The optimum time for treatment of perennial broad-leaved weeds e.g Docks or thistles is from flowering, but before the plant dies back, usually July to October. This is when the natural flow of sugars is downwards to the root sink, which ensures maximum translocation and efficacy of Roundup.
Perennial grass weeds must have full emergence of healthy green leaf. Common Couch for example becomes susceptible at the onset of tillering and new rhizome growth, which usually occurs when plants have 10-15 cm of new growth.
Where perennials are a major problem before planting in the spring, treat pre-planting and then follow up pre-harvest or in the stubble before the following crop.
Annual grasses and broad-leaved weeds should have at least 5 cm of leaf or two expanded true leaves, respectively. The diagram below illustrates that treating too early means that all weeds may not have emerged or have sufficient leaf area to absorb the herbicide. Any weeds not emerged at the time of application will not be controlled.
NB: Spray before emergence of the crop
as part of a 2 spray programme
Stem Extension Phase
Do not spray
End April - End May
Ears fully emerged but no seeds set.
End May - End June
The stem extension phase describes the growth stage when assimilates are pushing upwards very strongly to produce flower heads and translocation of glyphosate down to the roots is against the direction of that flow. Poor uptake from applications at this time can lead to variable control. When sprayed at this stage the usual effect is an initial good kill followed by re-growth from those tillers that were at stem extension. The re-growth typically goes on to produce more flowering stems and requires a further treatment to stop viable seed production. This phenomenon has been particularly noted in the pre-cut treatment of grassland, it is not related to product, dose rate or formulation.