Florida Agricultural Experiment Station


that has come under his observation, although the fruit is no
hardier than that of other oranges. Among grapefruit trees, he
considers the Duncan the hardiest that has come under his ob-
servation and the Triumph the most tender.
 Aside from the variety, the resistance to cold will depend
 largely upon the age of the trees. The ability of a citrus trunk
 or branch to resist cold increases directly with the diameter.
 Large branches and trunks, unless in an active state of growth,
 are injured only by intense cold and the top of a tree may be
 killed back by cold before the large branches or trunk are ef-
 fected. Even the twigs, however, may resist a considerable de-
 gree of cold if sufficiently dormant.
 The degree of dormancy is also a very important factor in de-
 termining cold resistance. Hume reports that dormant sweet
 orange trees have been known to stand a temperature of 18
 to 200F. for a short time, with no injury beyond the loss of a few
 leaves. This is probably the limit of their cold resistance. He
 also reports that completely dormant Owari Satsuma orange
 trees on trifoliate orange stock have withstood a temperature of
 150F., with the loss of not more than 25 percent of their foliage
 and no damage to twig growth. Unfortunately, however, citrus
 trees in central and southern Florida rarely become completely
 dormant and the weather conditions are often such as to induce
 more or less growth during the winter. When the trees are in
 a growing condition they may be severely injured or killed to
 the ground even at much higher temperatures than those just
 cited.
 Trees partially defoliated by cold are in great danger of severe
injury from a second cold period, since the thick foliage helps
to insulate the frame-work of the tree from cold. The presence
of foliage is also very beneficial in shading the inner portion of
the tree crown from the injurious effect of the sun after a freeze.
By reason of their additional protection to the trunk, low-headed
trees are preferable to high-headed ones.
 The extent to which citrus trees may be injured by freezing
also depends upon the weather conditions following the cold
spell. It is invariably recognized that freezing is much less in-
jurious when followed by cloudy weather than when followed by
sunshine. This has given rise to the widespread belief that the
rapidity of thawing after freezing determines, to a large extent,
the severity of the injury. However, there is practical unanim-