Lemon trees are highly susceptible to frost. The formation of ice crystals in their cells deprives the lemon tree's tissue of water. Signs of a frost-damaged lemon tree include dark brown or black leaves. Frost damage to lemon trees can occur when the temperature drops to 29 degrees F for 30 minutes or longer. If you live in a region that sometimes experiences frosts, protect your lemon trees using some simple methods.
Wrap the trunk of the lemon tree from the ground level up to the lowest branches with cardboard, fiberglass insulation or corn husks. Wrap an outer cover on top of the insulating material with plastic during wet weather.
Hang Christmas lights or 100-watt lamps in the lemon tree. The lights produce enough heat to minimize the frost damage to the tree.
Clean the area around the tree, with as little ground cover as possible. Remove any mulch around the tree. Heat will radiate more from moist, bare soil than soil covered with mulch or other ground cover.
Moisten the soil around the lemon tree. Moist soil retains more heat than dry soil. Run a sprinkler system slowly through the night or water the soil lightly around the tree to keep the soil moistened.