High density planting systems of European plums - the effect of growth and productivity of three cultivars after nine years

An intensive field planting of 'Edda', 'Opal' and 'Mallard' plum trees ( Prunus domestica L.) all grafted on the semidwarf rootstock St. Julien A was established in 1993 at Ullensvang Research Centre, western Norway at 608 North. Four different single row planting systems ( vertical axis, free spindle, hedgerow and Y-trellis) and three planting densities ( 0.5, 1.0, 1.5 x 4 m for the Y-trellis and hedge trees and 2.0 x 4 m for the central leader trees) were evaluated. This field planting gave a rapid canopy development and crop yields at early tree age. Trunk cross sectional area of the trees increased with increasing tree spacing. 'Opal' was the most vigorous cultivar. During the first four cropping years, the yields/ha were positively correlated with tree density. After nine years, cumulative yield per ha was highest on the Y-trellis and hedge trees with the highest density ( 5000 trees per ha). The two central leaders showed similar yield pattern, but yielded less than the two other systems. Plums produced on these canopy systems showed minor differences in the average fruit weights, and no significant differences were found among the spacings. The soluble solid contents of the plums were in general high. Neither planting density nor system had much impact on the sugar content of these cultivars. 'Opal' had significantly smaller plums and lower contents of soluble solids than the two other cultivars.