By Wayne Flanary, CPAg and CCA Field Specialist in Agronomy, MU Extension

We often receive grower concerns that Johnsongrass is regrowing after being sprayed. Growers are concerned the lack of control may be herbicide resistance. However, MU research indicates, one should spray Johnsongrass in the fall as the plants are moving carbohydrates into the rhizomes to prepare for winter. This allows the herbicide to move into the plants, increasing the level of control.

Therefore, we need to prepare for controlling Johnsongrass this fall, once it cools off. September 20th would be a good target date to start spraying. Yes, this is during harvest, but if you have rhizome Johnsongrass, this is best time for control. If the weather forecast indicates a frost, do not spray. We have found that systemic herbicides will not translocate after frost and control will fail.

It is important to control Johnsongrass before heading out to prevent seed production. The reason this plant is spreading across Northwest Missouri is seed production.

When spraying rhizome Johnsongrass early, this helps limit seed production. Energy is moving out of the root system to grow and expand the plant with new tillers, so limited amounts of herbicide are moved into the rhizome root system, so rhizome Johnsongrass will regrow.

Seedling Johnsongrass can be controlled by a variety of herbicides. After controlling rhizome Johnsongrass by fall herbicide application, it will be easier to control seedling Johnsongrass in the spring. A single plant will become a clump of Johnsongrass and may make as many as 80,000 seeds. Also, Johnsongrass seeds may last more than 10 years in the soil.

The best way to control Johnsongrass is to prevent a single plant from growing on your farm. It can get out of control and then you will be fighting all summer. Don’t forget about field borders. Treat the borders like they are part of the field.

I will be offering several county meetings on the Johnsongrass control topic. Please attend one of these meetings as they will be advertised on the University of Missouri Extension website or contact your local Extension office for meeting dates later this fall and winter.

For more information, contact Wayne Flanary, Field Specialist in Agronomy, at 816-279-1691, University of Missouri Extension.