The effect of tillage on a perennial weed varies depending on the phenology of the species relative to the timing of tillage. In general, a perennial weed in a seasonal climate is most vulnerable to damage shortly after reserves in the per-ennating organs have been converted to new shoots.
In a series of studies in southern Sweden, Hákansson & Wallgren (Hákansson, 1963, 1967, 1969a; Hákansson & Wallgren, 1972b, 1976) found that although Allium vineale, Sonchus arvensis, and Agropyron (Elytrigia) repens perennate by different means, all were most susceptible to damage by burial at the point when their perennating organs reached minimum mass. For A. repens this occurred when three to four leaves had formed on the new shoots - just prior to initiation of tillers and new rhizomes (Hákansson 1967; Hákansson & Wallgren 1976). In Sweden the three-to-four leaf stage was reached in late May for undisturbed plants, but in warmer climates overwintering leaves may resupply rhizomes before the new growth reaches this size. Majek, Erickson & Duke (1984) similarly found that A. repens in New York was most susceptible to tillage in early May, just before formation of new rhizomes.
For Allium vineale, maximum susceptibility to tillage due to depletion of stored reserves occurred primarily in the spring following autumn germination of bulbs and was difficult to induce by cultural practices. In contrast, provided Agropyron repens and S. arvensis were metabolically active, fragmentation induced the plants to divert resources from storage organs to shoots. A second tillage operation could then kill the fragments at their most susceptible stage.
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