Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 150

Figure 4.2 Density ofshoots of Cyperus rotundus 14 days after irrigation in response
to the period ofdesiccation 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 freez-
ing 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 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 hardi-
ness in the autumn is an energy-consuming activity, and thus forcing
resprouting by autumn cultivation may increase sensitivity to freezing
(Schimming & Messersmith, 1988).

12 °

Physical removal of perennating organs from the field
Severe infestations of species with perennating organs near the
surface may be substantially reduced in density by removal of roots and rhi-
zomes from the field. Typically, the soil is plowed, roots or rhizomes are
worked to the surface with a spring tooth harrow and then pulled to the edge
of the field with a rake or harrow (Travers, 1950). Rhizomes should be broken
as little as possible during primary tillage, since longer pieces are more easily
sorted to the surface (Kouwenhoven & Terpstra, 1979). With current technol-
ogy, removal of roots and rhizomes is only practical on small fields; on large
fields,much of this material falls through the tines and remains in the field.In