Using Citrus Leaf Freezing Information to Determine Critical Temperature

Chris Oswalt, Polk County Extension

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 termed acclimation. Citrus trees proceed through many changes during acclimation. These changes include: increases in sugars and amino acids with decreases in starch levels within plant tissues. Tissue moisture decreases along with increases in the stability and binding of cell water. These factors combine to increase the ability of citrus tissues to withstand the formation and presence of ice.

Citrus trees acclimated to cold temperatures have survive 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, although the predominate factor, would be exposure temperatures. Studies of citrus leaf killing point temperatures clearly indicate that citrus trees grown in more northern growing areas acquire greater acclimation than trees grown in growing regions further south. Trees grown in 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.

Citrus Leaf Killing Temperatures for Florida Citrus

Location Variety/Rootstock 11/14/2016 11/21/2016 11/28/2016 12/05/2016 12/12/2016 12/19/2016
Balm Valencia/Carrizo 22 22 22 21 22 21
Ft. Meade Valencia/Cleo 21 21 21 20 21 21
Frostproof Hamlin/Swingle 21 23 23 22 22 22
Green Swamp Hamlin/Swingle 21 21 21 21 22 21
Location Variety/Rootstock 12/26/2016 01/02/2017 01/09/2017 01/16/2017 01/23/2017 01/30/2017
Balm Valencia/Carrizo 21 21 22 22.5 24.5 24
Ft. Meade Valencia/Cleo 22 22 23 22 24.5 22.5
Frostproof Hamlin/Swingle 22 23 23 23 24 23
Green Swamp Hamlin/Swingle 21 20 21 21 23 21.5
Location Variety/Rootstock 02/06/2017 02/13/2017 02/20/2017 02/27/2017 03/06/2017 03/13/2017
Balm Valencia/Carrizo 25 24 24 24.5 25 25
Ft. Meade Valencia/Cleo 24 23 23.5 24 25 25
Frostproof Hamlin/Swingle 25 24 24 24.5 25 25
Green Swamp Hamlin/Swingle 24 23 24 25 25 25

Thanks to the Southwest Florida Water Management District for funding this research.

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