Abstract
The infestation of R. obtusifolius (Ezonogishigishi) and distribution pattern of its individual plants were investigated in sown grasslands in eastern Hokkaido during 1983 to 1985.
1) About 60% of sown grasslands were infested with R. obtusifolius, one -third of them seriously. High infestation of R. longifolius (Nodaio) occurred only in peat soil areas.
2) The occurrence of R. obtusifolius was positively associated with that of annual arable weeds such as Capsella bursa-pastoris (Nazuna), Chenopodium album (Shiroza), Rorippa islandica (Sukashita-goboo), Polygonum longisetum (Inutade) and Stellaria media (Hakobe).
3) There was no significant difference of soil pH and nitrogen content on grasslands irrespective of the infestation of R. obtusifolius.
4) Individual plants of R. obtusifolius distributed unevenly in sown grasslands with a L-type distribution.
5) The distribution pattern of dry matter weight per plant was approximately logarithmnormal with a high proportion of smaller plants.
