But far more important than any of the above phases are the extremes in temperature and more particularly of cold. A plant can not be grown profitably where the temperature, even occasionally, falls below the point where winter-killing results to tree or bud, or where the blossom is injured by frost. Extremes of heat are disastrous usually only when long continued. For each fruit, too, there must be a total amount of heat available to carry it from the setting of the fruit to maturity, in which respect varieties of any fruit may differ materially. Of the injurious effects and of the necessary amounts of heat, however, we know but little.

These general considerations of temperature lead us to their application to the plum and especially to a discussion of the most important of the several factors—hardiness.

Hardiness to cold, a matter of prime importance with all fruits, is especially so with the plum because of the many different species, each with its own capacity for withstanding cold. As the different species are taken from their natural habitat to other regions, there to become acclimatized, and as new forms originate by hybridization, the matter becomes more complicated and more important. Waugh[145] has investigated the hardiness of plums and we publish a table given by him showing the hardiness of representative varieties of the species most generally cultivated. We have taken the liberty of adding a few plums not given in the original table and have also made some changes in the nomenclature of the groups.

Table Showing the Hardiness of Representative Varieties of Plums at Various Places.

h—hardy; hh—half-hardy; t—tender.

OhioIndianaIllinoisKansasColoradoNebraskaW. New YorkVermontOntarioMaineWisconsinIowaMinnesota
Americana Plums:
De Sotohhhhhhhhhhhhh
Hawkeyehhhhhhhhhhhhh
Wolfhhhhhhhhhhhhh
Domestica Plums:
Lombardhhhhhhhhhhhhhh
Reine Claudehhhhhhhhhhhhhht
Bradshawhhthhhhhhhhhhhhhhht
Arctichhhhhhhhhhhhhh
Hortulana Plums:
Moremanhhhhhhhhh?hhht
Golden Beauty?hhhh?hh???hhht
Waylandhhhhh?hh??hhhht
Insititia Plums:
Damsonshhhhhhhhhhhhhh
Munsoniana Plums:
Pottawattamiehhhhhhhhhhthhh
Newmanhhhh??h?tthhtt
Robinsonhhhh??h?ttthht
Nigra Plumshhhhhhhhhhhhh
The Simon Plumhhhhhhhhhhhhhtttt
Triflora Plums:
Abundancehhhhhhhhhhhhhhhht
Burbankhhhhhhhhhhhhhhht
Satsumahhhhhhthhhhhhhtthht
Kelseyt?thhttttttttt

A few general statements in addition to the above table will help to make plain the comparative hardiness of the different groups of cultivated plums. The Nigras may be rated as the hardiest of the plums to be considered though the Americanas are but slightly less hardy. The plums of these species are the hardiest of our tree-fruits and are able to resist nearly as much cold as any other cultivated plant. The Insititias, as represented by the Damsons, at least, come next hardiest after the above species, with varieties of Domestica, as Arctic, Lombard and Voronesh, nearly as hardy. So far as resistance to cold is concerned the Domestica plums as a class are less hardy than the apple, ranking in this respect with the pear. Of the Domesticas the Reine Claude plums are as tender to cold as any though some consider Bradshaw as more tender. Between these last sorts and the hardiest varieties there is a great range in capacity to endure cold, as would be expected with so large a number of varieties originating in widely separated climates. The Triflora plums vary more in hardiness than any other of the cultivated species. Speaking very generally they are less hardy than the Domesticas, the hardiest sorts, Burbank and Abundance, being somewhat hardier than the peach, while the tenderest varieties, of which Kelsey is probably most tender, are distinctly less hardy than the peach. Of the remaining plums, the Hortulana, Munsoniana and Watsoni groups, there are great diversities in opinion as to hardiness. Probably all of the varieties in these last groups are as hardy as the peach with a few sorts in each more hardy than the peach. It is to be expected from the more northern range of the wild prototypes that the Hortulana and Watsoni plums are somewhat hardier than Prunus munsoniana.

The rainfall is of comparatively small concern to plum-growers in America, since, with now and then an exception, in eastern America it is sufficient under proper cultivation, and on the Pacific Coast the crop is largely grown under irrigation. Summarized statements of annual rainfall are of little or no importance since almost all depends upon the distribution of the amount throughout the year and upon the manner in which it falls. Monthly and seasonal “means” of precipitation, as in the case of temperature, may be of considerable importance in determining the desirability of a locality for plums.

Air currents are of local or regional occurrence and though not often the determinant of profitable culture of plums have sometimes been important factors in choosing a location to grow this fruit. The occurrence, direction, moisture condition and temperature are the attributes of air currents usually considered. The failure of many plums to grow in the prairie region of the Mississippi Valley and the Great Plains is no doubt due in some measure to winter winds. The problem of varietal adaptation is more or less complicated in any region by the nature of the air currents.