2 BULLETIN 1477, IT. S. DEPARTMENT OF AGRICULTURE 
knowledge of the behavior of his different varieties if the best results 
are to be obtained. 
Plums, as well as other fruits in this section, are quickly affected 
by lack of water for irrigation, variation in depth and fertility of 
soil, and extremes in summer temperatures; and various combina- 
tions of these conditions are found in the plum-growing localities. 
The great diversity found between the performance of different 
commercially grown varieties may also be explained in part by the 
fact that among them are represented several species including 
Prtcnus domestica, P. salicina (trifiora), P. simoni, and probably 
native American types either by true forms or hybrids. 
When planting a western plum orchard for the purpose of pro- 
ducing fruit for fresh shipment, varieties are chosen which will 
ripen their fruit in such order as to make an uninterrupted and 
extended harvest. As the ripening period of a variety continues for 
a few days only, depending upon the locality and the variety, several 
varieties, usually from 1 to 24 or more, are needed to extend the 
harvest over a satisfactory period. The orchard of only a few acres 
often has as many varieties as the large one, if plums are the main 
crop or are grown exclusively. Where the fruit is grown for drying 
to prunes, only a small number of varieties is chosen, often only one 
or two; but here, as with the fruit for fresh shipment, the large- 
sized fruit is usually the most profitable and the variations in habits 
of growth of individual varieties are striking, and a real knowledge 
of them is needed by the grower and primer just as with fruit for 
fresh shipment. 
If pollination is provided for, there need be little fear in the Pacific 
States as to the setting of a large crop of plums. Unless frost 
injures the blossoms or small fruit soon after it sets, the crop will 
be excessive and an expensive fruit-thinning operation necessary. 
This is regardless of any of the common pruning practices. Though 
the pruning is often relied upon to remove some of the bearing wood 
in order to lighten the crop and to distribute the load over the tree, 
a variation in the pruning method to encourage fruitfulness is sel- 
dom, if ever, necessary when bearing plum trees are pruned. 
The general pruning practice which has been evolved to meet 
the various conditions consists largely in the cutting back of the 
previous season's growth and some thinning out of branches. The 
quantity of wood removed is determined very largely by local con- 
ditions and the use which is to be made of the fruit. When water 
for irrigation is scarce or soils poor, severe and regular heading 
back of the previous year's growth is needed to induce the fruit to 
grow to sufficient size to be profitable for fresh shipment (pi. 1, 
A and B). Where soils are fertile and well irrigated, regular head- 
ing back and thinning out of fruiting branches is done for the 
purpose of preventing too tall a growth of the tree, to admit light, 
to lighten the crop on the tree, and to insure large fruit (pi. 1, 
C and D). By following these general rules in pruning, the habits 
of growth and production peculiar to the variety, although often 
very pronounced and important, are liable to be overlooked or con- 
sidered in only a general way. 
In the preparation of the following discussion of the principal 
commercial varieties of plums grown in the Pacific States, it has 
