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 sail (Durixerollic Calciorthids; coarse-silty,
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 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
