Because citrus trees are subtropical plants, all varieties are subject to damage when temperatures drop below 32 degrees F. But if you purchase a grafted citrus tree, it will be a bit more tolerant of frost than trees you begin from seed, according to Phoenix (Arizona) Tropicals. Lemon and lime trees are most likely to suffer frost damage, while kumquats are the hardiest citrus. You can protect your citrus from cold weather by planting it in a protected area and by providing cover, lights or a spray of water.
Purchase a grafted citrus tree of a frost-hardy variety. For example, Satsuma mandarins are more tolerant of hard, cold temperatures than other types of citrus, according to the University of California; ripe fruit can withstand 25 degrees. Kumquats are also frost tolerant.
Plant your young tree in an area that receives some protection from cold weather. Avoid planting your tree in open areas where there is no protection against wind. Do not plant citrus in low areas. Favor locations near buildings, fences or walls and areas with a southwest exposure.
Keep your tree strong and healthy by giving it sufficient water and fertilizer; good cultural practices reduce frost's impact. Water your tree deeply once a week throughout winter if rains are slight. Fertilize your tree four times each year with a special plant food developed for citrus; these products normally have an N-P-K ratio of 8-8-8. Limit your application of fertilizer to spring and summer.
Spray your citrus tree with a garden sprinkler all night long when temperatures are forecast to drop below freezing--moving water cannot freeze, so this will protect your tree.
String Christmas lights over your tree and turn them on to provide warmth on cold nights. You can also hang a 100-watt incandescent light bulb from the upper branches of your tree to generate warmth that will protect it.
Use an anti-transpirant spray product such as "Moisturin" or "Cloud Cover"--these products prevent your tree from the dehydration that frost causes.