Bulletin 229, Diseases of Citrus in Florida


 FACTORS INVOLVED IN OCCURRENCE
 While the cause of cold injury is obvious, the conditions that
contribute to its occurrence and severity are not generally under-
stood. The extent to which citrus trees are injured by freezing
depends upon a great many factors and conditions, among which
may be mentioned the degree of cold and duration of the same,
local conditions such as elevation and natural protection afforded
by water or forests, wind velocity, air flow and other atmospheric
conditions, degree of dormancy, variety, rootstock, soil and soil
moisture conditions, vigor of the tree with regard to fertiliza-
tion and freedom from diseases and insects, and the weather con-
ditions immediately preceding and following the cold spell. Space
will allow only a brief discussion of the more important of these
factors.
 One of the most important factors in cold protection is air
drainage. To have good air drainage, we need slope, elevation,
and an outlet for the cold air. Low areas or valleys without
outlets or areas of a valley below the level of the outlet should
be avoided for grove sites unless provision is made for heating
or unless there is water protection. In the case of open slopes
a southerly exposure is preferable to a northerly one. High
cover crops left in the grove may also be a factor in cold injury
to the lower parts of trees, since they retard the movement of
cold air and tend to hold it around the trees.
 Combined with the factor of air drainage is the question
 of water protection, which is a very important factor in minimiz-
 ing cold injury. The effectiveness of water protection will de-
 pend upon the size, depth, and temperature of the body of water,
 combined with the amount and direction of slope to the water.
 On the larger lakes it is desirable to have groves located on the
 south side of the lake, where the wind may reach them only
 after having traversed several miles of water surface. Timber
 protection is also of importance, especially in cold spells accom-
 panied by considerable wind or when firing is undertaken.
 The different kinds of citrus trees commonly in cultivation in
 Florida show a considerable range of resistance to cold. Hume
 arranges them in order of decreasing hardiness as follows: (1)
 trifoliate orange, (2) Satsuma orange, kumquat, and calamondin,
 (3) sour orange, (4) tangerine, (5) sweet orange, (6) grape-
 fruit, (7) lemon, (8) lime, citron, and shaddock. Among sweet
 oranges, Hume states that the Lue Gim Gong is the hardiest tree