Performance of Fall-sown Onion Cultivars Using Four Seeding Dates

As fall-sown onions are seeded earlier, an increase in premature seedstalk (bolting) incidence is observed for bolting-susceptible cultivars. The mechanism of resistance for bolting-resistant cultivars is not well known. Four fall-sown cultivars (Daybreak, NuMex Mesa, NuMex Sweetpak, Texas Early White), that differed in their bolting susceptibility, were seeded on four separate dates in September, each one week apart, in Las Cruces, N.M. , to observe their growth and performance at each seeding date. Plant height and leaf number were measured monthly throughout the growing season from 10 plants in each plot. Prior to harvest, the number of plants that bolted were counted. When 80% of the plants in a plot were mature, the bulbs were harvested and the maturity date, disease resistance, bulb yield, and percentage of single centers were recorded. In general, earlier seeding dates resulted in larger plants with more leaves than later seeding dates when compared early in the growing season. By harvest time, plants from later seeding dates were comparable in height and had produced more leaves than earlier seeded plants. Plant height of 'NuMex Mesaʼ (bolting resistant) was less than the plant height of bolting-susceptible cultivars. Bolting-resistant and bolting- susceptible cultivars produced similar numbers of leaves throughout the season. The mechanism of bolting resistance for 'NuMex Mesaʼ may be a smaller plant size and/or a greater plant size required for receptivity to bolting-inducing temperatures as compared to bolting-susceptible cultivars. In general, cultivars exhibited less bolting, later maturity dates, and an increase in bulb yield with a delay in seeding. During the months of June and July, New Mexico supplies most summer nonstorage onions (Allium cepa L.) grown in the United States (U.S. Department of Agriculture, 2000). Most onion cultivars grown in New Mexico are short- or intermediate-day in their bulbing daylength response. The fall-seeded crop is planted in late September and early October, overwintered, and harvested in late May and June of the following year. Temperatures are high enough during the winter season that onions can be overwintered without substantial plant losses. Conversely, temperatures during the winter season are low enough to induce premature fl ower- ing or bolting, and yield loss occurs unless bolting resistance is present. Planting date recommendations for fall-seeded cultivars are based on bolting resistance (Corgan et al., 2000). Bolting- susceptible cultivars are seeded in early October to reduce bolt- ing. As cultivars are seeded later in October, plants are smaller in size going into the winter season and these plants are more susceptible to winter injury than earlier-seeded, larger-sized plants (Corgan et al., 2000). However, as cultivars are seeded earlier in the fall season, larger plants are produced before the winter season, less winter injury occurs, but more bolting is observed. Other researchers have observed increases in bolting with earlier seeding dates of fall-sown onions (Allen et al., 1978; Brewster and Salter, 1980; Holdsworth, 1945; Matlob and El-Haber, 1979; Passam et al., 1995; Salter and James, 1975). Determining the proper date for fall seeding to minimize winter injury and bolt- ing, while maximizing yield, can be diffi cult and is cultivar and environment dependent. Bolting-resistant cultivars have reduced bolting incidence and winter injury, increased bulb yield, and allowed for earlier seeding. Onion fl ower induction is caused by low temperatures after a