like inflorescences (Missouri Botanical Garden, 2018). Maiden grass produces long-lasting dried flowers for winter interest. Panicum virgatum ‘Northwind’ (switchgrass), an introduction of Northwind Perennial Farm (Burlington, WI), is a warm-season grass with finely textured, pink-tinged, branched panicle inflorescences and olive- to bluish green foliage forming a compact, narrow, erect clump (Missouri Botanical Garden, 2018). Schizachyrium scoparium (little bluestem) is one of the dominant grasses of the tallgrass prairie region and features upright clumps of slender, flat, linear green leaves with purplish bronze flowers in racemes (Missouri Botanical Garden, 2018). These four ornamental grass species belong to the grass family (Poaceae). They tolerate a wide range of soils and drought conditions and are accent specimens for garden and urban landscapes.
The salt tolerance of a few ornamental grasses have been evaluated. In general, cool-season grasses usually have less salt tolerance than warm-season grasses (Schiavon et al., 2012, 2014). In a 4-month experiment, Leymus arenarius (L.) Hochst. (sand ryegrass), Muhlenbergia capillaris (Lam.) Trin. (pink muhly grass), and Pennisetum alopecuroides (L.) Spreng. (fountain grass) showed a very strong salt tolerance with an acceptable visual quality, although plant growth was inhibited by the increasing salinity (Sun and Palmer, 2018). Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths (blue grama) has moderate tolerance to salinity when grown at a saturated soil extract (ECe) of 4 to 8 dS·m–1 (Kratsch et al., 2008), and its salt tolerance varies within ecotypes and is greater at the germination stage than the mature stage (Zhang et al., 2012). P. alopecuroides ‘Hameln’ is slightly more tolerant to salt spray than M. sinensis ‘Gracillimus’ (Alvarez, 2006). Pennisetum clandestinum Hochst. ex Chiov. (kikuyugrass) is a suitable candidate for the saline–sodic water reuse system with a threshold ECe of 8.0 dS·m–1 (Grieve et al., 2004, 2012). Pink muhly grass can survive at sodium chloride (NaCl) irrigation rates of 10,000 mg·L–1 (corresponding to an EC of ≈12.5 dS·m–1), which is up to 20 times greater than graywater (Christova-Boal et al., 1996; LeCompte et al., 2016). Because diverse ornamental grasses are produced by the green industry, further research is needed to identify salt-tolerant ornamental grasses for landscape use. This study was designed to assess the salt tolerance of four commonly used ornamental grasses in response to saline solution irrigation through comparing their growth, gas exchange, and mineral nutrients.
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