Onions are grown throughout the United States using multiple production systems in a range of climates. Onions are typically divided into short-day, intermediate-day, and long-day types; with bulbing occurring in response to daylengths greater than or equal to 11–12 h, 13–14 h, and greater than 14 h, respectively (Rubatzky and Yamaguchi, 1997). Daylength during the winter and early spring months in southern regions of the United States allows short-day types to be grown for overwinter production in areas such as the Vidalia region of Georgia (Boyhan and Torrance, 2002). Long- and intermediate-day types are used for spring and summer production in more northern latitudes of the United States.
Onion production in states such as Kentucky and Tennessee has more in common with northern parts of the United States than with southern regions. Although Kentucky has a relatively mild winter climate, short periods of killing temperatures for onion occur. Many onion cultivars grown in the United States are killed when exposed to temperatures below −10 °C (Corgan and Kedar, 1990). Therefore, onions are typically transplanted in April and harvested in July and August in Kentucky (Coolong et al., 2011). However, on-farm observations by the authors suggest that onions may be overwintered in Kentucky. Intermediate- and long-day types are often grown as they are appropriate for the daylengths experienced in this region during late spring and early summer (Coolong et al., 2011). Short-day onions are not currently recommended for spring plantings in Kentucky, because of premature bulb initiation. However, growing plants during winter months when daylengths are shorter may allow for successful production.
Although onion bulbing is primarily a photoperiodic response, it can also be influenced by environmental factors such as temperature, light intensity, nitrogen (N) fertility, and irrigation (Brewster, 1990). Once exposed to a minimum photoperiod, bulbing increases with older or larger plants (Sobeih and Wright, 1986). Therefore, fall-planted onions mature earlier than those which are spring planted, allowing harvests in cooler temperatures. Bacterial pathogens of onion such as sour skin (Burkholderia cepacia) and center rot (Pantoea ananatis), which thrive in hot weather, may also be avoided (Schwartz and Mohan, 1995). In addition, earlier harvests may provide marketing opportunities for growers. Furthermore, onions have been shown to have a milder flavor potential when grown under cooler temperatures (Coolong and Randle, 2003a).
Despite positive aspects of overwinter onion production for growers in Kentucky, limitations exist. Onion cultivars can vary significantly in their ability to withstand freezing temperatures (Corgan and Kedar, 1990; Gill and Waister, 1983). Short-, intermediate-, and long-day onion cultivars have been developed for production in distinct environments (Hanelt, 1990) and may vary in their survival ability and yield potential when overwintered in Kentucky.
Observations suggest the use of plastic mulches and rowcovers may enhance the winter survival of field-grown onions in Kentucky. Although the use of black-polyethylene mulch has been reported to increase yield in overwintered onions in northern Florida (Hochmuth et al., 1990), elevated temperatures, which are detrimental to growth, were reported for onions grown using black-plastic mulches in Georgia (Diaz-Perez et al., 2004). Walters (2008) reported improved overwinter production of garlic (Allium sativum) in southern Illinois when grown on black-plastic mulch compared with plants grown with straw mulch on bare soil. Mansour and Hemphill (1987) reported a yield improvement for spring-planted green onions grown in Oregon under a spunbonded polyester rowcover compared with those grown on bare ground. Onions were successfully overwintered in New Hampshire using a combination of 6-mil plastic and rowcover (1.25 oz/yard2) in a low-tunnel structure (Sideman and Bryant, 2013). Increase in air temperatures of more than 20 °F were reported for the low-tunnel structure in that trial.
The objective of this research was to optimize overwinter onion production for growers in Kentucky and similar growing regions. Plastic low tunnels, although effective in New Hampshire, were not used in this study as there were concerns about excessive temperatures occurring under the plastic during sunny days in the winter. Several onion cultivars, representing short-, intermediate-, and long-day types were grown.
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