With the arrival of cooler fall temperatures, Poa annua control programs have become a common talking point on recent site visits. For obvious reasons, fall preemergence applications are a critical part of a control program. Poa annua control is a complex topic and an entire book could be written on the subject. For the sake of this brief update, a few thoughts on Poa annua control in ultradwarf greens will be shared.
The prevalence and overall pressure of Poa annua in ultradwarf greens varies significantly between courses, even those in the same ZIP code. There are many reasons for this, but the size of the seed bank and possible herbicide resistance issues in each population are often to blame. With herbicides, 100% control is rarely the outcome. For that reason, 90% control of a small population might be acceptable, but if the seed bank is much larger, 90% control could mean a significant infestation. This issue is exacerbated when herbicide-resistant populations develop.
The size of the seed bank can vary based on the age of the greens, history of herbicide programs and overall control of Poa annua in other areas of the course, not just on the greens. The Poa annua that germinates in collars, surrounds and rough might be in those areas for the time being, but the seeds those plants produce will surely find their way onto putting greens.
So I’m sure by now you’re asking yourself, “Enough already, what do I spray to kill this stuff?” Herbicides are certainly part of the equation but here are some other important factors to consider as well:
- Be sure to promote healthy and dense turf in late summer and early fall. Poa annua is an opportunistic plant. If the desirable turf is thin, Poa annua will be happy to fill the void. Raise mowing heights to reduce mechanical stress and limit traffic stress as much as possible.
- There are limited herbicide options for Poa annua in ultradwarf greens so implementing precautions to delay resistance is critical. This starts with proper calibration of application equipment. Product rate, application timing and placement are also key.
- Proper application timing is important for both pre and postemergence products. Don’t expect postemergence applications to be effective against gorilla-sized plants in late winter. Effective programs begin well before plants are visible.
- Tank mix and/or rotate products with different modes of action to reduce the likelihood of herbicide resistance.
At courses with significant Poa annua issues on greens, a successful herbicide program will require several applications using multiple modes of action. Making one preemergence application or continuously applying the same postemergence product is not going to work well in the long term. Relying on just a couple of products or applications is one of the most common reasons for poor weed control.
If you have taken all these steps to optimize efficacy but herbicides are still not providing acceptable control, resurfacing greens may be an option for you to consider. Harvesting the upper portion of the profile removes a significant amount of the Poa annua seed bank, which can reduce weed pressure and help promote healthier turf.
There is no silver bullet when it comes to Poa annua control and it’s unlikely there ever will be. Management requires a programmatic approach that includes factors other than herbicide selection alone. If you have struggled to control Poa annua at your course, feel free to contact your regional USGA agronomist to develop a plan that is tailored to your specific needs. Also, check out the USGA Green Section podcast episode "Poa annua: Problems and Solutions With Dr. Jim Brosnan."
Central Region Agronomists:
Paul Jacobs, agronomist – pjacobs@usga.org
Zach Nicoludis, agronomist – znicoludis@usga.org