September 2020 FINAL STUDY REPORT Lockeford Plant Materials Center Lockeford, California Milkweed Establishment in California’s Central Valley: II. Comparison of Asclepias eriocarpa, Asclepias fascicularis, and Asclepias speciosa by Seed Valerie Bullard, Jessa Kay Cruz, Margaret Smither-Kopperl ABSTRACT Populations of milkweed species in California have declined in recent years resulting in fewer breeding sites for monarch butterflies. Milkweed species are frequently included in pollinator mixes, although poor establishment of milkweed species were observed at the Lockeford Plant Materials Center and in plantings by the Xerces Society. The objective of this replicated study was to compare establishment of three species of milkweed: woollypod milkweed(Asclepias eriocarpa); narrow leaf milkweed (Asclepias fascicularis), and showy milkweed (Asclepias speciosa), from seed in a randomized complete block design with four replications. The site at the Lockeford Plant Materials Center was solarized in the summer of 2017 and plantings made into a firm seed bed with a no-till drill at 15 seeds per square foot after a rainfall event. Plants were irrigated during the first year of the trial and frequent weeding was required. Evaluations were made during 2018 and 2019 for germination and survival, plant height, disease resistance and insect resistance. Woollypod and narrow leaf milkweed were first to emerge in early April 2018, and showy milkweed emerged in early May. By the end of the 2018 growing season, narrow leaf milkweed had a significantly higher survival rate (moderate to good survival), than showy milkweed (fair survival). In 2019, showy milkweed emerged in mid-April, while woollypod and narrow leaf milkweeds emerged in late April 2019. Overall, showy milkweed had higher survival rates, lower insect and disease damage and was significantly taller in height than the other two species. INTRODUCTION. Milkweed species are critically important for survival of monarch butterflies (Danaus plexippus) as they are the primary larval host for the caterpillars. Western populations of monarch butterflies overwinter on the California Coast. The butterflies set out each spring on their annual migration to other western states and seek populations of milkweed plants on which to lay their eggs. There are numerous, overlapping generations of monarch butterflies each year as they complete their migration throughout California, into Nevada and Idaho, returning to their overwintering sites along the California Coast late in the fall. Xerces volunteer Thanksgiving counts of overwintering western monarch butterfly populations in groves on the California coast have declined precipitously in recent years (Pelton, 2017; Xerces Society, 2020). The 2018 Xerces Thanksgiving count was less than 30,000 butterflies, a 99% drop since 1997. This large drop causes concern of a quasi-extinction event for the western monarch (Pelton et al. 2019). A major factor contributing to these declines is the loss of milkweed plants across the western monarch’s breeding range. A year-round population model showed that losses to breeding sites Margaret Smither-Kopperl, Manager; Valerie Bullard, Agronomist, Lockeford Plant Materials Center, 21001 N. Elliott Rd., Lockeford, CA 95237. Jessa Kay Cruz, Senior Pollinator Conservation Specialist Xerces Society, [email protected]. 2 had four times the impact as the loss of overwintering sites (Flockhart, et al. 2015; Pelton et al. 2019). California’s Central Valley has recently been suggested as habitat of historic importance for milkweed populations, necessary for egg laying and successful survival of monarch populations (Espeset et al., 2016). The Central Valley is one of the most intensively farmed areas in the world resulting in major loss of native habitat. Extensive use of herbicides and cultivation is linked with loss of milkweed in fields and field borders (Pleasants and Oberhauser, 2012). This study focused on establishment of three milkweed species, woollypod, narrow leaf and showy milkweed, which are widely distributed and commercially available. Woollypod milkweed is distributed through the Coast Ranges and the Sierra Foothills of California (Calflora, 2017). It is a perennial with rarely branching, densely hairy stems up to 3 feet tall (Borders, 2012; Rosatti and Hoffman, 2017). The dark green leaves are opposite and oblong to oval and covered above with dense hairs and prominent yellow mid‐veins. The umbels emerge from the uppermost nodes, the flowers moderately large and greenish to yellowish cream, while flushed with rose. Woollypod milkweed produces upright pods that are densely hairy, tapering to a tip, these from 1 to 3.5 inches in length (Rosatti and Hoffman, 2017). Narrow leaf milkweed is distributed throughout California, apart from the desert regions (Calflora, 2017). They are erect perennials with smooth stems, with both axillary and terminal umbels with many greenish white and purple tinged flowers, 2 to 3 feet in height (Borders, 2012; Rosatti and Hoffman, 2017). The persistent leaves are whorled with 3 to 5 leaves at each node, each of these are narrowly linear with a tapered base, these are less than one inch wide but usually smaller and 4.5 inches long, smooth on top. The umbels are rounded on top and made up of numerous flowers with grayish pink to nearly white but greenish corolla. The upright pod is smooth and 2 to 5 inches long and less than a half inch wide (Rosatti and Hoffman, 2017). Showy milkweed is distributed in the Coast Ranges and Sierra Foothills of California mainly north of Merced and the San Francisco Bay (Calflora, 2017). The plant is sturdy, single or multi- stemmed, up to 4 feet tall, with large opposite leaves that terminate in several umbels near the top of the stem with large pink to purplish flowers (Borders, 2012; Rosatti and Hoffman, 2017; Stevens, 2010). The umbels are between 1.5 and 3 inches across with flowers about a half‐inch across that have elongate hoods. The stems have opposite spade‐shaped leaves that are fuzzy on both sides with rather woolly hairs below. The pods are upright, 4 to 5 inches long and covered with hairs and some have warty projections (Rosatti and Hoffman, 2017). The Xerces Society recommends milkweed species in hedgerow and pollinator plantings (Borders and Lee-Mader, 2014). Observations by the Xerces Society in pollinator plantings in California and at the Lockeford PMC indicate that milkweed establishment, as part of a pollinator mix, is frequently unsuccessful. Likewise, milkweed transplants are less likely to establish than other pollinator plants. The reason for the lack of success of milkweed establishment is not clear as guidelines used successfully for other species are followed (Borders and Lee-Mader, 2014). The objective of this study is to compare the establishment of three different milkweed species, woollypod, narrow leaf, and showy milkweed, after fall seeding . The results reported here are part of a larger milkweed establishment study at the CAPMC. The additional reports are: Milkweed Establishment in California’s Central Valley: I. Showy Milkweed, Asclepias speciosa 3 by Seed, Rhizome and Transplants, and Milkweed Establishment in California’s Central Valley: III. Comparison of Milkweed Species Asclepias eriocarpa, A. fascicularis, and A. speciosa by Transplants. MATERIALS AND METHODS The milkweed establishment trial was conducted over two years (2017-2019) at the Lockeford Plant Materials Center (CAPMC). The CAPMC is located on the eastern side of the San Joaquin Valley in central California and sits on a historical flood plain on the west bank of the Mokelumne River. The soil series is a Columbia fine sandy loam on 0 to 2 percent slopes. It is a very deep, well-drained soil with pH ranging from moderately acid to slightly alkaline. The mean annual maximum temperature in this area is 73.6°F and minimum temperature is 46°F (WRCC, 2018). The mean annual precipitation is 17.24 inches, mainly occurring between the months of December and March (WRCC, 2018). Precipitation totals were just below average (16.8 inches) between September 1, 2017 and August 1, 2018 and were above average (24 inches) between September 1, 2018 and August 1, 2019 shown in Figure 1 (WWG, 2019). 7 120.0 6 100.0 F) ° 5 80.0 4 60.0 3 40.0 2 Monthly Total Precipitation (in) 20.0 1 ( & Min Max Temp Monthly Mean 0 0.0 Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Total Precip 2018-2019 Total Precip 2017-2018 Mean Monthly Precip Avg Max Temp 2018-2019 Avg Max Temp 2017-2018 Mean Monthly Max Temp Avg Min Temp 2018-2019 Avg Min Temp 2017-2018 Mean Monthly Min Temp Figure 1. Mean minimum and maximum temperatures in the 2017/2018 and 2018/2019 growing seasons generally followed annual averages. Total precipitation was just below average in 2017/2018 and above average in 2018/2019. Monthly weather data from September through August was provided from Western Weather Group Lockeford Weather Station located directly across the river from the CAPMC. Average weather summaries from 1893-2015 for the Lodi area were provided from Western Regional Climate Center. Site Preparation Site preparation began in June 2017. Weeds and the remaining residue from the previous fall planted cover crop were disked. The area was chisel plowed and cultipacked to prepare a firm seed bed and smooth solarization surface. Sprinkler handlines were laid to provide adequate soil 4 moisture. The UV-treated, 4 mm plastic was installed in June for maximum solar incidence for weed control. The solarization treatment effectiveness was marginal as nutsedge (Cyperus spp.) was growing under the plastic. Nutsedge was dug up and removed prior to planting and additional harrowing and cultipacking was also required to control the germinating weeds prior to planting the milkweed trial. Recycled solarization plastic was used for this project resulting in patching together multiple small pieces of plastic which may have decreased the efficacy of this usually effective site preparation method.