Abstract
The experiment was conducted at the field station of college of Agriculture Forestry / Mosul University (Iraq) during the growing season 2011 on water hyacinth .The purpose of the research is to study the effect of frequency of clipping at different duration to control this weed .It was used Orthogonal contrast analysis between mean treatments in order to determine the best time for mechanical clipping of water hyacinth .Results showed that the clipping treatments of July (Q3) had to significantly decreased in plant high, leaf area ,root length and dry weight comparing with treatments had done on May or June (Q1,Q2).On other hand Q3 had superiority in number of inflorescences /m² and number of daughter plant/m² .In general ,the percentage of killing for Q3 was superior than Q1 or Q2 which had reach up to 16% and 12.76% respectively. In general, the Orthogonal contrast had give the beast treatment for clipping at the end of July which caused the higher of killing reached up to 40% .Although the water hyacinth plant had tendency to produce a high number of seed and also the vegetative reproduction was highly increased .As a result at this study ,it is not useful to consider the mechanical method as a reasonable way in controlling water hyacinth .
