Best Practices for Postemergence Weed Control

By Eric Yu and David Nicolai et.al

With temperatures rising and adequate soil moisture across much of the region, weeds are growing rapidly, creating ideal conditions for early-season competition in row crops. Even if a pre-emergence herbicide was applied, it is important to scout fields now for weed emergence. Early detection and timely intervention are key.

Postemergence herbicide applications are essential for maintaining crop yield and protecting farm profitability. Weeds that emerge alongside crops compete aggressively for sunlight, nutrients, and water, especially during critical early growth stages. Targeting weeds while they are still small improves control effectiveness, reduces the risk of seed production, and limits the need for costly rescue treatments later in the season.

Timing is everything: Target weeds when they're small

Postemergence herbicides are most effective when weeds are small, ideally no more than 3 to 4 inches tall. This size threshold is critical for achieving consistent control and minimizing the risk of yield loss. Once weeds grow beyond this stage, control becomes more difficult and less reliable.

Larger weeds are more likely to survive an herbicide application, especially under less-than-ideal conditions such as poor coverage, incorrect spray rates, unfavorable temperatures, or suboptimal timing during the day. Bigger plants often have more growing points than smaller seedlings, allowing them to better tolerate or recover from herbicide injury. In addition, herbicide uptake and coverage tend to decline as plant size increases, reducing overall efficacy.