Flowering dogwood is native to the eastern United States from Texas to Massachusetts (Witte et al., 2000). Flowering dogwood continues to be a valuable crop for nursery producers. It is ranked third in total sales value among flowering trees, behind only crape myrtle (Lagerstroemia indica) and flowering cherry (Prunus serrulata), and it accounts for more than 7.5% of the total value of all flowering trees sold in the United States (U.S. Department of Agriculture, 2019). Although flowering dogwood can be grown throughout a wide geographical region, three states (Tennessee, Florida, and Oregon) produce more than 75% of the plants within the nursery industry (U.S. Department of Agriculture, 2019). Field production is the most common method used for commercially grown flowering dogwood in Tennessee (Halcomb, 2002). Plants can be harvested as either bare-root or balled and burlap (B&B), depending on the plant size and intended use. Although field production is the least expensive method of growing flowering dogwood, the long production cycle and limited transplant window for B&B nursery stock have led to increased interest in growing flowering dogwood in containers. Additionally, the market for container-grown flowering dogwood has expanded due to year-round sales of plants and demand from garden centers and large retailers.
Container-grown flowering dogwood can be a challenging crop due to a number of factors. Container-grown liners of flowering dogwood seedlings and cultivars are not widely available; therefore, most growers must transplant bare-root liners into containers. Bare-root liners have fewer roots and less root biomass compared with containerized liners due to harvesting, which can delay shoot growth and development. Burrows et al. (2015) reported that improper irrigation management, excessive fertilization, poor root development, and delayed budbreak can hinder development of bare-root liners during container production. In addition to problems soon after transplantation, flowering dogwood plants are sensitive to overwatering and highly soluble salts during production. Recommended practices include the use of cyclic irrigation, low to medium rates of controlled-release fertilizer, and frequent monitoring of water needs and soluble salt levels in the substrate (Fulcher and White, 2012). Powdery mildew is also a major issue during flowering dogwood production. Powdery mildew may cause cosmetic damage, including red–brown patches, reduced growth, and premature defoliation. A routine fungicide spray schedule is recommended between May and October in Tennessee to minimize powdery mildew severity and maximize crop growth and quality (Halcomb, 2002).
Most commercial nurseries produce flowering dogwood in full sun and plants are subjected to stress with long-term sun exposure and supraoptimal root zone temperature (RZT). RZT in nursery containers can reach 54 °C during the summer, but plant growth can cease at RZTs above 38 °C (Ingram et al., 2015; Markham et al., 2011). RZTs during container production commonly reach over 42 °C in the southeastern United States, and they can reach over 50 °C in other parts of the country, including Ohio and Washington (Mathers, 2003). Moderating RZT during production would reduce plant stress and improve crop growth and quality. Nevertheless, methods of reducing RZT must be practical for adoption at commercial nurseries. Growing crops under shadecloth or delaying spacing of containers can reduce RZT by minimizing solar radiation of the container sidewall, which is the main source of heat buildup in nursery containers (Ingram et al., 2015). Montague et al. (1992) reported that flowering dogwood had greater growth (plant height and stem diameter) when grown under black or white shadecloth compared with plants grown in full sun. Additionally, the container substrate temperature was 6 °F lower under shade compared with full sun. Burrows et al. (2015) also reported greater flowering dogwood growth under different types of shadecloth (black 30%, black 50%, and white 50%) compared with full sun, although the differences occurred later in the season (August), suggesting that plants grown in full sun were exposed to longer durations of high temperatures that were detrimental to plant growth. Burrows et al. (2015) also noted that the RZT was greatly reduced under 50% shadecloth (regardless of color) compared with 30% shadecloth and full sun. Shade culture could also aggravate the powdery mildew incidence in flowering dogwood. Powdery mildew develops and spreads more readily under high humidity conditions (Baysal-Gurel and Fare, 2016; Baysal-Gurel and Gunter, 2018). Plants under shade culture benefit from the reduced light intensity, but the shadecloth limits air flow, thus contributing to increased humidity and incidence of powdery mildew.
Liner type/size, transplant container size, and finished container size will vary by crop species and grower preference. Nursery growers may choose to transplant liners into the finished container size and hold the plants until the crop reaches a marketable size, whereas others prefer to transplant into an intermediate container size to potentially avoid crop stress and shorten the crop cycle (Beeson, 1991). Flowering dogwood has a moderate growth rate but can vary by cultivar. No published reports have detailed the production schedule for flowering dogwood in large (no. 7 or no. 15) containers. Demonstrating effective cultural practices that maximize flowering dogwood growth and quality will assist nursery growers in selecting proper production schedules for container-grown flowering dogwood. The objective of this research was to evaluate growth and powdery mildew severity of two cultivars of flowering dogwood bare-root liners over a 2-year production cycle in no. 7 and no. 15 containers under varying periods of shade exposure.
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