Poa annua seedheads…a recurring problem!

Poa seedhead with and without

As spring arrives, golf course managers face the recurring issue of Poa annua seedhead emergence. Notable for their quick proliferation at mowing heights below 3 mm, these seedheads, though interesting in their biology, frustrate golfers and greenkeepers alike. The seedheads disrupt the smooth, uniform surface of the putting greens, creating an unwelcome silvery shimmer on the turf. Poa annua seeding not only affects the visual uniformity of putting greens but also impacts playability, a crucial factor for all golfers and turfgrass managers!

Turfgrass managers’ ongoing battle with Poa annua seedheads is driven by the grass’s adaptability and competitiveness in challenging environments, including areas with heavy traffic, compaction, and moisture. Interestingly, Poa annua’s ability to thrive has led it to become the sole example of a successful alien plant invasion in Antarctica, which scientists predict will take over a decade to manually eradicate. Yet, its resilience and playability are evident in many golfing venues worldwide, many renowned for their pure Poa annua greens! The species demonstrates a remarkable ability to adapt and compete, especially under short day lengths and cool temperatures, making its control a complex issue.

Let’s look at the biological cycle of Poa annua, its resilience under varying environmental conditions, and the effectiveness of current cultural and chemical control methods. 

Biology of Poa annua

Poa annua is a winter annual plant that typically starts germinating in late summer or early fall when soil temperatures drop below 70°F (21°C). The seedlings grow through the fall, remain in a vegetative state during winter, and bloom and create seeds in late spring and early summer. The formation of seedheads is the final stage in Poa’s short yet highly productive reproductive cycle. With both male and female flowers on each plant, Poa annua can either self-pollinate or cross-fertilize with nearby plants. 

After pollination of the female flowers, seed development begins. Poa annua is known for its abundant seed production, with flowering and seed formation possible at any mowing height. A single plant can yield over 360 viable seeds, which can lie dormant in the soil for many years before germinating. Flourishing in short days and cool environments, Poa annua can dominate over other turf species in late fall and early spring.

The growth and appearance of Poa annua seedheads are significantly affected by air temperatures, with the optimal range for growth and seedhead development falling roughly between 50°F (10°C) and 70°F (21°C). Variations from this temperature range can lead to slower seedhead development, making their emergence timing uncertain. This variability poses a significant challenge for turf management, especially in predicting annual seedhead appearances. 

The genetic diversity within Poa annua adds to its unpredictability, as it consists of various sub-varieties or biotypes. These biotypes range from annuals that die after a short burst of seedhead production in spring, to perennials that flower later and for longer periods. The broad spectrum of biotypes makes it difficult to forecast the specific characteristics of Poa annua on a particular turf. This diversity underscores the importance of flexible and responsive turf management practices that account for both environmental and genetic factors influencing Poa annua seedhead production.

Controls

Battling Poa annua seedheads is notoriously difficult. Extensive research has been invested in exploring ways to enhance common control methods, identifying several key factors that turf managers must consider before devising the most effective seedhead suppression strategy for their courses. No one method provides 100% seedhead control.  For this reason, all strategies – cultural, chemical, and otherwise, should be employed for optimal results. 

In many legislative regions the use of chemical controls is restricted or banned completely, and the only control option for turfgrass mangers are cultural means. The focus of these controls are not to suppress seeding, but to reduce the impact of seeding on playability and aesthetic appearances.

Cultural Practices

For emerging seedheads, physical removal through grooming/verticutting or brushing offers the only viable cultural control method, enhancing surface smoothness while minimizing harm to the turf. A cultural program of brushing (daily during peak seedhead periods) and/or periodic vertical mowing is typically used to pull Poa seedheads upwards, making it easier to remove them via mowing.

Apply Silica

Several turfgrass managers use foliar applications of Silica to promote vertical growth. Silica is deposited in the leaf cells, cell walls and cuticles resulting in a more erect leaf blade and standing the seed heads upright, allowing for their easier removal when mowing.

Chemical Controls

Chemical control of Poa annua can be attempted with either preemergence and/or postemergence herbicides. Seedhead suppression can be obtained with plant growth regulators.

Preemergence herbicides

Preemergence herbicides, such as dithiopyr or prodiamine, can be used in Poa annua control programs. Application timing is very important, so herbicides must be applied in early fall (early-September) prior to Poa annua germination. A second application will be needed in November or March to control spring germinating Poa annua. This technique may take many years to reduce the Poa annua populations and it will not be effective on the perennial type of Poa annua.

Postemergence Herbicide

A postemergence herbicide, ethofumesate exhibits some residual preemergence control. Two or three applications of ethofumesate applied between September and December are recommended per year. The applications should be approximately four weeks apart. Results are rarely seen that Fall; but are usually observed the following spring. Bispyribac-sodium is labeled for control of Poa annua in creeping bentgrass and perennial ryegrass fairways. Multiple applications at low rates provide effective control of Poa annua. Refer to label recommendations for specific instructions.

PoaCure

Methiozolin in the last couple of years has been approved as a pre- and post-emergence herbicide for selective control of Poa annua in golf course turfgrass in the US. The product is applied at very low rates and permits slow removal of Poa annua with minimal disruption of play. For successful use it is necessary to know the level of Poa annua infestation and its predominant phenotype (annual vs. perennial), the prevalent environmental conditions, and pertinent maintenance practices.

Plant Growth Regulators

PGR’s such as Ethephon stands out as the singular plant growth regulator proven to consistently suppress Poa annua seedheads on golf courses. Its success hinges on various factors, including how well the plant absorbs it. The flag leaf of Poa annua, being highly metabolically active and a primary energy source for flower development, facilitates the most efficient distribution of Ethephon to the seedheads. Absorption by lower leaves tends to result in reduced effectiveness, hence the advice to avoid mowing for 24 hours post-Ethephon application.

The timing and frequency of Ethephon applications are crucial for suppressing seedheads. Studies indicate that including a late Fall application alongside a standard Spring regimen significantly boosts seedhead control. Recommended practices now include three Ethephon applications: one in Autumn, another in early Spring, and a final application three weeks later. Key to the Spring applications is timing them when the majority of Poa annua plants are in the “boot stage,” critical for optimal timing. The initial application should occur at a cumulative 120 GDD (Base = 6 °C) for the annual Poa annua biotype and 160 GDD for the perennial type, with GDD accumulation starting from January 1st. Once seedheads emerge, PGRs become ineffective, and only cultural practices like vertical cutting can mitigate the issue.

Combining Ethephon with other substances

This can further enhance seedhead control and improve turf quality. Mixtures including Ethephon with Trinexapac-ethyl and with mono- and di-potassium salts of phosphorous acid have shown to provide consistent control and boost turf health beyond Ethephon alone. More recently, combining Ethephon with prohexadione calcium in Spring applications has proven to enhance Ethephon’s effect by delaying seedhead production, thereby allowing more time for Ethephon absorption and activity.

Summary

Poa annua seedheads are a common issue faced by golf course managers, especially during spring. These seedheads, which emerge at mowing heights below 3 mm, disrupt the smooth surface of putting greens, affecting playability and aesthetics. Due to the grass’s adaptability, competitiveness, and abundant seed production, controlling Poa annua seedheads poses challenges. The species exhibits genetic diversity, making it unpredictable and requiring flexible management practices.

Cultural and chemical control methods are employed to manage Poa annua seedheads. Cultural practices like grooming and brushing help maintain the smoothness of the turf surface. Chemical controls involve preemergence and postemergence herbicides, with timings crucial for effective control. Ethephon stands out as an effective plant growth regulator for suppressing Poa annua seedheads. Combining Ethephon with other substances has shown promising results in enhancing seedhead control and improving turf quality. However, once seedheads emerge, only cultural practices can mitigate the issue effectively.

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