Controlled Cooling of Onion Umbels by Periodic Sprinkling¹

Abstract

Low onion (Miura cepa L.) seed yields uoccur without obvious reasons izy=cularly with hy. varieties. Solutions to the need to be found to insure seed for bul re Water and ttemperaturestress in the um b el may be factors during hot, dry, sunny weather. We investigated the relationship of umbel temperature to ambient and umbel conditions and the effects of periodic wetting of the umbels. Onions were grown in a field n'iment at Kimberly, Idaho on Panne& silt loam sail (Durixerollic Calciorthids; coarsesihy, mixed, metric) using commercial practices except that periodicsling was provided with a rotating-head sprinkler irrigation system when temperatures exceeded certain levels. Sprinkling for five min reduced floret temperatures as much as 15 C below ambient. The florets remained cool for 20 to 40 min, depending on umbel maturity, wind speed, and air temperature and humidity. Nearly mature umbels were hotter and uired more sprinkling to keep cool. Five min of sprinkl ing every 20 to 30 min usually provided adequate cooling. A speciallydesigned temperature sensor was found to successfully simulate the behavior of an onion floret during wetting and drying, permitting direct control of the Initiation, duration, and frequency. of winkl. Excessive tem.ratures did not occur d the ex t, so seed yields were similar on and unsyrinkled plots. While we have shown that periodic sprinkhn will provide umbel cooling and minimize undesirable effects on pollination, similar studies need to be conducted in hotter, major seed producing areas to establish economic benefits.