Ecology and Control of Knotgrass (<i>Paspalum distichum</i>)

Knotgrass (Paspalum distichum) has been a serious weed under a humid soil condition, regardless of crop and cropless lands. Recently in southern Japan, Kyushu, this weed is invading in rice fields without satisfactory tillage and/or winter cropping. Once when it invades in rice, it is very hard to weed out perfectly. Thus, we have studied the ecological and anatomical characteristics of this grass weed, concerning its control, and have made the preliminary evaluation test of several herbicides. The results are summarized as follows:<br>1) Knotgrass can root easily from the nodes of a runner and elongate rapidly. High temperatures are more suitable for elongation than low temperatures, showing the elongation velocity of 15 to 20cm per week under the most favourable condition. The velocity may be about two times or more of rice in the summer season. However, the sunlight control of about one-fifth to one-eighth provides no effect on the elongation of regrowing shoots.<br>2) The emergence of knotgrass in the soil is made from deeper sites in a dry condition than in a flooded one, but there is no essential difference between sandy and silt-clay soils. Generally speaking, this grass weed seems to be rather shallow emergeable, when compared with other perennial such as perennial cyperaceaes found everywhere in Japan. The depth of emergence is 3.25cm on the average. This fact indicates us that this grass should be controlled by deep-plowing.<br>3) Some anatomical natures in matured stems which tell us resistability against chemical substances were found. That is to say, thickening of the cell membrance of epidermis, cortex, and central portion, poor development of vascular bundles in the central portion, no air capacity in the central portion, and much accumulation of starch grains in the cortex and central portion.<br>4) In the cross section of a leaf, distinct differentiation of bundle sheath cells surrounding a vascular bundle is found. This indicates the strong competition ability of this grass with rice.<br>5) Several herbicides were evaluated at soil-, foliage-, and soil and foliage treatments. Acute action on foliage was found in paraquat, dalapon, NaClO₃, and AMS+2, 3, 6 TBA. Strong suppression of regrowing shoots after cutting was seen in 2, 4-D+2, 4, 5 T and 2, 4-D+amitrole besides the herbicides named above. Generally speaking, promising herbicides in practice may be considered as follows: dalapon, 2, 4-D+amitrole, NaClO₃, AMS+2, 3, 6 TBA, paraquat.