Onion Plant Size and Timing for Ethephon-induced Inhibition of Bolting1

Abstract Late-winter application of 5000 ppm (2-chloroethyl)phosphonic acid (ethephon) to fall-seeded short- day onions ( Allium cepa L.) retarded leaf growth and inhibited bolting. Bolting inhibition was significantly correlated with bulb diameter at time of treatment. Bolting inhibition was greatest when bulb diameter was 0.9 to 1.6 cm at time of treatment. Bulb diameter in late February or early March was an effective criterion for the effectiveness of ethephon treatment to reduce bolting. When ethephon treatments greatly reduced bolting, the number of harvestable bulbs and yields were increased, but when bolting percentage in control plots was low, ethephon reduced yield by decreasing average bulb weight at harvest.