Conspicuous population declines of the iconic monarch butterfly are the result of loss of summer breeding habitat, extreme weather events, and deforestation of overwintering habitat (Brower et al., 2002, 2012; Casner et al., 2014; Flockhart et al., 2015; Hartzler, 2010; Pleasants and Oberhauser, 2012). Two migratory populations of monarch butterfly exist in North America. The eastern population is the larger of the two and migrates over successive generations ≈4000 km from overwintering grounds in the high-elevation oyamel fir (Abies religiosa) forests of central Mexico to summer breeding grounds in the United States and southern Canada (Brower, 1995). The relatively smaller western population spans ≈500 km from overwintering sites in coastal California to summer breeding grounds west of the Rocky Mountain Range (Dingle et al., 2005).
Monarch larvae (caterpillars) feed almost exclusively on milkweed, a genus of perennial dicotyledonous plants with more than 130 known species native to North America (Woodson, 1954). Larvae acquire protection from predators as a result of the toxic cardenolide glycosides in the milkweed foliage they consume (Malcolm et al., 1989). Monarch migration is closely tied to the timing of milkweed development across the summer breeding range (Stevens and Frey, 2010). Shrinking populations of milkweed due to a combination of extensive planting of genetically modified herbicide-resistant soybean (Glycine max) and corn (Zea mays), widespread herbicide use, and land-use change have reduced the suitability of summer breeding grounds (Pleasants and Oberhauser, 2012). Climate change is predicted to further reduce summer breeding habitat suitability (Lemoine, 2015).
Given the need to restore monarch butterfly summer breeding habitat, recent efforts have promoted milkweed propagation (Cutting and Tallamy, 2015; Dumroese et al., 2016; Landis, 2014; Landis and Dumroese, 2015; Luna and Dumroese, 2013). Much of what is known about milkweed biology and propagation is based on work done with common milkweed [Asclepias syriaca (reviewed in Bhowmik, 1994)]. Propagation protocols have also been developed to facilitate production of other milkweed species, including the federally threatened mead’s milkweed [Asclepias meadii (Baskin and Baskin, 2002)]. Our research focuses on two milkweed species, showy milkweed and narrowleaf milkweed, common to western North America, essential to the western monarch population, and in high demand for habitat restoration efforts.
Showy milkweed is a species distributed across western North America from British Columbia to Manitoba and south to Texas and is found in well-drained soil in open habitats from 0 to 1900 m elevation [U.S. Department of Agriculture (USDA), 2017a]. Narrowleaf milkweed has a much smaller range, covering California, Idaho, Nevada, Oregon, Washington, Utah, and Baja California and is found in dry, barren areas from 50 to 2200 m elevation (USDA, 2017b). Published protocols for the two species use a variety of seed cleaning techniques, either with or without a stratification treatment, and employ an assortment of containers and media, either with or without fertilizer amendments (Bartow, 2006; Leigh et al., 2006; Skinner, 2008; Tilley, 2016). In one protocol for narrowleaf milkweed, seeds were sown directly into flats (1.5 inches deep) containing soilless media, which were then stored in an outdoor coldframe from autumn though the following spring (Leigh et al., 2006). Transplantable seedlings developed within ≈8 weeks of germination and were transferred to a variety of larger pots containing the same soilless media, where they developed into seedlings with fibrous root systems (10–20 cm in length) by the end of the 8-month growing season. In one protocol for showy milkweed, seeds were sown into 10-inch3 containers filled with soilless media amended with fertilizer (Bartow, 2006). Under controlled greenhouse conditions, seeds germinated within 1 week (80% germination). Developing plants were hand-watered as needed and received 20N–8.8P–16.6K water-soluble fertilizer (JR Peters, Allentown, PA) twice during the 5-month growing period. Skinner (2008) sowed showy milkweed seeds directly into 10-inch3 containers filled with soilless media and noted germination was complete within 2 weeks. Plants were watered deeply every 2 d and fertilized (formulation unspecified) weekly for ≈3 months. Tilley (2016) provided a third protocol for showy milkweed, directly sowing untreated seeds into 10-inch3 containers filled with soilless media without fertilizer in a controlled greenhouse environment, providing 20 min of daily irrigation from overhead sprinklers for the first 30 d. Plants were then fertilized [15N–13.1P–12.5K (Miracle Grow All Purpose Plant Food; Scotts, Marysville, OH)] weekly and watered 40–60 min every 2 d for ≈4 months. Although protocols vary, one commonality is the use of small volume (i.e., 10 inch3) containers. Information on the production of milkweed by directly sowing seeds into larger volume containers is lacking.
In restoration plantings, particularly where competition is a factor and environmental stress limits establishment success, larger seedlings may be superior because of the competitive advantage conferred by greater heights and increased root growth potential associated with larger root systems (Grossnickle, 2012). Evidence suggests that not only do larger volume containers yield larger nursery seedlings, but these larger seedlings also have higher survival rates and continue to realize greater growth rates in the field after planting (Pinto et al., 2015; Sutherland and Day, 1988). Similar to increased container volume, identifying optimal species-specific fertilizer application rates will aid in the propagation of superior seedlings in the nursery (Cardoso et al., 2007; Clark and Zheng, 2015), which may confer these same advantages on outplanting. However, a concern in the production of container milkweed plants from seeds is the formation of a firm root plug; i.e., one in which the root system remains intact when removed from the container (Landis, 2014). Because milkweed plants develop rhizomes without many fibrous roots, transplanting or outplanting seedlings can be challenging if root systems are poorly developed. Thus, the objectives of this research are 1) to develop propagation techniques that yield firm plugs for two species of milkweed common to western North America through the manipulation of container volume and fertilizer application rate, and 2) to assess first year field survival of these seedlings.
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