Technical “
The maximum kill effect can only occur when a fatal dose actually reaches the growing points. Following the product label recommendations is a must for best results
Importance of growth stage
In any plant, the natural flow of sugars from photosynthesis in the leaves changes during its life cycle and, because glyphosate moves in the phloem along with the sugars, the movement of glyphosate inside the plant will vary according to growth stage.
Treatment as the stem is extending rapidly to produce flowers - often in early spring - will mean most glyphosate goes to the growing points in the developing stems, leading to a rapid dieback of the stem but little movement downwards and subsequent re- growth from the base.
This can be seen most obviously in the invasive alien Japanese Knotweed, but it is often a factor in the control of Rose Bay Willowherb, Common Ragwort, Creeping Thistle, Common Nettle, Brambles, Bracken, Canadian Fleabane, Common Couch and Ryegrasses
Once the flowers are formed, the next physiological stage for perennials is to move sugars back down to the roots for storage. Spraying at or just after flowering will give excellent translocation down to the roots and a much higher level of long-term kill.
Avoid spraying perennials too early
Often, weeds are sprayed too early due to worries about seed proliferation once the plant starts to flower. Good examples are Common Ragwort, Giant Hogweed and all species of thistles. However, it takes several weeks after flowering before seeds are mature and application of glyphosate before maturity will still render the seeds non-viable.
Weed programmes involving early spring treatments will give good suppression of perennials, but will require another treatment later in the season for complete control.
Best practice
Factors which may affect the final weed control results fall into three main areas - plant, climate and application.
PLANT FACTORS Species
Certain species are less susceptible to glyphosate, such as wild white clover, horsetail and ivy. This may be
caused by a physical barrier - thick waxy cuticles, hairy surfaces - or by a growth habit which provides a poor target, such as small leaves.
Species also vary in the speed they react to glyphosate. Most notable is the slower symptoms evidenced on broad-leaved plants when compared to grasses.
Resistance
A plant is only defined as resistant to glyphosate when dose rates, which used to give good control, no longer work and this characteristic is heritable.
This is exceptionally rare and, although there is documented resistance in eight plant species (15 biotypes) across the world, there are none recorded in UK.
However, we must not be complacent and users need to be aware of the risk, especially if they continually use glyphosate and no other active method of weed control - for example, on railways, perennial plantings and plant-free areas. The chances of resistance developing are higher if use is in sub-optimum circumstances.
Resistance is not the same as tolerance, where some species are not controlled by leaf uptake of label doses, usually due to poor uptake through waxy cuticle. But, unlike resistant plants, these species can be controlled if glyphosate is directly applied by methods such as stump treatment, chemical thinning or stem injection.
Growth stage and timing
Glyphosate only controls weeds present at the time of application, and has no effect on seedlings which emerge after treatment.
Operators should ensure that weeds are green and growing to ensure uptake and translocation. Annual weeds should have at least one pair of true leaves and grasses 5cm growth before spraying begins.
Activity/stress
Reduced metabolism leads to reduction in transport to the growing points and will give poor results.
Stress can be from extremes of temperature, seasonal die-back, waterlogging, drought or systemic disease like viruses.
Grasses - two weeks Grasses - four weeks
Worth waiting for ... seasonal conditions may mean that glyphosate products take longer to work. Results at two and four weeks on dandelion, nettles and grasses
PC JUNE/JULY 2015 I 125 Nettles - two weeks Nettles - four weeks
Weeds under stress have a reduced metabolism which leads to reduction in transport to the growing points and means poor results. Stress can be from extremes of temperature (hot or freezing), seasonal die-back, waterlogging, drought or systemic disease like viruses
Treatment of Japanese Knotweed as the stem is extending rapidly to produce flowers - often in early spring - will mean most glyphosate goes to the growing points in the developing stems, leading to a rapid dieback of the stem, but little movement downwards and subsequent re-growth from the base
Dandelion - two weeks
Dandelion - four weeks
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