Seeds can germinate and seedlings emerge from a depth of 6 inches, but most arise from a depth of 2 to 3 inches or less. Johnsongrass plants developing from seed begin to emerge when soil temperature approaches 57F and the optimum germination temperature is 75F. Plants that emerge when the soil temperature is 75 F can flower in six weeks and have viable seeds two to three weeks later. Seedling johnsongrass can produce rhizomes as early as three weeks after emergence. With this rapid growth, a single plant can produce more than 100 stems, several thousand seeds, and over 100 feet of rhizomes in a five to six month growing season. Most of the rhizomes are found in the top 8 inches of soil.
Rhizomes are thick and scaly.
The vast rhizome system largely accounts for the difficulty in controlling johnsongrass. In addition, johnsongrass has a fibrous root system and can develop roots at nodes on lower stems that come into contact with the soil. Decaying johnsongrass rhizomes release chemicals that can inhibit the growth of other plants. Rhizomes will die if exposed to freezing temperatures for a few hours or become dehydrated. Small rhizome segments are more susceptible to dehydration than larger segments.