Millions of boxes of fruit and thousands of acres of citrus trees have been lost in freezes and frosts. More than nearly any other factor, freezes have caused some of the most dramatic changes in fruit supply, availability, and price. Thus, any method that provides some cold protection can be of major importance to citrus growers.
Undertree Microsprinkler Irrigation
Low volume undertree microsprinkler irrigation is an alternative method for partial frost protection and can be more affordable than other methods. Microsprinklers have proven effective during several freeze nights in central Florida tests. In addition to frost protection, microsprinklers can provide effective year-round irrigation.
What is a Microsprinkler?
Microsprinklers, or spray jets, are small, low volume irrigation sprinklers that discharge 5 to 50 gallons/hour. In citrus groves, the most commonly used spray jets discharge from 5 to 25 gallons/hour and cover a diameter of 5 to 21 feet. Usually 1 or 2 microsprinklers per tree are installed at the ground level or on short risers. Unlike overhead sprinklers, microsprinklers do not commonly wet leaves and branches above a height of about 3 feet and do not usually cause serious limb damage.
How Irrigation Works for Cold Protection
When used properly, water can provide partial or complete protection for a number of crops. Improper use of water can increase cooling or ice loading and cause greater damage than if no water were used at all. It is important to know what principals are involved in cold protection.
Effectiveness of Microsprinklers
Microsprinkler irrigation is more effective for cold protection when high volumes of water are used. A system that delivers the maximum amount of water per acre and is practical or affordable is best for frost protection. If there are 200 trees per acre, then one 10 gall/hour jet is adequate. Rates below this level will provide some protection but not as much as higher rates. Microsprinklers can provide some protection to leaves and wood, particularly on the lower and inner part of the canopy. Since fruit is more sensitive to cold, microsprinklers do not protect the fruit. At higher volumes, spray jets will help protect fruit a litter better than no irrigation. It is better for tree protection rather than fruit protection. There is a limit the effectiveness of microsprinklers. Factors such as tree health, rootstock, and cold acclimation affect tree survival. All depending on the amount of water applied.
Young Tree Protection
In young trees, the microsprinkler protects the lower trunk by the direct application of water. When water freezes, it releases heat. If the application rate is high enough, the freezing water will maintain the trunk temperature at 32F.
Insulating tree wraps placed around the trunks of young trees slow the rate of temperature fall. Tree wraps in combination with microsprinkler irrigation provide even better cold protection insurance.
Operation on Freezing Nights
For cold protection, a microsprinkler system must be designed to provide water to the entire block or grove all at once. A system designed for freeze protection is initially more expensive to install because it handles a larger volume of water. Once the microsprinklers are turned on, the system must keep running all night. If the irrigation stops due to failure or any other reason, the trunk can rapidly drop below the air temperature by evaporative cooling. Even if the system stops for a short amount of time, damage can happen. The system can be turned off in the morning when the temperatures warm, ice does not have to be melted. If the air temperature has risen to 40 or 45F, the system can be turned off safely.
Operating on Windy Nights
Like other methods used for citrus protection, microsprinklers are less effective during windy or advective freeze nights. They provide little to no protection to mature trees. A higher application rate can protect trees to lower temperatures. One way to increase the application rate is to reduce the spray pattern size. With a higher application rate, the protection level is better.
If used and designed properly, microsprinklers can provide cold protection to citrus trees depending upon the severity of the weather conditions.
If you would like to know more about microsprinklers and cold protection for citrus, visit edis.ifas.ufl.edu or contact your local extension office.