Bill Johnson
Extension Weed Science

Purdue University

Created 6/10/2011

Information listed here is based on research and outreach extension programming at Purdue University and elsewhere. The use of trade names is for clarity to readers of this site, does not imply endorsement of a particular brand nor does exclusion imply non-approval. Always consult the herbicide label for the most current and update precautions and restrictions. Copies, reproductions, or transcriptions of this document or its information must bear the statement 'Produced and prepared by Purdue University Extension Weed Science' unless approval is given by the author.
To get the most out of a postemergence herbicide application, timing for weed height is critical. Most herbicide labels state that broadleaf weeds should be no more than 4 to 6 inches tall and grasses even smaller. That does not mean that you cannot spray taller weeds and be successful, but the taller the weed gets over label recommendation, the less the performance will be to your liking. Small weeds are easier to cover with an herbicide spray and their cuticles are thinner. Thus, more herbicide inters the plant. The taller the weed the less coverage you get, and the hotter and drier the conditions, the thicker the plant cuticles will be; thus the less efficacy you will get. Herbicide application timing is not always convenient to achieve, and some of the later emerging weeds may not fit the niche for a one-time postemergence application. You end up with some weeds being too tall, some not emerged, and some just right for the application. Did you ever wonder why an 18 inch to two foot or taller giant ragweed plant was so hard to control (let's not mention three to four feet tall)? The only way you got one that tall was to first have one three or four inches tall. The control of the small ones will always be greater than that of the taller ones.

In a year like this one, where heavy rains are sure to produce a multitude of late emerging weeds, it is imperative that scouting is done to identify individual weed species as they emerge and select the correct herbicide to control them. This may mean that you will need to make an additional application on a timely bases before the corn or beans are over the growth stage needed to ensure crop safety while catching the weeds at a small enough stage to get maximum weed control.

Timing for crop height is as critical as timing for weed height with postemergence herbicides when it comes to products like the plant growth regulators such as 2,4-D or dicamba and related herbicides or with amino acid inhibitor herbicides like the SU's and related products. Consider an area of about three square feet. When a corn plant is only about four inches tall, it occupies only a small portion of the area and the crop plant intercepts only a small fraction of the spray solution that covers that area. As the plant grows it occupied a much larger portion of the area and thus intercepts a higher quantity of the spray solution. These types of translocated herbicides move to the newly developing plant tissue in the crop plant. If brace roots are being developed, then an over application of a growth regulator herbicide can cause brace root damage. If the plant is three feet or taller it can act like a funnel and collet the spray solution directing it to the whorl of the plant where the newly developing tissue is likely to be the developing tassel or ear, causing malformed tassels and or deformed ears.

Years in which crops are delayed due to heavy rains and flooding creates a difficult situation for weed control due to a number of weeds that would normally not be a problem in a field becoming more of a dominate problem. These weed will invariably germinate when you lest want them to do so, causing your application timing to be off. Effort is needed to time postemergence herbicide application to control late emerging weeds while they are small and likewise not spray crop plants that are taller than the label recommends. Sometime this may call for drop nozzles.