It is impossible to define in few words the climatic conditions most favorable to the avocado, since the different races do not always succeed under the same conditions. The subject must, therefore, be considered from the standpoint of races.
The West Indian race, which comes from the moist lowlands and seacoasts of tropical America, is more susceptible to frost than the others. Hence, when grown near the northern limit of the subtropical zone, it requires more protection from possible severe frosts than the Guatemalan race, which comes from the highlands of southern Mexico and Guatemala, or the hardy avocados from central and northern Mexico which constitute the Mexican race. Not a few losses have already resulted from attempts to grow West Indian avocados in locations in California subject to occasional severe frosts. In this state, the Guatemalan and Mexican races are the only ones to plant. The same is true of central and northern Florida, where the West Indian race has nearly always succumbed to cold.
In Florida, the region in which avocado culture is at present conducted commercially lies south of Palm Beach on the east coast and south of Tampa Bay on the west. Of the orchards which are now in bearing, the largest are situated close to Miami and Homestead. On the west coast the most important plantings are near Fort Myers. Most of the orchards in Florida are planted to Trapp, a variety of the West Indian race. The planting of hardy Guatemalan kinds will probably extend the commercial culture of this fruit many miles to the northward of the present limits of the zone. In addition, it will make avocado growing safer in all regions by lessening the possibility of frost injury. The Mexican race is known to have fruited as far north in Florida as Gainesville and Waldo.
In California, most of the young orchards, as well as the old seedlings which have fruited for some years, are in the vicinity of Los Angeles, Orange, and Santa Barbara. The coastal belt between Santa Barbara and San Diego, including the foothill region some distance from the coast, has been tested sufficiently to show that planting may proceed with confidence. In the interior valleys comparatively few trees have been planted, and these mainly in recent years. Much less is known, therefore, regarding the adaptability of the avocado in these situations. Old seedlings are to be seen at Visalia, San Luis Obispo, Berkeley, Los Gatos, and Napa, indicating that some varieties may be grown successfully as far north as the Sacramento Valley. Sections of the San Joaquin Valley which have proved suitable for citrus culture, such as the Porter-ville district, should prove safe for the hardier varieties of avocados as well. Experimental plantings in the Imperial and Coachella Valleys have up to the present served only to indicate that the atmosphere of these regions is too dry. The leaves turn brown and fall off, no matter how much water is applied at the root.
Plate II. Four or five tortillas (corn cakes) and a good-sized avocado are considered a good meal by the Guatemala Indians.
As a guide to planters in regions where the avocado has not been tested, it may be said that experience has shown the Guatemalan race to be about as hardy as the lemon. Certain kinds probably are hardier than that, while others are known to be more tender. The Mexican race, in its hardier varieties, withstands a little more frost than the orange. The West Indian race is distinctly more tender than either the Guatemalan or Mexican.
W. J. Krome's experience at Homestead, Florida, leads him to say: "As a general rule West Indian avocados, beyond one year old, will not be damaged by a temperature of 32° unless that temperature holds for a longer period than two or three hours. When four or five years old they will stand 26° or 27° without injury, except to tender growth, but below that temperature there is likely to be considerable damage. At 22° five-year-old Trapp trees were killed back to wood one inch in diameter. At a temperature somewhere between 22° and 24° Guatemalans have, with a few exceptions, shown almost no damage beyond a slight singeing of the leaves."
In the Report of the California Avocado Association for 1917, H. J. Webber publishes the following summary of the effect of different temperatures on avocado trees in California in the cold weather of the winter of 1916-1917:
"30° F. Nothing injured so far as could bo observed.
"29° F. No injury of account; only traces on most tender growth of West Indian and Guatemalan varieties.
"28° F. New foliage scorched on Guatemalan types; West Indian varieties showing considerable damage.
"27° F. Mexican varieties with new tips slightly scorched; Guatemalan with almost all new foliage injured; West Indian badly damaged.
"25° to 26° F. Mexican varieties with new foliage injured but some dormant trees uninjured; all Guatemalan sorts with new foliage badly injured and some old foliage scorched.
"24° F. Some dormant Mexicans uninjured; Guatemalan varieties badly injured, small limbs frozen back.
"21° F. All Guatemalan types killed to bud; a few of the hardiest Mexicans, such as Knowles and San Sebastian, with young leaves only injured."
The observations reported to Webber showed that young trees were injured at higher temperatures than older ones, when the variety was the same in both cases. It was also observed that trees in rapid growth were more severely injured than those which were in semi-dormant condition. Krome of Florida reports an opposite state of affairs. He says: "At the time of the January 1918 freeze, Trapp trees which had borne heavy crops and were in a hard, completely dormant state suffered a great deal more injury than trees which, owing to light crops the preceding season, were in full growth." Possibly the trees were weakened by over-production of fruit, and thus more susceptible to frost-injury. The subject demands further investigation.