This chemical is used to control herbaceous weeds and some woody plants without killing desirable lawn and turf grasses. According to the US. Getting Rid Of Johnson Grass Thriftyfun Ad Find China Manufacturers Of Lawn Grass. Johnson grass killer for lawns . You can prevent the grass from spreading by plowing immediately after you harvest till frequently and use Johnson grass-free seed and feed. You can also use herbicides to kill already growing Johnson grass. Illinois Natural History Survey recommends treating Johnsongrass in June with 2 percent Roundup a. Like most other chemical herbicides Grazon must be used according to package instructions and does carry some risks. Ad Find Grass Lawn Care and Related Articles. Post emergence products may be effective in outlying areas of the property. In tall fescue there are no selective herbicides for post-emergent Johnsongrass control. Chemical Johnsongrass Killer Chemical control is usually the most effective Johnsongra...