Temperature Thresholds to Freeze Damage in Cranberry

Sprinkler irrigation is required for frost protection of cranberry and is arguably the most important cultural practice used in production of the crop.  Growers struggle however with questions on what temperatures to begin sprinkler frost protection, how much water is needed, and at what stages of development are the plants most susceptible to frost.  If applied too early or too often, irrigation water may run out.  When missed or applied too late, crop damage will result.  A study was initiated in a commercial bed of ‘Stevens’ cranberry located in Langlois, OR.  The objectives are to identify temperature thresholds to freeze damage and to determine the amount of sprinkler water required to protect the plants from frost.  Temperature-control units were developed to expose the cranberry vines to freezing temperatures at various stages of plant development under field conditions, and a grid of thermocouples was installed to monitor temperature within the bed.  Ambient temperatures within the bed were similar inside and on top of the crop canopy (approximately 7.5 and 15 cm above the soil surface) but as much as to 2.4 °C warmer at 15 and 30 cm above the canopy surface.  Ambient temperature on top of the canopy was also as much as 1.1 °C warmer than cranberry leaf temperature at the same location.  Temperatures thus varied with both height and location in the bed, illustrating the importance of thermocouple position when setting up a frost alarm. The temperature data will be used next to evaluate frost prediction models for cranberry.  The temperature-control units were tested at –12 °C on 9 June 2011.  Bed temperature was maintained near the set temperature but fluctuated as the freezer motor cycled on and off.  Plants were also frosted following the test and died within a few days.  The units were modified and tested again at –5 °C on 15 Nov. 2011. Chamber temperature varied < 1.5 °C during the second test and the plants all survived following the test.  The units were used in Spring 2012 to expose different patches of plants within the bed to a range of temperatures above and below ambient.  Tests were run during frost events whenever possible.  Evaluation of the plants following each test is ongoing and will be completed this fall.