Unwinter, Poa seedheads and Growing Degree Days, Oh MY!!! – Turfgrass Science at Purdue University

Unwinter, Poa seedheads and Growing Degree Days, Oh MY!!!

As the “un-winter of 2011-2012” rapidly winds to a close golf course managers are already bringing their playing surfaces into peak form. For those managing putting surfaces containing annual bluegrass (Poa annua) this means addressing concerns that seedheads are affecting surface smoothness. With the lack of snow cover this past winter, the seedhead season is already off to a rapid start and seedheads in putting green turf are developing quickly! There are several cultural practices that can help minimize seedheads but treatment with a chemical plant growth regulator (PGR) provides the most consistent suppressive results.


Product choices: PGRs have been used for many years and currently there are not many new chemistries on the market. Historically, the most reliable choices have been; mefluidide (Embark or Embark T/O = less formulation) and ethephon (Proxy, several other brand names) or an ethephon + trinexapac-ethyl (TE:Primo Maxx, T-Nex and many other brand names) tank-mix. Mefluidide is normally a one time application thus, proper application timing is essential to success (unfortunately all plants do not produce seedheads at the same time/rate). Many consider it the “gold standard” for seedhead suppression. Any dissatisfaction with this product has been related to the missing the ideal application window and the risk for turf yellowing, particularly for Kentucky bluegrass surrounds. Many managers have moved away from using mefluidide on putting surfaces but it is still frequently used for higher cut fairway turf.

A popular alternative with very good suppressive properties is the ethephon + TE tank mix. The purported main advantage to the ethephon + TE combination is that the addition of TE with ethephon will minimize the yellowing (a “Granny Smith” apple green color) compared to ethephon applied alone. Additionally, the ethephon + TE combo is involves multiple applications (a second application should occur approximately 21 days following the first) which brackets seedhead development. Thus, many managers report more consistent seedhead suppression with this tank-mix compared to the single mefluidide application.

The use of other plant growth regulators like paclobutrazol, flurprimidol or TE alone, only slightly suppresses or delays seedhead emergence. These PGRs will not, however, minimize the seedheads to the extent of a mefluidide or ethephon + TE tank-mix. The addition of TE with ethephon also enhances turf color, however, research has shown where ethephon applied alone or tank-mixed with TE significant scalping (Dernoeden and Pigati, 2009) can occur when a TE program is not continued. Another consideration with any of the seedhead products would be the inclusion of a chelated iron source. This will further enhance green color and mask any potential PGR discoloration.

Timing: There are a number of managers who still use the “feel”, experience or local knowledge method to determine application timing. Another suggestion has been to initiate a seedhead program based after the second “true” mowing or when the turf is actively growing. While prior experience should still be a part of the decision to initiate, there are also more quantitative methods to help make this decision. Several models are available based on accumulating growing degree days (GDD) or heat units. The GDD models are based on the high and low air temperatures and utilize a “base” temperature. The most common method for calculating GDD is:

GDD = (max temp + min temp)/2 – “base temperature” (normally either 32 or 50 F)

Example: If we had a day with a maximum temperature of 76 and a minimum of 50 and used a base temperature of 50 the calculation would be: GDD = (76 + 50)/2 – 50 = 13, or for this single day 13 GDD. Remember there are no negative GDDs, a negative number is considered a zero and GDDs are summed across time (e.g. GDD day 1 = 3, day 2 = 4, day 3 = 15 would be 22 GDDs). You can do this on your own with a spreadsheet program and the weather data, but in our region (IN, IL, MI, OH) the growing degree day tracker located at www.gddtracker.net is an extremely useful tool that calculates for you.

Differences in base GDD temperatures? In prior years a base temperature of 50 was once used. Research conducted at Michigan State suggested that the GDD 50 base model often overestimated a few early warm days in the season and due to the higher base temperature and low number of GDD units required to reach the application threshold the model quickly passed the application target range. Thus, the GDD50 model is no longer used and a base temperature of 32 now used for the www.gddtracker.net model.

What to do? This spring (2012) GDDs have accumulated quite rapidly. The target for initiating an ethephon + TE program is normally between 200 and 250 GDDs, some suggest a specific number 220 and applications will be out of range at 500 GDD. In other words managers that start once 500 GDD accumulate should expect less than satisfactory results.

For many, in the region it is already time to apply the initial ethephon + TE tank-mix. For example, in Indianapolis this past Sunday (11 March) saw GDD exceed the initial threshold of 200 at 228 GDD and today (15 March) GDDs are predicted to be 320. In Lafayette, the initial threshold was met on Monday (3-12) at 236 GDD and today we will see 300 GDD. Way up north in Bristol the initial threshold will be met today at 212 GDD.

The take home point is that it is time to start treating for seedheads and as I talk to people in Indiana this initial application is about 11 to 14 days sooner than when they applied last year. The models are tools along with experience and as you gather “quantitative information” for your 2012 agronomic plans don’t forget to check out our tools on the Purdue Turfgrass Web-site https://turf.purdue.edu/ as well as other online tools like Syngenta’s Pest Outlook Maps http://www.greencastonline.com/tools/PestOutlooks.aspx


Dernoeden, P. H., and R. L. Pigati. 2009. Scalping and creeping bentgrass quality as influenced by ethephon and trinexapac-ethyl. [Online]Appl. Turfgrass Sci. p. 1-7.

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