The effect of plant spacing on the yield and bolting of two cultivars of overwintered bulb onions

SummaryThe total and ware bulb yields of both an early-and a late-maturing cultivar of overwintered onion increased linearly with increasing plant densities between 43 and 129 plants m-2. A rectangularity of 1.0 (square arrangement) gave yields 15 to 20% higher than a rectangularity of 8.0 over the range of densities studied. Sowings made in mid-or late-August gave the highest bulb yields whereas early August sowings resulted in a high proportion of plants bolting (flowering) rather than bulbing. The cultivars differed markedly in their susceptibility to bolting but the proportion of plants bolting was not affected by spacing. An equation with two fitted constants relating bulb yield to plant density and the proportion of plants bolting accounted for 82% of the variation. The variability of bulb weight was significantly greater in the later maturing cultivar which was open pollinated than in the earlier cultivar which was a hybrid.