Effect of Spacing on the Bolting of Onions

THE effect of spacing on the incidence of double and 'bottle-necked' onions has been reported in the literature ; but the effect on bolting is not mentioned. Wilson1 found in his Utah experiments that thinning stimulates the formation of double onions and scallions. He maintains that doubling is a type of vegetative growth and is stimulated by the increased supply of available nitrogen resulting from thin stands. Bremer2 reports that under the climatic conditions of mid-Europe wide spacing increases the incidence of bottle-necked onions. The relation does not apparently occur in Egypt, where wide spacing is customary, presumably owing to a difference in varietal responses. In Bremer's opinion the wider spacing associated with outside rows discourages ripening, since the effects of accelerated carbon assimilation are outweighed by the increased uptake of nitrogen.