The United States is the third leading worldwide producer of citrus, following China and Brazil, at an average of 10 million tons annually. This represents a $3 billion annual industry whose greatest threat is not economics, but weather. Citrus trees are prone to an issue called "frost burn," in which the fruit hanging on the trees becomes dehydrated in low temperatures and useless for sale or juicing. Naturally, producing an effective insulation for citrus trees against sudden frosts is a high priority for citrus growers.
Insulation for citrus trees consists of long strips of foam-backed, water resistant urethane which are wrapped around the limbs of the tree and tied in place. Combined with pockets in the foam backing into which are placed packets of exothermic chemical reactants using the same technology as heating pads, this can insulate and maintain the internal temperature of the tree for up to three days. After which, the chemical reactants have been spent and must be replaced.
Due to cost restrictions, many citrus growers still do not use insulation specifically designed for their trees. Enough heating packets and insulation to cover a mature orange tree, for example, can run over $1,000. For that reason, other makeshift materials are used. Polyethylene wrapping can protect trees from wind shear, as can standard housing insulation.
While keeping the tree and its fruit warm is the primary concern, citrus growers also take measures to warm the ground of their citrus groves as well. Sudden changes in temperature created by a frost can have deleterious effects on the tree's roots, which in turn affect the overall health of the tree. Water to the ground is an effective means of insulation against root damage. Water creates a hard layer of ice within the sod and prevents the cold temperatures from actually progressing down to the tree's roots.
Many people believe that frost damage is the result of ice, snow, and generally cold temperatures. While this was true at one time, thanks to selective breeding, the majority of citrus trees grown commercially are proof against ice and temperatures as low as 20 degrees F. Citrus trees are still susceptible to frost burn from high winds at extremely low temperatures. Cold winds leach moisture from citrus, making fruit all but useless when it comes to selling them commercially. Citrus growers who're caught flat-footed by a sudden frost and lack the time to insulate their crops through other methods will turn on their sprinkler systems. The water freezes, creating a layer of ice which protects the fruit from the wind and only marginally reduces crop yield.
Solid insulation cannot be placed around citrus trees year round. The bark and fruit are prone to harboring a number of fungal diseases, among which citrus canker is the greatest threat. Should insulation be left around citrus trees for too long, water can pool and create a prime breeding ground for these diseases.