Kristen Obeid, OMAFRA’s weed management specialist for horticulture is looking at the May 8 weekend forecast and giving timely advice. 

“I’ve been getting a lot of questions on how well certain herbicides will work in our colder than normal weather,” says Obeid.  “There is no question that cold weather will impact post-emergent herbicide performance.”

Optimal temperatures are between 60 – 80°F or (16 – 27°C). Cold weather slows weed growth and hardens cell walls. This affects uptake of the herbicide by the weed which can lead to reduced weed control. Plants degrade herbicides by metabolism, but plant metabolism slows during cool or cold conditions, which extends the amount of time required to degrade herbicides in plants.

Cold temperatures following a herbicide application influences crop safety or injury and weed control from herbicides. Rapid degradation of herbicides under warm conditions allows crop plants to escape herbicide injury. Slow degradation of herbicides during cold weather could result in crop injury depending on the herbicide. 

Grass and broadleaf weeds are controlled more effectively when plants are actively growing. Cool or cold conditions at and following application of products containing fenoxaprop-p-ethyl (Puma) give greater grass weed control but also may cause crop injury. Other ACCase herbicides such as quizalofop-p-ethyl (Assure II), sethoxydim (Poast Ultra), and clethodim (Select, Arrow, Centurion) provide better grass control in warm weather when grasses are actively growing.

Cold temperatures, including freezing conditions following application of ALS herbicides, that contain metribuzin (Lexone, Sencor, Tricor, Squadron) and bromoxynil (Pardner, Bromax, Brotex, Buctril) may increase crop injury of respective crops but weed control will be maintained.

Bentazon (Basagran), fomesafen (Flexstar), glufosinate-ammonium (Ignite) and aciflurofen (Ultra Blazer) may not cause crop injury when cold temperatures follow application but less weed control may result.

2,4-D, dicamba, MCPA, clopyralid (Lontrel), fluroxypyr (Starane), glyphosate (tolerant crops) have adequate crop safety and provide similar weed control, but weed death is slowed when cold temperatures follow application.

The recommendation for applying products containing fenoxaprop ad ALS herbicides that contain metribuzin is to delay application until daytime temperatures exceed 60 oF (16 oC) and after active plant growth resumes.

Cold weather generally is not a concern for pre-emergence herbicides and can actually help certain products. Volatility losses are less in cold temperatures, a benefit for products such as trifluralin (Treflan, Bonanza, Rival) and dichlobenil (Casoron) which can volatilize during warm temperatures. Also, herbicide breakdown by microorganisms is slower in low temperatures, increasing herbicide longevity and, thus, the length of weed control for pre-emergence herbicides.

So, the bottom line … if you haven’t applied your pre-emergent herbicides, now is the perfect time and if possible try to apply all post-emergent herbicides when temperatures are above 60°F (16°C). 

Source:  Kristen Obeid, OMAFRA May 8, 2020 blog