Imazapyr will work just fine but I'd be willing to bet that most of the pines you plant will out compete most of the competition in a few years if you are able to do some bush hogging or mowing on the edges or in between the rows.I need to plant some rows of pines for screening and windbreak around my house build. The field is very fertile so grass/weeds takes them over very quick. Is imazapyr what I could spray directly over them to control the grass/weeds?