Understanding Acclimation and Protecting Trees From Freeze

By Davie Kadyampakeni, Chris Oswalt and Muhammad A. Shahid

With the onset of cooler temperatures, citrus trees cease active growth and become quiescent. This continued quiescence at lower temperatures results in a subsequent increase in cold hardiness, known as acclimation.

Citrus trees proceed through many changes during acclimation, including:

• Sugars and amino acids increase.
• Starch levels decrease.
• Tissue moisture decreases.
• Stability and binding of cell water increases.

The above factors combine to increase the ability of citrus tissues to withstand the formation and presence of ice.

Citrus trees acclimated to cold temperatures have survived temperatures as low as 14°F. Acclimation is affected by exposure temperatures, scion cultivar, rootstock cultivar, rootstock/scion combination, tree nutritional status, crop load and water stress. Acclimation is dynamic and will change during the winter in response to warming exposure temperatures with a possible resumption of growth.

Leaf-killing points vary in magnitude in response to the above conditions, but the predominant factor is exposure temperatures. Studies of citrus leaf-killing-point temperatures clearly indicate that citrus trees grown in more northern areas of Florida exhibit greater acclimation than those grown in regions further south. Trees grown in the southern regions of the state are also more susceptible to active growth due to favorable growing conditions during the winter.

Non-acclimated citrus leaves will generally survive to temperatures of 24°F. New spring flush leaves formed in April will rarely survive temperatures of 31°F. By mid-May, these leaves will have similar leaf-killing points to mature leaves. Research studies indicated that citrus leaf-killing points can range from 16°F to 24°F during the winter with a satsuma cultivar reaching 14°F during one year. Field observations indicated that these leaf-killing-point values hold up in a number of freezes.

In the winter months of December–February, you can save time, water and money with the Florida Automated Weather Network (FAWN) freeze alert tool. Download the FAWN app to get the alerts by text message.

The FAWN app offers the following benefits:

• It saves time. As an active and mobile alert system, you can receive important weather-related information while in the field, instead of having to get it in front of your computer.
• It saves water. One of the most critical aspects of what FAWN does is help growers save water. Depending on the farm size, you can save a lot of water. With the mobile platform, the app can provide time-critical information for when to start and stop your water, all the while maximizing protection of your crop from cold air damage.
• It saves money. By helping you turn on/off your water at optimal times, you will save money.
• It protects crops. With water being the most widely used method of protecting crops during a freeze event, FAWN provides real-time feedback on the best actions to protect your crops.

Davie Kadyampakeni is an associate professor at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center in Lake Alfred. Chris Oswalt is a UF/IFAS citrus Extension agent for Polk and Hillsborough counties. Muhammad A. Shahid is an assistant professor at the UF/IFAS North Florida Research and Education Center in Quincy.