Ask Sally - Spraying efficacy

Q. I have used Roundup in an attempt to kill weeds this spring, but control seems patchy. Is there any reason why some weeds have totally escaped? Should I be spraying with something else?

A. Check the sprayer is working properly. But, I admit, I have had patchy results this year too. I blame myself for being too eager, too early. Some of the weeds are eventually succumbing to the chemical, but in the meantime a host more have germinated. In truth, spraying cannot be done by the calendar. It needs to take account of the weather and this year has been problematic.

It is important to understand how weedkillers work to get the best from them. Most sprayer operatives will appreciate that glyphosate works by leaf uptake and translocation to the growing points in shoots and roots. It blocks the biochemical pathway producing amino acids - the building blocks for protein - hence it causes a slow death by starvation.

Plants, of course, need to be healthy and have sufficient leaf growth to absorb the chemical. But what we may not appreciate is that different weeds emerge at different times - the time also varying with conditions. Perennial weeds such as thistles, nettles and dandelions were late to emerge from winter dormancy this year because seed germination was delayed by cold soil temperatures.

Variable weather can also slow the effects of herbicide applications, as Monsanto technical manager Manda Sansom explains: "Even with good leaf coverage, low soil temperatures and high diurnal air temperature ranges result in slow plant growth and mean visible symptoms can take up to four weeks rather than the expected 10-14 days. However, the end result will be the same."

Also, it is important to ensure the weedkiller is not washed off before it has entered the plant. Roundup ProVantage may help. It has been developed for these conditions.

At 480g glyphosate per litre and with an optimised blend of surfactants as well as the high loading, this advanced formulation shows superior performance, including improved rainfastness and speed of uptake. In fact, this formulation is rainfast within an hour for annuals and within four hours for perennials. It reduces the risk of run-off from the unexpected shower that the weathermen failed to mention and ensures maximum glyphosate reaches the roots, where it will be effective.

Furthermore, this formulation has improved reliability in hot and dry as well as cool and dry conditions, helping increase opportunities to spray and be successful.

Sally Drury, Technical editor

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