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 Mesa' (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 Mesa' 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.
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