[header] How to control johnson grass [title] Plow immediately after you harvest. [step] Johnson grass has rhizomes that allow the weed to spread into uninfected areas. If you plow your fields immediately after you harvest your crops, it breaks up the rhizomes and prevents them from getting a good hold and then spreading to other areas. 
[title] Till your soil a few times per week. [step] You should till the soil of crops where johnson grass grows more frequently than other areas.