Ornamental kale is widely used as a cool-season bedding plant but there is also a growing interest in selected cultivars, which feature tall, thin-stemmed, cabbage-like heads, as cut flowers (T. Spencer, personal communication) (Fig. 1). In the United States, ornamental kale is increasingly incorporated into bouquets, especially during the fall and winter holiday season. One to three stems of ornamental kale are generally used in mixed flower bouquets and/or single stems are sold directly to consumers, so they can make their own arrangements. Major ornamental kale plant breeders include Takii Seeds (Kyoto, Japan), Sakata Seed (Yokohama, Japan), Ishii Seed Growers (Shizuoka City, Japan), and Evanthia (Monster, The Netherlands) (H. Furuichi, personal communication).
In North America, ornamental kale is grown year-round in California, as a fall crop in Canada, and in small areas along the U.S. east coast; it is also grown year-round in Colombia and Ecuador for export (T. Spencer, personal communication). The U.S. domestic wholesale cut flower market, for growers having $100,000 or more in sales, is valued at $374 million [U.S. Department of Agriculture (USDA), 2016]. It is difficult to more specifically assess the value of this crop since ornamental kale is represented in the “other cut flowers” category estimated to be worth $130 million (USDA, 2016).
Specialty and/or locally grown cut flowers may be profitable for their unique qualities, faster harvest to consumer transition resulting in a longer post-harvest vase life, and for delicate flowers which are difficult to transport without damaging (Ortiz et al., 2012; Owen et al., 2016; Wien, 2009; Yue and Hall, 2010). The increasing interest among U.S. consumers to purchase American-grown and/or locally grown flowers alongside other agricultural products (Connor, 2018; Feldmann and Hamm, 2015; Low and Vogel, 2011; Yue et al., 2011) is a value-driven trend. Support from floriculture industry groups is evident. For example, the Association of Specialty Cut Flower Growers (1988) states that it strives to “help foster and promote the local availability of high quality floral material” and the third-party verified label American Grown Flowers (2014) promotes “bouquets and bunches of domestic origins with high quality, freshness and consistency.”
Ornamental kale is a popular crop in Japan, where it is grown primarily under high tunnels (T. Spencer, personal communication). High tunnels reduce weather-related production risks, improve product quality and consistency, assist with organic growing efforts, help increase productivity on limited amounts of land, and amplify season extension opportunities (Carey et al., 2009; Jayalath et al., 2017; Lamont, 2009; O’Connell et al., 2012). The use of high tunnels has been growing steadily in the United States although primarily for vegetable production (Carey et al., 2009; Hochmuth and Toro, 2014; Knewston et al., 2010). Inclusion of cut flowers in high tunnel systems that also produce vegetables, herbs, fruits, etc. provides additional options to rotate crops which can help address disease, pest, and nutrient management issues. In addition, cut flowers may benefit from increased stem elongation compared with the open field. This is due to decreased light intensity under high tunnel environments due to a semitransparent polyethylene plastic roof as well as reduced wind.
Seasonal production strategies in Japan appear to be adaptable to the southeastern United States, including Georgia, which shares a humid subtropical climate according to the most frequently used Köppen climate classification system (American Takii, 2013; Kottek et al., 2006). In Japan, seedlings are transplanted to high tunnels during periods of high temperature and humidity (T. Spencer, personal communication). This period of warm weather (between 55 and 80 °F) is reported to encourage vegetative growth and stem elongation (American Takii, 2013; Sakata Seed America, 2018).
Generally, ornamental kale reaches an optimum stem length (60 to 70 cm) ≈2 to 3 months after transplanting (American Takii, 2013; Kristl, 2001; Sakata Seed America, 2018). Optimum stem length criteria is based on Japanese flower market standards that consider shipping, processing, and ultimately use in a variety of applications such as bouquets and table arrangements (H. Furuichi, personal communication). The United States does not have its own ornamental kale standards yet.
The consensus is that a minimum of 10 to 14 d with nighttime temperatures ≤55 to 60 °F is required to achieve good leaf color contrasts (American Takii, 2013; Kristl, 2001; Sakata Seed America, 2018). Even more intense leaf color contrasts may develop if nighttime temperatures remain between 35 and 45 °F for a few weeks (Sakata Seed America, 2018). Cold-storing ornamental kale between 45 and 50 °F is recommended post-harvest (H. Furuichi, personal communication).
There are very limited studies exploring ornamental kale and/or cut flowers under high tunnels in the United States. In 2002, a field and pot study evaluated ornamental kale production in North Carolina using conventional practices (Greer et al., 2003). Results indicated that ornamental kale should be planted ≈75 d before consistently cool temperatures (i.e., end of June planting date in North Carolina), that applications of gibberellic acid to the leaves did not result in increased stem length, and that plant spacing affected the average stem length (Greer et al., 2003). The most generous plant spacing trialed in North Carolina (8 × 8 inches) yielded the longest average stems (Greer et al., 2003). Johnny’s Selected Seeds (Fairfield, ME) conducted a trial to provide growers with guidelines about planting dates and management practices applicable in Maine (Kristl, 2001). Results demonstrated that transplanting ornamental kale in June resulted in an early October harvest thus requiring ≈120 d in the field in Maine (Kristl, 2001).
High tunnel vs. field system experiments for selected cut flowers have been published primarily for New York and Indiana. A high tunnel vs. field comparison of six cut flower species in New York, ornamental kale not included, suggested that similar transplant dates yield earlier and more concentrated harvest periods under high tunnels but did not always increase the number of stems per plant (Wien, 2009). Owen et al. (2016) conducted a high tunnel and field system comparison of seven conventionally grown cut flowers from July through October in Indiana. Although ornamental kale was not included, selected cut flower species demonstrated increases in the total number of stems per week when grown under high tunnels, and stem length was greater for most flower types compared with the field (Owen et al., 2016). Overall, the number of flowers per plant was not different between the high tunnel and field but flower quality was greater for selected species (Owen et al., 2016). Lastly, Ortiz et al. (2012) found that high tunnels could result in earlier yields, more stems, and longer stems for selected cut flowers including snapdragon (Antirrhinum majus ‘Rocket Red’), dianthus (Dianthus barbatus ‘Amazon Neon Cherry’), stock (Matthiola incana ‘Katz Lavender Blue’), zinnia (Zinnia elegans ‘Benary Giant Scarlet’), dahlia (Dahlia ×hybrida ‘Karma Thalia Dark Fuschia’), lisianthus (Eustoma russellianum ‘Mariachi Blue’), and sunflower (Helianthus annuus ‘Sunrich Yellow’ and ‘Premier Lemon’) compared with the field in Indiana.
To our knowledge, no studies have been published about growing ornamental kale under high tunnels in the United States although it is common practice in other parts of the world. Therefore, the objectives of our research were to evaluate the effects of planting date and cultivar on ornamental kale crop yields grown under an organic high tunnel system in Georgia. Ornamental kale cultivars included in our experiment were selected based on similar advertised stem height (≈60 to 90 cm), differing color characteristics, and the availability of untreated seed suitable for certified organic production until organic seeds are available. Additional mustard family (Brassicaceae) crops including broccoli (Brassica oleracea var. italica), cauliflower (B. oleracea var. botrytis), and turnip (Brassica rapa var. rapa) were planted under the high tunnels around the same time. Results for the broccoli and cauliflower study are available separately (O’Connell and Tate, 2017).
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