Ready-to-eat salad mix has experienced more than a 5-fold increase in supermarket sales over the last 20 years in the United States, increasing from $197 million in 1993 to $2.7 billion in 2008, while the sales of iceberg and romaine head lettuce have decreased (Cook, 2008). Bagged ready-to-eat salad mixes were first widely distributed in supermarkets in 1989, and were composed of mature heads of lettuce (Lactuca sativa; primarily iceberg type) cut into bite-sized pieces using specialized head lettuce-processing equipment (Thompson and Wilson, 1999). Today, a diverse array of leafy green salad crops are grown for baby-leaf salad mix in the United States, including lettuce, spinach (Spinacea oleracea L.), mustard greens (Brassica juncea L. and Brassica oleracea L.), pak choi (Brassica rapa L.), kale (Brassica oleracea L.), arugula (Eruca sativa L. and Diplotaxis tenuifolia L.), and beet greens (Beta vulgaris L.) (Hardesty and Leff, 2009). Although the widespread consumption and distribution of baby-leaf salad mix is a relatively recent development in the United States, the practice of harvesting leafy greens at an immature stage of development originated with the concept of mesclun, or “spring mix” in Provence, France, in the 1700s and was introduced to American cuisine in metropolitan restaurants in the late 1960s (Hardesty, 2010).
Growers in California and Arizona are able to produce leafy green salad crops year-round because of the region’s mild winter climate (Koike et al., 2011; Reader, 2003; Smith et al., 2011). These two states accounted for 96% of all lettuce grown in the United States in 2012, and the Salinas Valley alone accounted for 70% of leafy green salad crops (City of Salinas Economic Development, 2013; USDA, 2010), which included 57,200 t of spring mix, 15,100 t of baby-leaf spinach, 11,800 t of radicchio (Cichorium intybus L.), 21,900 t of kale, and 945,000 t of leaf lettuce (Monterey County Crop Report, 2012). In most other states, leafy green salad crops are grown seasonally on small- and moderate-sized farms, with over 70% of lettuce and over 60% of spinach producers growing less than one-half hectare (USDA, 2010, 2012b, 2014). In Washington State, lettuce was produced on 222 farms totaling 83 ha in 2012 with leaf lettuce as the dominant lettuce type, followed by crisphead and romaine types (USDA, 2012b). Spinach was produced on 44 farms totaling 64 ha in 2012.
There is strong demand in northwest Washington for locally grown baby-leaf salad greens. While the market is well supplied throughout the summer in this region because of the mild climate that is conducive to baby-leaf salad green production, growers require production information to enable them to extend the season as much as possible in the spring and fall. Research on salad crop production has historically focused on crop performance at the mature growth stage (Clarkson et al., 2003; Farnham and Garrett, 1996; Hoque et al., 2010; Sanchez and El-Hout, 1995; Zhao and Carey, 2009). Baby-leaf salad greens are harvested between 30 and 45 d after planting, and in recent studies it has been shown that cultivar performance at the baby-leaf stage differs from performance at the mature stage because of physiological and morphological differences between the developmental stages (Borrelli et al., 2013; Coolong et al., 2013; Egea-Gilabert et al., 2013; Wallace et al., 2012).
The objective of this experiment was to evaluate nine salad crop cultivars for suitability as baby-leaf salad greens in the spring (April–June) and six cultivars for the fall (September–November) growing seasons in northwest Washington. Overall productivity, earliness, and ability to compete with weeds were measured.
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