Understanding Poa annua
September 15, 2023
David R. Huff, Ph.D., Penn State University
Poa annua can produce an exceptional putting surface given the right conditions. In fact, it’s the primary putting green turf at many U.S. Open courses. So why is Poa annua loved by some and disliked by many, and why is it so difficult to understand what makes this grass tick?
Key Takeaways
· All Poa annua plants are both annual and perennial at the same time, all the time. How it behaves on that spectrum is dictated by the growing environment and management program.
· Properties induced by the parent plant’s environmental growing conditions, like adaptations to mowing, are inherited by its seeded progeny.
· Poa annua adjusts its adaptation differently to different management strategies. This is the root of why it is so difficult to control, and why many have found it better to work with this grass rather than trying to eradicate it.
· Golf course trials are underway to evaluate improved varieties of Poa annua that could become commercially available.
Paul R. Latshaw, the legendary golf course superintendent who hosted a combined nine majors including the U.S. Open, PGA Championship and Masters, once told me: “Poa annua has probably saved more superintendents’ positions than it’s destroyed.” That’s a powerful statement and implies that the invasive nature of Poa annua often outweighs any lack of biotic or abiotic stress tolerance. He shared this insight with me back in 1998. At the time, I’d just started my Poa annua breeding program at Penn State to develop improved varieties for greens and fairways. He called to encourage me to “give it all you got, and never give up!” That’s because he knew what I was about to learn – being a proponent of Poa annua in the golf industry is like swimming against a riptide of disapproving popular opinion, and such rip currents can end badly for swimmers unless you know how to navigate them.
Poa annua, as I’ve come to learn, is not only our most controversial turfgrass species – in that a majority of the industry strive to eradicate it while many of the top-ranked golf courses in the U.S. propagate it – but it’s also one of the most scientifically interesting grasses I’ve ever worked on and it’s one of the hardest to understand agronomically. Poa annua is unlike any of our other turfgrass species, but once you begin to understand some fundamentals of its biology it can become a reliable ally, as Latshaw knew so well. In this article, I’ll attempt to explain the key aspects of Poa annua biology to help superintendents either propagate or eradicate it.
Researchers at Penn State have been exploring the genetics of Poa annua to better understand the biology and adaptability of this enigmatic grass.
Poa annua Unplugged
To begin, let’s examine where Poa annua grows naturally, unaided by golf course management practices. In 18th century England, Poa annua was observed to rapidly fill in the paths where humans regularly walked but was noticeably absent from the adjacent hillside meadows. Because of this habit, Poa annua became known as “path grass” in the U.S. during the early 1900s. Today, Poa annua continues this behavior by dominating hiking trials on Mount Kilimanjaro in Africa, campsites in Australia and dirt roads in China, all while being absent beyond a yard’s distance from the edge of these disturbed areas. Thus, Poa annua’s presence seems to be a biological indicator of human activity.
In addition, Poa annua has very few herbivore resistance mechanisms and serves as a common food source for grazing animals and migratory waterfowl like ducks and geese. Knowing its association with human travel, combined with its beneficial feed value for animals and birds with regional and global migratory routes, leads us to the unsurprising result that Poa annua has a world-wide distribution. However, what is surprising and what we need to learn is why Poa annua is so broadly adaptable to such a wide range of environmental conditions around the globe.
One of the main stumbling blocks that most superintendents and scientists have regarding Poa annua’s basic biology that limits their ability to either manage or control it is they’ve been taught that Poa annua exists in only one of two forms, it’s either annual or perennial. In fact, all Poa annua is both annual and perennial at the same time, all the time. Let me explain.
Poa annua has the genetic programming to behave like an annual and as a perennial depending on the situation.
Genetically, Poa annua is an allotetraploid species, which simply means that each of its cells contains the genetic material (genomes) of two different species. For Poa annua, these genomes have not had sufficient time to become blended together. The “mom” of Poa annua is Poa infirma, a truly annual species native to the Mediterranean where it grows and sets seed during the mild, moist winters and survives the cloudless, rainless summers as seed. The “dad” of Poa annua is Poa supina, a stoloniferous perennial species native to the mountainous regions of central Europe where it’s often managed as soccer fields and golf course fairways. Thus, in every cell of every Poa annua plant is the genetic programming to behave as an annual (like mom) and as a perennial (like dad), and in reality, it behaves as both at the same time, all the time.
Poa annua’s Superpower
Having annual and perennial life history characteristics in each of its cells enables Poa annua to exhibit some truly remarkable behaviors. For example, inheritance of “annuality” versus “perenniality” in Poa annua does not follow the three laws of Mendelian genetics – i.e., dominance, segregation and independent assortment – but rather by a new field of genetics known as “epigenetics” (La Mantia & Huff, 2011). In addition, management practices like mowing not only influence the growth habits of the mowed plants, but also influence the growth habits of their next generation’s seed progeny (Mao & Huff, 2012; Benson et al., 2021). Such phenomenon is known as “transgenerational memory," which is when epigenetic properties induced by the parent plant’s environmental growing conditions, like mowing, are being inherited by its seeded progeny. In other words, Poa annua plants “remember” if their parents were mowed or not. As I said, Poa annua is scientifically very interesting.
"In other words, Poa annua plants “remember” if their parents were mowed or not."
Finally, the underlying genetic programs of annuality and perenniality inherent in Poa annua have not had sufficient time to have become blended together but rather remain distinct and operate simultaneously under contrasting environmental conditions (Benson et al., 2023). Thus, it may be surmised that Poa annua’s “superpower” stems from a non-segregating, supercharged hybrid vigor for all of its life history characteristics. Hybrid vigor, or heterosis, is a powerful genetic feature resulting in enhanced performance and happens to be the basis of the modern U.S. corn industry. However, unlike corn where the effects of heterosis are lost after one generation due to segregation, heterosis in Poa annua does not segregate away and is thus maintained generation after generation.
As a result of its permanent heterosis, Poa annua has the ability to outperform either of its parental species. Poa annua can be a more successful annual than its annual parent (Poa infirma) by producing more seed at a faster rate, and it can be a more successful perennial than its perennial parent (Poa supina) in extreme environments, like low putting green mowing heights. In addition, because Poa annua has this supercharged hybrid vigor permanently built into its genetic makeup, it also has a supercharged ability to adapt to very different environmental conditions. For example, in the non-irrigated,
Mediterranean climate of California, Poa annua functions more successfully as an annual. However, in the consistently dry, windswept regions of Antarctica, Poa annua functions more successfully as a perennial. What’s truly remarkable is that this genetic potential for such extreme and varied adaptation is inherently present in each and every cell, of each and every Poa annua plant.
How To Manage Poa annua
Given this new understanding of Poa annua’s basic biology, let’s consider how it might be applied for its encouragement or discouragement on greens and fairways. For management purposes it’s more useful to think of Poa annua as having two different modes of reproduction: sexual reproduction via seed which is an annual life history characteristic, and vegetative reproduction via tillers which is a perennial life history characteristic. Poa annua modulates between these two modes of reproduction differently depending on the environmental conditions it encounters.
Whether Poa annua is considered a weed depends on the situation. This plant growing in a ‘419’ bermudagrass fairway in Florida needs to be eradicated.
The two main management parameters that guide Poa annua adaptation are mowing and disturbance. Without mowing, Poa annua has a difficult time outcompeting other grass species, it is after all “path grass.” However, under mowing heights of a few inches down to less than one-tenth of an inch, Poa annua proportionally increases its production of daughter tillers allowing it to compete against other grasses. When these mowed turfs are disturbed by core cultivation, vertical mowing, traffic and other stresses, Poa annua’s ability to flower and set seed under any mowing height gives it a competitive edge by quickly filling in the resulting voids with seed. Without regular disturbance, the opportunity for seedling establishment is reduced and Poa annua begins to reallocate its photosynthetic resources away from seed production and more toward vegetative daughter-tiller production, resulting in very high shoot density plants that are very low seed yielders. Thus, the type of Poa annua you have will depend on how you manage your playing surfaces.
Superintendents first need to determine the function they want their Poa annua to serve and then manage for that function, because Poa annua will adapt itself to any set of management conditions. When management plans are unclear and inconsistent, a highly variable population of Poa annua will result. Additional factors such as temperature and available soil moisture throughout a growing season will also tell Poa annua at which point along its spectrum of annual and perennial life history characteristics it needs to be. Therefore, these additional factors also need to be considered. For example, on non-irrigated bermudagrass fairways in Australia, Poa annua behaves as a winter annual and is actively managed by Australian superintendents as a low-cost winter playing surface, a technique once practiced long ago in the southern U.S. Don’t get caught up in wondering whether you have the annual or the perennial Poa annua, because unlike what we’ve been taught, it is not one or the other but rather a spectrum. The more appropriate questions to ask are, “How is Poa annua adapting to my current management practices and how can I best change my management strategies to either propagate or eradicate it?”
It should be clear by now that no matter what you do or how you manage your mowed playing surfaces, Poa annua has the ability to adapt. Thus, eradicating Poa annua is an extreme challenge and according to the International Herbicide-Resistant Weed Database, it has developed herbicide resistance to nine modes of action involving 23 different active chemistries, so chemical control seems limited. The best Poa annua-free bentgrass courses I’ve visited all have a “zero-tolerance” policy and physically remove each and every Poa annua invader with a knife or plugger and then transport it away from the playing surfaces.
"It should be clear by now that no matter what you do or how you manage your mowed playing surfaces, Poa annua has the ability to adapt."
Dr. Joe Vargas, a renowned turfgrass pathologist at Michigan State University, once said, “If Poa annua is so weak, what happened to your bentgrass?” Which was his way of letting people know that not all Poa annua is intolerant of stress. In my Poa annua breeding program, we’ve discovered naturally occurring resistance to biotic diseases like anthracnose, dollar spot and bacterial wilt along with sufficient variability for tolerance to abiotic stresses like heat, cold and salinity. These are the elements necessary to breed improved stress-tolerant varieties. While I fully appreciate how difficult it is to control, clearly one of the most significant problems with Poa annua is that there are no commercial seed sources of improved varieties for superintendents who want to actively manage it, but that situation is about to change. Currently, trials are underway at five golf courses across the U.S. to evaluate the performance of the commercial variety ‘Pa-33’ Poa annua. Highlands CC is one of the 5 golf courses.
A Final Thought
The amount of information remaining to properly discuss the management and control of Poa annua would fill a book, but unfortunately I’ve reached my editor’s word limit and so I’ll finish with a quote from Dr. Leon C. Megginson, a Louisiana State University business professor, who wrote: “According to ‘Darwin’s Origin of Species,’ it is not the most intellectual of the species that survives; it is not the strongest that survives; but the species that survives is the one that is able best to adapt and adjust to the changing environment in which it finds itself.”
Poa annua’s heterosis superpower, which underlies its extraordinary ability to adjust its annual and perennial life history characteristics to changing environmental conditions, supports Megginson’s statement perfectly. This is the central feature of its world-wide distribution and serves as the foundation for the challenges and opportunities superintendents face in either its propagation or eradication. This article is based on research funded by the USGA’s Mike Davis Program for Advancing Golf Course Management and the Pennsylvania Turfgrass Council.
