Golf greens and fairways planted to creeping bentgrass (Agrostis stolonifera), colonial bentgrass (A. capillaris), and velvet bentgrass (A. canina) require overseeding to reestablishment areas damaged from winter injuries. Cold soil temperatures (≈10 °C) typical of early spring plantings in the northern United States reduce seed germination and establishment vigor of cool-season grasses when compared with more favorable soil temperatures of 20 to 30 °C (He et al., 2013; Liu et al., 2001; McGinnies, 1960; Wright et al., 1978). Delayed growth shortens the growing period of seedling grasses and results in lower plant numbers in turf stands (Larsen et al., 2004), which are more susceptible to competition for moisture and summer drought (Young and Evans, 1982) and to competition from weeds during the establishment phase (Murphy et al., 2005).
Seed germination is the initial step in establishment, and rapid establishment can mitigate costs and revenue loss when golf courses openings are delayed. High-quality seed and seedling vigor is characterized by high viability (more germinating seeds). Emergence is the point that the primary leaf emerges from the soil surface followed by growth and development of roots, stolons, and rhizomes to form a dense turf. Seedling emergence, seedling vigor, and mature plant growth are distinct but related stages. Numerous studies have investigated laboratory seed germination characteristics such as final germination percentage and mean germination times that are conducted under controlled temperatures and moisture. It has been suggested the rate of growth after emergence is determined largely by the genotype and environment and not by the rate of seed germination (Argerich et al., 1990; Scott and Jones, 1985; Yamamoto et al., 1997a). For example, Yamamoto et al. (1997b) found the rate of emergence of the second and third leaves in kentucky bluegrass (Poa pratensis) to be under genetic control.
Unlike controlled environments, seeds are exposed to variable and unfavorable temperature and soil moisture in the field that delay and postpone germination, thereby affecting establishment (Happ et al., 1993). Germination of rough bluegrass (Poa trivialis) in the growth chamber at 10 °C provided two to three times the germination rate observed in the field when rough bluegrass was over-seeded into hybrid bermudagrass (Cynodon dactylon × C. transvaalensis) putting greens (Liu et al., 2001). Emergence of perennial ryegrass (Lolium perenne) seeded on compacted soil cores was only 40% to 69% of that observed under optimal germination when assayed on moist paper (Snapp et al., 2008). Total emergence and emergence rates in the field have been shown to be lower than emergence responses in the laboratory for cool-season range grasses (Hardegree and Van Vactor, 2000). These same authors reported greater germination in laboratory tests than measured in the field during early spring planting dates. Laycock and Shildrick (1979), however, found some positive correlation between perennial ryegrass laboratory germination and initial establishment measured in the field during the time between the sowing date (seed germination and emergence phase) and before the first mowing (maturity).
Soil temperature, water potential, and oxygen levels are important environmental factors affecting seed germination and influencing the seedlings to emerge from the soil (Burris, 1976; Pollock, 1972; Powell, 1988). Beard (1979) found the base temperatures (Tb), below which no germination is observed, to be ≈7 °C for perennial ryegrass and 11 °C for creeping bentgrass. Trudgill et al. (2000) reported colonial bentgrass Tb of 8 °C, and perennial ryegrass exhibited a Tb of 5 °C. The final stage of establishment involves mature roots, shoots, and lateral stems to form, which is a distinctly different phase from seed germination and emergence (Beard, 1973). As such, constant temperature and moisture regimes typical of germination laboratory tests are not necessarily predictive of field performance (Steiner et al., 1989; Trawatha et al., 1990).
Seed requires an accumulation of time and temperature (“thermal-time”) above Tb to germinate (Larsen and Bibby, 2005; Larsen et al., 2004) and in turn, to establish. Nonlinear regression thermal-time models have been used for seed germination and emergence (Larsen and Bibby, 2005; Larsen et al., 2004; Zhang et al., 2013). Monitoring soil temperatures after seedling emergence in the field until a mature stand is formed (i.e., requiring frequent mowing) has not been investigated in bentgrass species (Agrostis sp.) and cultivars. Growing degree day (GDD) models have been computed using soil or air temperatures collected daily. Base temperature varies with the crop with GDD for warm-season turfgrass establishment calculated using Tb close to 5 °C (Frank et al., 1998; Patton et al., 2004; Shahba and Qian, 2008; Unruh et al., 1996), while for cool-season lawn grass species, values of Tb for seed germination ranged from 1.6 to 3.6 °C (Larsen and Bibby, 2005), with bentgrass species exhibiting Tb closer to 10 °C (Heineck et al., 2018; Trudgill et al., 2000).
Identifying bentgrass species and cultivars with the potential to establish under low soil temperatures would be beneficial toward achieving more mature stands earlier in the spring. Simulated field-temperature studies have been conducted in cool-season range grasses (Hardegree and Van Vactor, 1999), cool-season lawn grasses (Larsen and Bibby, 2005), and in cool-season turfgrass for golf use such as bentgrass species and cultivars (Heineck et al., 2018). However, these responses were not validated in the field. Subsequently, Hardegree and Van Vactor (2000) reevaluated their previous findings and found simulated soil temperature models predicted greater germination than measured in the field, and they reported total emergence in the field to be more variable. While bentgrass species and cultivars have been tested in laboratory settings, there is a need to study cold temperature germination of these grasses in the field. Our research objective was to identify bentgrass species and cultivars with the capacity to establish to maturity in the shortest time and under cooler soil temperatures during an early spring planting date.
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