MARSH SOW-THISTLE, SONCHUS-PALUSTRIS L (ASTERACEAE) IN ONTARIO - AN ADDITION TO THE INTRODUCED FLORA OF NORTH-AMERICA

Three small populations totally less than 100 plants of Sonchus palust ris, a perennial native to western Eurasia, were discovered along the disturbed edge of a wet fen complex in Gloucester, Regional Municipali ty of Ottawa-Carleton, Ontario. This constitutes the first report of S . palustris in Canada and North America. Plants from this population w ere diploid (2n = 18) and had the characteristic diagnostic features o f Sonchus palustris including exceptional height (>1.5 m), densely cor ymbose inflorescence of pale yellow flowers and long-pointed stem leaf bases. These, as well as ecological characteristics, were used in the separation of this from the smaller tetraploid (2n=44) S. arvensis. I t is speculated that the distinctive and particular habitat requiremen t of S. palustris likely precludes it from becoming a widespread invas ive or troublesome weed in Canada.