Weather Conditions


The optimum temperature range for efficacy is 15-25C. Low temperatures, even light frosts give a good, but slow kill. Symptoms can take several weeks to show over the winter months, but the end result is the same. Hard or long-term frosts, when the plants go floppy and the metabolism shuts down, will result in poor chemical uptake and consequentially poorer performance.

High temperatures, in excess of 25°C, can cause scorch and stress - the plants ‘shut-down’. This leads to a reduced uptake and movement of chemical within the plant resulting in poorer performance.

Rainfall, relative humidity, dew and fog

Modern Roundup formulations are rainfast in as little as one hour, depending on the target species. Uptake is quicker in grasses than broad-leaved weeds and in annuals compared to perennials. See product labels for further information.

  • High humidity leads to good control, providing run-off is minimal. Spraying on a morning dew or in fog will give excellent results, providing it dries out during the day
  • Avoid spraying in the evening because of the risk of run-off from rainfall during the night

Rainfall challenges performance most when the performance is restricted by other factors.


Target plants require light to move Roundup around the plant to the growing points. Best results come from morning applications due to the long light period before darkness. Spraying in the evening requires a longer rain-free period for uptake. Optimum results also come from application during high light intensity, long days.

Do not cut out the light by ploughing, cultivating, applying lime or organic manures until the cultivation period on the label has passed.


Reduced plant metabolism for whatever reason leads to reduction in transport to the growing points and will lead to poorer results. Stress can be from:

  • High temperature
  • Drought
  • Frost
  • Die-back
  • Water-logging
  • Dust
  • Disease