December 2015



Citrus trees can be susceptible to freeze damage when temperatures reach below 32 degrees Fahrenheit for greater then 8 hours continuously.  Fruit damage can be incurred when fruit is exposed for greater then 6 hours.  Keep in mind, sections of a tree or only certain fruit that are more exposed to the elements could be damaged, NOT the whole tree nor all the fruit. The varieties of citrus that can withstand freeze temperatures better are kumquats, grapefruits, oranges, tangerines/tangelos. Lemons and Limes have a softer, more porous wood and their tissues will expand and contract with the freeze temperatures, and can burst inside the branches, creating dead sections, or killing the whole tree. 

December and January are the coolest months of the year in the Salt River Valley, but we can experience the worst freezes into the first two weeks of March, with March 15th usually being the cut-off date. The coolest over night lows normally occur from 30 minutes before sunrise to 30 minutes after sunrise.  It is the duration of the cold, below 28 degrees Fahrenheit, that does the most damage.  On December 1st, the Phoenix sunrise and sunset is 7:14AM and 5:20PM respectively.  Weather conditions that favor the coolest temperatures include clear skies, calm conditions (no breezes), lower dew point temperatures (in the 20's and lower), and air flow out of the north-northwest.  If a light breeze develops over your area during the night, the air temperatures will rise because of the mixing of the air.  However, as day break approaches, the breeze will drop off to calm, or nearly calm, allowing the temperature to drop down to the previous low, or worse, even cooler than recorded earlier in the evening.  The national weather service puts out a new forecast daily at 4 am & 4 pm, with updates as needed.  Check them out. 

Factors that can increase the tree's susceptibility to freeze include the tree's location in the backyard gardener's yard and or citrus orchard acreage. Those trees on the outermost edges, less proximity to a building, block wall, or home can freeze easier.  Younger citrus trees (or newly planted citrus trees) do not have the established root system of a mature tree, so are more sensitive to freezing.  Trees in an irrigated lot or field are generally more resistant to freezing as the trees have deeper roots from the quantity of water received with flood irrigation versus drip irrigation used by backyard gardeners. Fruit on the outer perimeter of the tree's canopy, or on the "colder" sides of the tree can be damaged by freeze temperatures, but the fruit at eye level and lower to the ground will be more protected, and should survive the cold. Any citrus trees under stress caused by, sunburn from the previous summer/summers, trees watered too often, citrus tree trunks beat up by the weed eater each week with grass cutting, or trunks being cooked by rocks under the tree inside the tree well, could be more vulnerable to the cold.

To help minimize frost damage on all citrus trees, fill the well with water the night before the frost.  However, a newly planted citrus tree does not have the established root system of a mature tree to absorb water fast enough to offer sufficient protection. For that reason, we recommend covering your new citrus tree to help keep frost from settling on their defenseless branches and leaves.  Cover your young citrus trees with burlap or old sheets. Do not use plastic or material that does not "breathe". Mature trees can be protected by filling the wells of the trees on the night of the freeze. Lemon and Lime varieties exposed to the elements, need more attention. If younger in age or transplant, let the water hose drip into the well of these varieties during the freeze night, after first filling the well or basin with water, late the evening of the freeze.

Frost Damage - citrus trees may not show freeze damage right away.  A few days later, you may see leaves become blanched, whitish-yellow, even a light beige color from a freeze.  Don't trim off this freeze damaged area until after March 15th. The danger of a freeze occurring is minimal by that date.  A second or third freeze could possibly occur and damaging the tree further. Once the freeze has passed, some damaged wood may recover.  Wait until new growth appears before any pruning of dead wood.  Citrus fruit damaged from a freeze will exhibit a soft or mushy tissue inside, and the fruit will be flat in taste.  Also, the fruit rind may appear translucent after freezing. Again the fruit most apt to freeze, is located on the outer extremities of the canopy. Fruit growing closer to the ground or inside the tree's canopy, may be spared from freeze damage.