1. Field of the Invention
This invention relates to devices for protecting the trunks of growing trees, and, more particularly, to such devices that automatically expand as the trunks increase in diameter.
2. Description of the Prior Art
Many woody plants, including several species of fruit trees, especially citrus trees, are much less cold hardy when young than they are after they attain some size. In order to preserve these young plants during short periods of killing cold, which most often occur late at night, it is common practice to insulate the trunk or main stem of the plant. Thus, if the exposed top is killed by exposure to cold, the plant will sprout from the trunk new growth during the next growing season. Protection of the trunk is particularly important where the fruiting wood on fruit trees, or the blooming wood in plants that are raised for their flowers, is of a different variety than is the root stock because a plant that sprouts below the graft or the bud union, if a budded tree, is of no value.
Probably the most commonly used method of protecting the trunks of plants from cold is to pile dirt up around the trunks, often called "banking", to act as an insulator. The disadvantages of this practice are many and include: (1) banking and unbanking with its associated cost must be performed each fall and spring, respectively; (2) wind and rain may remove the bank; (3) rodents may burrow into the bank and nest, damaging the plant, and often using the bark for food; (4) if the plants are not unbanked before the weather becomes warm, the bark may be damaged by "steaming" caused by the hot moist soil packed up against it; (5) roots are often damaged from digging too deeply when removing the bank; (6) in the case of citrus trees, soil placed above the bud union promotes the growth of a fungus disease, known as "foot rot", which destroys the bark and frequently girdles the tree, killing it, and soil is often not completely removed below the bud union when the bank is removed; and (7) tools often strike the trunk, damaging the tender bark.
Work has been carried on over the years, particularly at Texas A & I University, to develop an insulator for young trees with fewer faults. These have included rock wool wrapped in roofing felt; fiberglass held in place with chicken wire; and finally, a polyurethane foam blanket wrapped around the trunk of the tree and secured with metal straps. The rock wool was found to harbor rodents; the fiberglass sagged under heavy rain; and the polyurethane foam tended to deteriorate from exposure to the sun. This work is reported by R. A. Hensz of the Texas A & I University Citrus Center, Westlaco, Tex., in a paper entitled "The Use of Insulating Wraps for Protection of Citrus Trees from Freeze Damage". All three of these materials readily saturate with water from rain or from irrigation. This is a particularly serious drawback in that the thermal insulating properties of these materials, which is in the range of K=0.2 to 0.3 BTU per hour, per .degree.F., per square foot of area, per inch of thickness, when the material is dry becomes more nearly that of water which transmits heat at the rate of K=4.1 BTU per hour, per .degree.F. per square foot per inch of thickness, or about 20 times the rate of heat transfer of the insulating material. Further, it has been reported by Robert Leyden and R. H. Hensz of Texas A & I University in a publication entitled "Effects of Conventional Cold Protection Systems in a Radiational Freeze", that irrigation prior to a freeze results in a warmer orchard during a freeze than one that is dry, because the water in the ground increases its heat capacity. Following this practice results in wet insulation at the time of the freeze. In addition, these wet insulating materials are slow to dry out and they are in intimate contact with the tree, which promotes the growth of undesirable fungus.
Of the insulating wraps tried, only the polyurethane foam has achieved any degree of commercial importance. A sheet of this material is usually wrapped around the tree several times and held in place by one person while a second person straps it with two inelastic metal bands or with nylon string. The urethane foam is easily compressed, and, in applying the metal straps this frequently occurs, reducing the effective thickness of insulation. Also, as the girth of the tree increases, it compresses the insulation if the straps are not manually adjusted, and this reduces the thickness and effectiveness of the wrap as a thermal insulator.