Leon Warren, Fred Yelverton and Travis Gannon

Presenting at block T14, location A13B


Controlling smooth crabgrass presents different challenges with each growing season.  Rainfall and / or irrigation greatly influence if and when products actually get activated in time for initial control and also how long the control will last once activated.  In general, the wetter and hotter the season, the quicker the products will break down due to early increased soil microbial activity.

Neither heat or rainfall has been in short supply this season.   Rainfall records at the station indicate that May received 7.57 inches, June received 3.51 inches and July received 10.75 inches.  Factor in some supplemental irrigation and this location has seen more than enough rainfall to ensure optimum turf and weed growing conditions.  June temperatures were the hottest on record and July consisted of many days in the 90’s also.

Older dinitroaniline (DNA) herbicides such as oryzalin (Oryzalin 4 Pro) and benefin + trifluralin (Team Pro 0.86G) have been tested for over 20 years for smooth crabgrass control and have been known to breakdown quickly in wet growing seasons.  Because of this, split applications are recommended to provide extended season-long control.  From June 10 to July 8, Oryzalin 4 Pro control dropped from 100 to 62% with a single application rate of 3 lb ai/A.  Control was 98% when applications were split at 1.5 lb ai/A each at 8 week intervals.  On July 8, Team Pro 0.86G provided 89% smooth crabgrass control applied at 3 lb ai/A and 95% control when split at 1.5 lb ai/A each.

Newer DNA herbicides such as prodiamine (Barricade) and pendimethalin (Pendulum Aquacap) are more stable than the older DNA herbicides and hold up better with single applications.  However, it is still recommended to split these applications to protect against herbicide break down for whatever reason.  As of July 8, all Barricade formulations were providing 99+% smooth crabgrass control regardless of application timing.  Pendulum Aquacap was providing 94 to 99% control at this same timing.  End of season ratings sometimes are quite lower than mid season ratings with these DNA herbicides as they tend to remain more effective at a longer interval than Oryzalin 4 Pro or Team Pro 0.86G but can still break down before frost.

Dithiopyr (Dimension Ultra 40WP) mode of action is similar to DNA herbicides and split applications are recommended with this herbicide also.  However, as of July 8, smooth crabgrass control was 94 to 100% and holding up very well.  Dimension Ultra 40WP is very effective when properly irrigated or watered in very soon after application.

Oxadiazon (Ronstar Flo, 2G, 50WP; Quali-Pro Oxadiazon) is a shoot-inhibiting herbicide that requires a single full rate initially with sequential applications not an option.  In heavy infestations of smooth crabgrass, 3 lb ai/A is recommended.  Ronstar Flo applied at 2 lb ai/A resulted in 56% smooth crabgrass control on July 8, while the 3 lb ai/A rate resulted in 93% control.  Ronstar 50WP applied at 3 lb ai/A provided 91% smooth crabgrass control.  Granular oxadiazon formulations did not hold out very long in this growing season.  Smooth crabgrass control ranged from 51 to 59% by July 8 and I am at a loss as to why control was so low this early in the growing season.

Specticle 20WP is a newly registered herbicide from Bayer CropScience with excellent smooth crabgrass activity.  On July 8, 1 oz ai/A was providing 91% control while the split application rates of 0.5 oz ai/A were resulting in 98% control.  Specticle 20WP is labeled in bermudagrass but not tall fescue.  Tower 6L is a newly registered herbicide from BASF Corporation with smooth crabgrass activity also.  However, control tends to be more short-term with Tower 6L as compared to Specticle 20WP.  From June 10 to July 8, smooth crabgrass control dropped from 94 to 23% with split applications of 1.5 lb ai/A each.

These early data suggest that Specticle 20WP may be as consistent as Barricade or Pendulum Aquacap in providing season-long smooth crabgrass control whether using single or split applications.  Tower 6L may only provide smooth crabgrass suppression or very short-term control based on rates tested of 1.5 lb ai/A with sequential applications.