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Pre-emergents 101

Thursday 27, Aug 2020

With cool and wet conditions prevailing so far in 2020 and with it the early and prolonged emergence of Poa annua, some turf managers have noticed a lack of efficacy with their pre-emergent herbicide applications.  John Neylan takes a closer look at how these important chemicals work and some of the factors that can impact their success. 

Once the COVID-19 lockdown came into being, I found that I had some extra time on my hands to contemplate all things turf.  It provided the perfect opportunity to take the time to catch up on some long overdue education and reading.  It was also a good time to think through some of the more perplexing questions as to why certain turf management strategies do or don’t work.  One that I have spent a lot of time working in has been weed control which continues to throw up numerous anomalies around the reasons why a herbicide strategy may or may not have worked. 

During the enforced break I took the opportunity to work through a seminar series developed by WeedSmart ( which is an industry-led initiative to promote best practice as it relates to herbicide resistance management.  While WeedSmart has been developed for Australian cropping systems, the basics around herbicide resistance, how does resistance develop and how herbicides work are all relevant topics to turf. 

The two courses that I have found most useful are ‘Herbicide Resistance 101’ and ‘Pre-emergent Herbicides 101’.  Having previously undertaken the former, I was particularly interested in the topic of pre-emergent herbicides as I have recently seen what appear to be ‘failures’.

Pre-emergent herbicides have become the most important element in an effective weed control programme.  If the weed can be controlled before it becomes established, the problem can be largely avoided.  With the developing resistance to a wide range of post-emergent herbicides, a preventative strategy makes sense.  However, we do see examples of where a pre-emergent herbicide may have failed in preventing a weed from developing.  Is it due to herbicide resistance, leaching, breakdown, attachment to organic matter or other unknown factors? 

Turf presents a unique set of circumstances compared to agriculture in that it provides a permanent vegetation cover, there is thatch to deal with and turf is irrigated regularly, to name but a few differences.  However, while there are differences, I was hopeful that by having a better understanding of how pre-emergent herbicides work may cast a light on my lack of knowledge.

The ‘Pre-emergent Herbicides 101’ course is presented by weed scientist Dr Chris Preston (University of Adelaide) and there were several key concepts that I noted and I have tried to relate to the turf situation for the purposes of this article.

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This article was originally published in Volume 22.3 of the ATM.