PLUMS IN RELATION" TO PRUNING 35 
the possibility of developing and maintaining the desired form of tree by 
consistent pruning. It is evident from the slow growth of branches and the 
dying out of shaded ones, as well as from the small-sized fruit produced 
by compact trees, that a moderate thinning out is necessary to maintain a 
thrifty, moderately open tree. 
WASHINGTON 
Tree upright spreading, vigorous, and produces numerous twigs and spurs. 
The spurs which form thickly on the vigorous 1-year-old wood are usually short 
lived if they fruit well, leaving much of the older wood barren if the tree is at 
all compact and prolific. The manner of growth is very readily influenced by 
cultural treatment and environment. If the soil is fertile and irrigated, regular 
thinning out and light heading back result in the growth of numerous vigorous 
twigs, but severe heading back results in the growth of many large vigorous 
shoots. The leaves on these shoots and in other parts of the tree are large 
and form dense shade in the interior of the tree. This is accompanied by loss 
of much fruiting wood, for if the branches become barren they usually remain 
so. If the trees lack for moisture the twigs are short and soon lose their 
fruit spurs. 
New shoots spring from near the point of heading back or at points where 
branches have been removed along the portion of the branch which is only a 
few years old. They are not common in other parts of the tree. Twigs and 
spurlike twigs form along the 1-year shoots and twigs, and if the tree is thinned 
out and moderately headed back in pruning many of the short spurlike twigs 
grow to thrifty twigs the following summer (pi. 38, A). If little or no prun- 
ing is done, few shoots will be produced and the terminals of shoots and 
twigs will be very short, especially on older trees, often of spur length only. 
Fruit is borne mostly on 1 and 2 year spurs and at nodes on 1-year-old twigs. 
Fruiting nodes seldom produce leaves, and many of the bearing spurs are 
leafless and die after the fruit ripens, especially on twigs which have made 
considerable terminal growth the previous year. If the tree is kept open and 
only moderately headed back at pruning time, many fruiting spurs make 
moderate growth in length. On thrifty young trees and vigorous shoots of 
older ones the crop on 1-year-old spurs is often light. 
In the cool coastal region of California, trees which have been severely headed 
back each year grow luxuriantly. In their vigorous growth they contrast with 
trees of Grand Duke and Diamond, also of the domestica group (pi. 3, B and 
C; pi. 20, B). 
YELLOW EGG 
Tree upright spreading, open, moderately vigorous, sensitive to poor soil and 
drought, and under poor growing conditions the branches soon become barren. 
Vigorous shoots spring from near the end of cut-back branches. Under good 
growing conditions these increase in length from year to year by the out- 
growth of the terminal bud, and they send out strong lateral branches. Where 
lightly pruned and not irrigated, the new growth on both large and small 
branches is very short. On the lighter soils there is less branching than else- 
where, short-lived spurs are formed in place of twigs and branches, the new 
growth is slender, and few fruiting twigs are retained. Under conditions 
favorable to wood growth only a few spurs are formed, as the lateral growth 
develops into twigs instead of spurs. On unpruned or lightly pruned trees or 
those making a slow growth, much of the lateral growth from the 1-year-old 
shoots is short and soon disappears, leaving much of the branch barren 
(pi. 38, B). The slender 1-year shoots produce few lateral spurs or twigs, so 
that the entire twig is barren with the exception of a few nodes near the tip. 
There is considerable difference in length and vigor of fruiting twigs. They 
are all slender, and the shorter ones often die after bearing one or two crops 
of fruit. This tends to keep the tree open and permits the larger twigs to 
continue their length growth. These slender twigs continue to lengthen and 
become drooping, and if the growth is encouraged by heading back they pro- 
duce lateral twigs and spurs which bear well (pi. 38, C). Twigs in the upper 
and outer part of thriftily growing trees which increase their length only a 
few inches each year soon die; whereas in the lower portion of open trees 
twigs are often retained for a considerable time, although making a short 
annual growth. If twigs or spurs are not formed, are broken off, or die, the 
branch remains barren, as new growth rarely appears on old wood except 
