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Controlled Cooling of Onion Umbels by Periodic Sprinkling¹

Low onion (Allium cepa L.) seed yields frequently occur without obvious reasons, particularly with hybrid varieties. Solutions to the problem need to be found to insure seed for bulb onion production. Water and temperature stress in the umbel 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 experiment at Kimberly, Idaho on Portneuf silt loam soil (Durixerollic Calciorthids; coarsesilty, mixed, mesic) using commercial practices except that periodic sprinkling 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 required more sprinkling to keep cool. Five min of sprinkling every 20 to 30 min usually provided adequate cooling. A specially-designed 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 sprinkling. Excessive temperatures did not occur during the experiment, so seed yields were similar on sprinkled and unsprinkled plots. while we have shown that periodic sprinkling 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.

Key Words: Evaporative cooling • Sprinkler cooling • Irrigation controllers • Umbel temperatures • Onion seed • Allium cepa L.

² Soil scientist, physical science technician, and soil scientist, respectively. Snake River Cons. Res. Ctr., Kimberly, ID 83341.

Received for publication April 25, 1980.