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Hakansson (1969&)

Notes:

* A strategy that minimizes the number of cultivations often requires longer to achieve complete eradication. 6 Sample size for means shown in the other columns.

c Interval between cultivations during the first year. Usually the interval was increased in subsequent years as the weeds weakened. d Plants buried to controlled depths.

* A strategy that minimizes the number of cultivations often requires longer to achieve complete eradication. 6 Sample size for means shown in the other columns.

c Interval between cultivations during the first year. Usually the interval was increased in subsequent years as the weeds weakened. d Plants buried to controlled depths.

Planting depth (cm)

Figure 4.1 Mass of new underground shoots per gram of rhizome planted for

Agropyron (Elytrigia) repens rhizomes of several lengths planted at various depths.

(Drawn from data in Hákansson, 19686.)

Planting depth (cm)

Figure 4.1 Mass of new underground shoots per gram of rhizome planted for

Agropyron (Elytrigia) repens rhizomes of several lengths planted at various depths.

(Drawn from data in Hákansson, 19686.)

shallow tillage, and these are then buried by deep plowing. Although field-scale trials of this procedure have not been published, the parameters of the method have been worked out for Agropyron (Elytrigia) repens (Vengris, 1962; Hákansson, 1968b, 1971), Sonchus arvensis (Hákansson & Wallgren, 1972b), Holcus mollis and Agrostisgigantea (Hákansson & Wallgren, 1976), Achillia millefolium (Bourdot, 1984; Field & Jayaweera, 1985), and Mentha arvensis (Ivany, 1997).

The procedure is illustrated by data on Agropyron repens. Hákansson (1968b, 1971) planted rhizomes of 4,8,16, and 32 cm length in October at 11 depths down to 30 cm. Rhizomes established poorly at the soil surface, and only the longest pieces produced a substantial weight of new rhizomes by the following autumn (Figure 4.1). Mechanically moving most rhizomes completely to the surface would be difficult, however, so this finding is not very useful for management. At 2.5 to 5 cm depth, the rhizomes established well, and the shorter pieces produced more shoots and new rhizomes per gram of rhizome planted because a greater percentage of buds sprouted (Figure 4.1). In contrast, however, productivity declined with deeper planting, and few shoots from the 4-cm fragments reached the surface from depths greater than 10 cm. This led to death of the original rhizomes, without replacement by new rhizomes (Figure 4.1). Similar results have been obtained for several different years, genetic materials, soils, and planting dates (Hákansson 1968a, 1968b, 1971; Hákansson & Wallgren, 1976), and for rhizomes that were allowed to sprout prior to burial (Vengris, 1962). Since most A. repens rhizomes occur in the top 10 to 15 cm of soil (Hákansson, 1968b), breaking them into small pieces by rotary tillage or disking is feasible. Subsequent burial will be most complete if plowing inverts the soil as fully as possible. Fortunately, the depth distribution of new rhizomes developed from fragments that managed to survive burial was little affected by the planting depth (Hákansson, 1969c).

In further work, Hákansson (1968a, 1971) found that a crop of white mustard caused a greater percentage decrease in the biomass of A. repens originating from small, deeply planted rhizomes relative to large or shallowly planted rhizomes. This happened because shoots from the deeply planted pieces emerged slowly, thereby giving the crop an opportunity to grow up and shade the weed, and the small reserves in the shorter fragments led to weaker growth. The synergistic interaction between crop competition and tillage in this study illustrates the importance of integrating ecological management procedures - simply chopping and burying the rhizomes would not be expected to decrease A. repens density in a fallow or a weakly competitive crop. The chop-and-bury plus crop competition approach to the management of shallowly spreading perennials is particularly notable because it results from understanding of weed ecology rather than from the development of new technology.

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