Herbicides, summer weeds and how to control them

By Doug Carter, Rankin County Extension Agent

Weed identification is important when applying selective herbicides. Weeds can be annual, biennial, or perennial and are classified as either grass, broadleaf weed, or sedge.

Annual weeds produce seed within one year of growth. They are those that emerge in the spring, grow through the summer, and produce seed in the fall.

Examples:  crabgrass, goosegrass, spurge and annual lespedeza.  Weeds that emerge in the fall, grow in the winter, and produce seed in the late spring are called winter annuals.  Examples: henbit, chickweed, and annual bluegrass.

Biennial weeds require two years of growth to produce seed. These plants growth the first year, develop a root system and a rosette of leaves, resumes growth the second year, produces seed and die. Examples: wild carrot, bull thistle.

Perennial weeds emerge, grow and produce some structure that enables the plant to overwinter and resume growth year after year.  Examples: bermudagrass, bahiagrass, dallisgrass, Florida betony, Virginia buttonweed, and nutsedge.

    Pre-emergent herbicides are applied to the soil surface or turf canopy before weeds come up. They will kill weeds soon after or before emergence. Chemical weed control begins with a pre-emergence herbicide. If you do not use a pre-emergence herbicide, chemical weed control becomes a defensive tactic. Apply pre-emergence herbicides from February to mid-March to control summer annual weeds.  

All pre-emergence herbicides must move into the supper portion of the soil to control weeds.  Herbicides not washed into the soil are decomposed by ultraviolet sunlight or lost as vapors. Some herbicides are more susceptible to decomposition than others.  The sooner a herbicide can be washed into the soil after application, the better the weed control

Post-emergence herbicides are applied after weed seedlings emerge. Post-emergence herbicides are most effective if applied to seedlings less than three inches tall.  Since weed seedling emergence occurs for several weeks, more than one application is needed to control weeds the entire season.  

Herbicides are available in several formulations: concentrate, water dispersible, flowable, and soluble powder. These formulations are mixed with water and sprayed onto the turf canopy or weeds.  

Other herbicides are available in granular formulations. The granules are applied directly to the turf or soil surface. Granular materials may be less difficult to apply but are more expensive per unit of active ingredient than the sprayable formulations.  

    Source:  Mississippi State Extension Publication 1322,  Establish & Manage Your Home Lawn