Frosts and freezes are a threat to citrus trees (Citrus spp.). Frost occurs when the weather is calm and clear and often when the air is dry. After what might have been a warm day, temperatures fall below 20 degrees Fahrenheit for a few hours overnight. Many citrus trees can survive a frost with good protection. Freezes are defined as constantly freezing temperatures day and night, and they can last for days. Citrus trees rarely survive freezes.
Citrus Tree Hardiness
Some citrus trees are more frost-tender than others.
- Kumquats (Fortunella spp.) are citrus relatives that are less sensitive to frost. Hardy in U.S. Department of Agriculture plant hardiness zones 8 through 10, kumquat tolerates temperatures as low as 10 degrees Fahrenheit.
- Mandarins, satsumas and tangerines (Citrus reticulata) are hardy in USDA zones 8B through 11. These trees suffer more damage with a sudden temperature fall than constantly freezing temperatures. The tangerine variety 'Changsha' tolerates 15 degrees Fahrenheit if the temperature falls slowly over several weeks.
- Sweet oranges (Citrus sinensis) have a medium tolerance for frost and need protection when temperatures fall below 24 degrees. These trees are hardy in USDA zones 9 through 11.
- Grapefruits (Citrus X paradisi) are hardy in USDA zones 9 through 11 and also have a medium frost tolerance. Mature grapefruit trees aren't affected by brief exposure to temperatures in the the low to mid-20s.
- Lemons (Citrus limon) are among the most frost-tender citrus trees. Hardy in USDA zones 9 through 11, lemons can suffer damage after 30 minutes at 29 degrees. Meyer lemons (Citrus meyeri), which are lemon hybrids, are less frost-sensitive. These trees are hardy in USDA zones 8B through 10 .
Growing citrus trees in the right spots and preparing them for winter gives long-term protection against frost. Evenly moist soil also gives protection. Grow citrus trees in sunny spots sheltered from winds and next to brick walls, if possible. Masonry walls absorb heat from the sun and radiate it at night, raising the temperature in the immediate area a few degrees. Sloping ground also offers frost protection because cold air flows away downhill. Do not plant the citrus tree in the lowest section of the yard where cold air accumulates as the tree is more susceptible to frost or freeze damage.
Fertilize citrus trees with a low-nitrogen, high-potassium or phosphorus fertilizer in late summer, to help toughen the trees up for winter. Apply a ready-to-use, granular 9-18-9 fertilizer at a rate of 1 tablespoon per 1 square foot beneath the trees' canopies in early fall. Gently rake the granules into the soil surface and water them into the soil.
Water citrus trees when the ground is moist to a depth of 1 to 2 inches. Moist soil loses less heat at night than dry soil.
Burlap sacks, blankets, quilts and other large cloth sheets can protect small citrus trees from frosts. Throw the sheets over the trees in the late afternoon or early evening before the temperature falls to freezing, and remove them in the morning when the temperature rises again.
Electric lighting in the branches of medium and large citrus trees provides additional heat and helps keep frost at bay. String holiday lights through citrus tree branches, or fix incandescent light bulbs in the canopies, and turn them on when night falls. One 100-watt incandescent bulb protects one tree. Covering the trees improves the protection.
Wrapping citrus tree trunks with insulating materials in fall offers protection over winter. Fiberglass, palm fronds, cornstalks and cardboard help protect citrus trees. Wrap the trunks from the soil surface up into the main branches, and tie twine around the insulating material to hold it in place. Is a spell of wet weather is forecast, cover the material with plastic to keep it dry.