Springtime is anything but predictable, and this can be frustrating when you’re trying to implement a successful weed control program. It’s a fact: cold, wet weather can negatively impact herbicide performance.
TEMPERATURE
Fortunately, glyphosate performs well under a wide range of temperatures. You’ll see the best performance when temperatures are in the 16-24°C range and remain there for a few hours after application. At this temperature, most plants will be actively growing and not experiencing temperature stress.
When it’s colder than 15 or 16°C, plant growth slows, resulting in slower herbicide uptake and translocation. This increases the required rainfast period, and slows down the onset of symptoms, however, overall weed control is not compromised.
FROST
The impact that a frost will have depends on the duration of the frost (hours), the severity (low temperatures), preceding weather and the weed species you’re targeting. Generally, perennials, biennials and winter annuals will possess a higher degree of frost tolerance than spring germinating annuals.
A light frost of -2°C to -3°C will not usually harm a plant. You can resume later in the day if temperatures are forecast to climb to a minimum of 10°C for at least 2-4 hours after the application.
Heavier frost (< -5°C) can cause more severe damage to most plants, including perennials. Avoid spraying for 1-2 days to assess any injury that has occurred. If at least 60% of the plant is still green and actively growing, and daytime temperatures are forecast to reach at least 8°C, you can resume spraying. Weeds that are under excess moisture or drought stress, disease or insect pressure will take longer to resume active growth after a frost, so you may need to observe a longer interval.
A frost that occurs soon after an application of glyphosate (within 24 hours on annuals, or within 3 days on perennials) can also impact control. If only a light frost is predicted, ensure at least a couple hours of above 8°C to allow the product to enter the plant. Once the temperature warms up the next day, translocation will resume. If a severe frost is predicted immediately following your intended application, you may want to avoid spraying. If the plant is damaged before the product has properly translocated, control may be reduced.
| Application |
Frost Severity |
Spray Interval |
Comments |
| Pre-seed or Post-seed |
> -4°C |
Can spray the next day. Need a daytime forecast high of >8°C for at least 2 hours after the application in order to resume spraying. |
Regardless of severity of frost, plants need to be actively growing or they won’t translocate. The target plants must have 60% green tissue for applications to be effective. |
| < -5°C |
Wait at least 1-2 days to assess damage to plants. Need a daytime forecast high of >8°C for 2 or more hours after the application in order to resume spraying. |
RAINFALL
Many herbicide products on today’s market come with recommendations on how much time must elapse between product application and subsequent rainfall to ensure good product performance. This time frame is often referred to as the rainfast period.
Generally, product rainfast ratings are based on good growing conditions. Poor conditions may require a longer interval between application and any rainfall to ensure adequate product translocation within the plant before wash-off occurs. For many products, any amount of rainfall soon after spraying has the potential to reduce control.
SUMMARY
- The optimal temperature range for spraying glyphosate is 16-24°C.
- A frost of 0°C to -3°C will not usually harm the target weeds and you can resume spraying later in the day if temperatures are forecast to reach 8°C or more for at least 2 hours after application.
- Heavier frost (< -5°C) can cause more severe damage to target weeds, so avoid spraying for 1-2 days to assess any injury.
If at least 60% of the plant is still green and actively growing, and daytime temperatures are forecast to reach 8°C or more for at least 2 hours after application, you can resume spraying.