Now that the cranberry fruit has sized up and has some color, it is more vulnerable to scald – the physiological disorder caused by overheating of the fruit. Hybrids in particular are more vulnerable to this type of damage. It is difficult to accurately predict scald since cloud cover, wind speed, ambient temperature, and solar radiation, and fruit color are all interacting factors.
From the informative scald fact sheet created by Peter Oudemans and Giverson Mupambi - https://njaes.rutgers.edu/fs1363/ :
Take home points: Critical factors for risk of scald:
- Ambient temperature above 90°F
- Low relative humidity
- Clear skies with minimal cloud cover
- High solar radiation
- Internal berry temperatures exceeding 108°F
“Environmental factors such as high ambient temperature, intense solar radiation, and low relative humidity contribute to fruit overheating in cranberry. New hybrid varieties tend to be more vulnerable since increased yields result in more berries being crowded into the upper canopy, where they are exposed to the sun. Fruit surface temperature in the upper canopy can exceed ambient temperature by more than 30°F during periods of high solar radiation.”
While the cranberry plant can cool itself down by evapotranspiration (pulling water up through the roots and releasing it from leaf stomata), the fruit does have stomata to cool itself the same way as the rest of the plant. Berry damage is thought to occur when fruit reaches 108°F for a period of time.
Brief intermittent sprinkling can help to physically cool the fruit by evaporative cooling (water evaporating on the surface of the fruit) – but it is always a balance between running long enough to take advantage of the evaporative cooling but not so long that your increase the moisture in the soil and increase your risk for fruit rot.
When weather conditions favor scald (and the risk of fruit reaching 108°F), the decision to run the irrigation should be based on a temperature threshold. There is no strict correlation between canopy temperature and fruit temperature! The actual berry temp will depend on more than the air temp (the size, color, location in the canopy, etc.). In terms of canopy temperature, some recommendations have been to trigger cooling irrigation when canopy temps are between 98 °F and 106 °F. In New Jersey, some growers are tracking internal fruit temps for a more accurate threshold. You can track the internal temperature of the top, reddest berries using an internal temperature probe with their weather stations and changing the probe to berries every 72 hours – or even just using a meat thermometer to periodically check the temp of a few berries. When berry temps reach 108°F for more than 10 minutes, they will initiate cooling irrigation.
If the heads on your sprinkler system complete their rotation quickly, that is advantageous as you get the evaporative cooling effect in a short time frame. The irrigation system can shut down sooner and minimize the increase in soil moisture. Run for 10 minutes.
With older systems and/or those with slow rotating heads, the opposite is true, and the balance is harder to maintain. Run for about 20 minutes.