Twinflower is a spreading, evergreen, circumpolar groundcover plant native to North America, Europe, and Asia (Sobey and Barkhouse, 1977). The species is declining in some regions of its native habitat and is considered threatened or endangered by eight U.S. states at the time of this publication (Tsaryk and Malynovs’kyi, 1995; U.S. Department of Agriculture, 2014; Wilcock, 2001). Twinflower’s subtle but attractive foliage and pale pink inflorescences appeal to homeowners and naturalists (Backlund and Pyck, 1998; Jacobs et al., 2010; Scobie and Wilcock, 2009), and the plant offers commercial and residential potential as a native, spreading groundcover for partly shaded landscape spaces (Wilcock, 2001). However, northern New England commercial and conservation growers report inconsistent success with cutting propagation, as well as labor-intensive and cost-ineffective methods (such as hand watering and daily bagging and opening for humidity control) used to improve results (W. Cullina, personal communication; S. Green, personal communication; M. Navazio, personal communication).
As a clonal plant, twinflower grows via spreading stolons. This growth habit, twinflower’s limited sexual reproductive mechanism (Barrett and Helenurm, 1987; Ericksson, 1992; Scobie and Wilcock, 2009), and its limited commercial value preclude more expensive techniques such as tissue culture, and propagation via cuttings has been the primary viable method used in horticultural production. Standard horticultural cutting propagation protocols emphasize shaded, moist environments to prevent excessive transpiration and promote turgor (Grange and Loach, 1983; Hartmann et al., 2011), but these conditions have resulted in problems with root and stem rot for regional twinflower propagators (S. Green, personal communication; M. Navazio, personal communication).
Data have indicated improved rooting rates in other species with increasing DLI under mist irrigation systems (Lopez and Runkle, 2008). Increased light intensity has caused notable shoot growth and branching in twinflower (Chavez and Macdonald, 2010; Ericksson, 1988, 1992; Niva et al., 2006) and other clonal, stoloniferous species (de Kroon and Hutchings, 1995; Dong and Pierdominici, 1995). Twinflower root growth in natural settings is generally poor, although this may be a result of ample moisture in shaded conditions where light is the limiting resource (Brouwer, 1983) or it could result from water resource sharing among clonal ramets (Alpert and Mooney, 1986; Chavez and Macdonald, 2010; Stuefer et al., 1994, 1996). Accordingly, Niva et al. (2006) found that decreased water alone had no effect on twinflower’s stolon architecture, but decreased water in conjunction with increased light intensity decreased its root:shoot ratio.
Little work has explored the impact of soil volumetric water content on root dry weight during propagation. Most of the research that has been conducted was done before low-cost, precise soil moisture sensors became available. For plants that have been researched previously in horticultural settings, even though, greater soil moisture generally resulted in greater root development. For example, chrysanthemum (Dendranthema ×morifolium) cuttings propagated in greenhouses produced the most roots when they were grown in a medium with the highest water content (Geneve et al., 2004). Volumetric water content as low as 0.1–0.2 L·L−1 for lavender (Lavandula angustifolia) and 0.2–0.3 L·L−1 for gardenia (Gardenia jasminoides) has been shown to negatively impact plant mortality and quality (Bayer et al., 2015; Zhen and Burnett, 2015).
Soil in the boreal forest that serves as twinflower’s primary habitat tends to be acidic and low in nutrient availability because of climate, high organic matter, low organic decomposition rates, and the soil acidification effects of moss and coniferous canopies (Bonan and Shugart, 1989; Hobbie et al., 2002). However, it is unknown if increased fertility during propagation would impact rooting. Twinflower does not fit neatly into herbaceous or woody plant categories, and little greenhouse research has been conducted on it; as a result, there was scarce information available that seemed relevant to our study.
Previous research demonstrates that rooting may be impacted by a variety of environmental factors, including light, substrate moisture, and nutrient concentration. There has been little research on the propagation of twinflower, in particular how the propagation environment will impact rooting. To develop an effective greenhouse propagation protocol for twinflower, we conducted two experiments. In our first experiment, we determined effective DLI and θ levels for propagation. Having identified the most successful DLI and θ regimens from those experimentally tested and applying those effective parameters, our second objective was to evaluate the extent to which preplant incorporated fertilizer influences root and shoot growth in twinflower. Results would complete the third standard component (along with light and water) of manipulation of the greenhouse production environment and yield an effective propagation protocol for twinflower.
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