When to spray weeds after a frost? After a light frost, spraying could resume when the following conditions are met: (1) A minimum of one night with minimum temperature of 5°C. (Note that this is a general comment, and applies to glyphosate. Tank mix partners may work differently under cool conditions.) (2) A minimum of one day of good growing conditions (warm and sunny) have passed. (3) Good growing conditions (warm and sunny) are present at the time of spraying. (4) You see no evidence of frost damage (blackening and water soaked appearance) on the crop or the weeds. “Crop” is included here because even a herbicide-tolerant canola crop requires that the metabolism of the plant be working at full capacity to enable it to effectively process the herbicide and prevent injury. Here are links for more on (1) spring frost and (2) fall frost and snow scenarios. Find a few more details at the Canola Encyclopedia.
Tips to spray in the wind. Spraying with wind speeds higher than 15 km/h will increase drift. Here are some tips to reduce the risk in slightly windier conditions (as long as wind is blowing away from higher risk areas, like the neighbour’s yard or recently emerged crops): Use low-drift nozzles. Increase water volumes to improve coverage with large droplets. Find a nozzle that can achieve a coarse spray at a broad range of pressures. Aim for 100% overlap nozzle to nozzle to provide equal coverage across the whole boom width. Keep the boom low. Travel slower to reduce the amount of fine droplets, and make it safer to drive with the boom lower. See the Canola Encyclopedia for more details. One option is to spray at night if the winds are more calm (but not completely calm). Learn more from Ken Cole’s Night spraying: Pesticide efficacy with night time applications study.
Tips for spraying in dusty conditions. Dry conditions and wind can cause dust build-up on weed leaves, which may limit herbicide uptake. Most products are not strongly affected by dust, but two important products are very dust-sensitive: glyphosate and diquat (Reglone and others). Full article.
Tips for spraying before a rain. Rainfastness is different for each herbicide. This article has more details but check the latest product labels for updates.
Efficacy when spraying in cool conditions. Herbicides in general tend to work best in warm sunny conditions when weeds are actively growing and cycling nutrients into their growing points. Performance is on a sliding scale. Cloudy days with highs of 10°C after a night near 0°C will tend to result in herbicide performance at the low end. Sunny days with highs of 15°C after a night of 3°C will provide improved control. Sunny days with highs of 20°C to 25°C after a night of 10°C will provide optimum control. This article has more details.
Efficacy when spraying in hot conditions. –Weeds: Warm, sunny days when weeds are activity growing are generally the best for herbicide activity, but hot weather reduces efficacy, especially if conditions are also dry. –Flea beetles: Flea beetle activity is lower under extreme hot temperatures and, perhaps more importantly, insecticide performance is lower in hot conditions. This article has a lot more detail on herbicide and insecticide performance.
When to spray post-emergence if you missed pre-seed burnoff? Seeding and then spraying means that winter annual and perennials will be that much larger, and harder to control, and makes it even more important (if that’s possible) to apply the in-crop application early. See the Canola Encyclopedia for more details.
Guides to crop protection/Blue Book
With any challenging spray scenario, it can help to check on restrictions and recommendations listed on the label. Here are good places to start: