Exposure of perennating organs to desiccation and freezing

Desiccation of roots and rhizomes by summer fallowing was a traditional method of controlling wandering perennials in Europe. The soil was plowed and allowed to dry into clods. These were stirred occasionally with a plow or heavy cultivator to completely desiccate roots and rhizomes (Bates, 1948; Travers, 1950). Foster (1989) indicated that the same procedure can be used to manage Rumex obtusifolius and R. crispus.

In Botswana, Phillips (1993) found that an extra moldboard plowing substantially reduced subsequent growth of Cynodon dactylon and increased sorghum grain yield, especially if it occurred during the dry season when tillage promoted desiccation of rhizomes. Similarly, in Nicaragua Vargas et al. (1990) found that plowing dry soil at the end of a four-month dry season caused Cyperus rotundus tubers to die of desiccation. This greatly reduced shoot density of the weed in the subsequent crop (Figure 4.2).

Figure 4.2 Density of shoots of Cyperus rotundus 14 days after irrigation in response to the period of desiccation between dry plowing and irrigation. All treatments were first irrigated on the same day. (Drawn from data in Vargas et al., 1990.)

Period of exposure to dry soil between plowing and irrigation (days)

Figure 4.2 Density of shoots of Cyperus rotundus 14 days after irrigation in response to the period of desiccation between dry plowing and irrigation. All treatments were first irrigated on the same day. (Drawn from data in Vargas et al., 1990.)

In regions with cold winters, some perennial weeds can be killed by freezing damage to the perennating organs. Schimming & Messersmith (1988) studied the temperatures required to kill overwintering perennial buds of four species. Cirsium arvense suffered 90% mortality at -12 °C, but the other species were more cold tolerant. Since minimum soil temperature increases with depth, effective exposure of roots and rhizomes to lethal temperatures requires working these organs to the surface. Development of winter hardiness in the autumn is an energy-consuming activity, and thus forcing resprouting by autumn cultivation may increase sensitivity to freezing (Schimming & Messersmith, 1988).

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