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Plant Growth Regulators

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Plant growth regulators (PGR) are organic compounds, either synthesized in the plant or as an applied substance, that in very low concentrations can either increase or decrease plant growth. New York State regulates PGRs as pesticides, as many herbicides are classified as growth regulators and therefore, they are registered with the EPA.

Plants create biomass (leaves, stems, roots, flowers) by producing new cells from existing cells that divide (cell division). Cell division increases the number of cells. Once the plant has new cells, these cells must stretch or elongate to make new organs (leaves, roots, flowers) by a process called cell elongation. These processes are indirectly regulated by a plant hormone, gibberillic acid (GA). As GA levels increase, growth (division & elongation) occurs and the plant creates new biomass.

Turfgrass growth regulation began as an idea some 50 years ago in an effort to reduce mowing. One of the first products used successfully in turf was maleic hydrazide (MH). MH suppressed foliar growth and seedhead formation of roadside vegetation. More recently, mefluidide has been used primarily for Poa annua seedhead suppression on golf courses. Mefluidide is absorbed by the leaves, most effectively at the base of the leaves, and does not move through the plant. This makes thorough spray coverage essential. Once it penetrates the leaf it begins to primarily affect cell division with a lesser influence on cell elongation. Consequently, when it contacts a flowering stem (Poa seedhead) during formation, it causes it to distort and prevents seedhead emergence from the leaf sheath. Products with this activity are classified as Type I growth regulators, of which mefluidide is the most common.

Type II growth regulators act by influencing cell elongation, primarily through inhibiting GA synthesis. Products in this class include flurprimidol, paclobutrazol, ethephon and trinexapac-ethyl. Because these products primarily affect cell elongation and not division, the number of new cells is only slightly reduced while their ability to elongate is significantly altered. Increased number of smaller cells explains the altered morphology of the turf leaves (wider leaf blades) and the short stumpy appearance of the plants (reduced internode length – the distance between new leaves). There is also some experimental evidence that indicates stolons of treated plants become more prostrate and rosette-like (like a witch’s broom).

Flurprimidol and paclobutrazol are both primarily root absorbed while trinexapac and ethephon are foliar absorbed. This is a significant strategic aspect in terms of the length and flexibility of regulation. The other significant factor in the use of PGRs is that they exhibit different degrees of regulation depending on the turf species.

The use of growth regulators in turf can cause significant reductions in turfgrass quality if not applied at the proper timing and at the proper rate to the proper turfgrass species, much like an herbicide. The most common uses of plant growth regulators are for mowing management, annual bluegrass seedhead suppression, and to selectively suppress and reduce populations of annual bluegrass.

Mowing Management

Plant growth regulation to reduce elongation of turf leaf blades can extend the time periods between mowing. Theoretically, this could reduce mower wear and tear, reduce fuel consumption, and minimize clipping handling problems. Additionally, with growing concern regarding carbon emissions contributing to climate change any reduction in mowing would be viewed as moving toward sustainability.

The effective use of plant growth regulators for mowing management depends on many factors: timing of the application (the earlier in the season the PGR is applied the greater the reduction), the turf species being treated (generally ryegrass and tall fescue require higher rates than Kentucky bluegrass and creeping bentgrass), and the rate and frequency of application.

In general, plant growth regulators can reduce mowing requirements by 40 to 60 percent without significant reductions in turfgrass quality. Environmental stress such as heat or cold can compromise turfgrass quality during regulation and should be considered prior to implementing a mowing management program.

Recent research has suggested the use of growing degree day models to proper time PGR application. For example, by maintaining a consistent physiological interval the growth of the turf can be uniformly suppressed without the associated “rebound” effect. The rebound effect is described as turf growth that exceeds the growth of an untreated plant. Specific research with trinexapac indicates applications made on a base 32 GDD model every 200 units will maintain uniform suppression both for golf turf ball roll distance and general mowing reduction.

Annual Bluegrass Seedhead Suppression

As a winter annual weed, annual bluegrass is triggered in the spring to shift emphasis from vegetative growth (leaves, stems, roots) to reproductive growth (flowers that bear seedheads). This shift has biological and physical consequences. Biologically, tillers that produce a seedhead will die off resulting in a natural thinning of the stand. Physically, seedhead production when plants are mowed at or below 1/8 inch reduces the playing quality of putting surfaces.

There is a continuum of life cycle types of annual bluegrass found on golf courses. It ranges from the weedy winter annual type that grows in clumps and produces enormous amounts of seed to the perennial type that grows laterally and produces less seed. Putting greens in northern climates consist of annual types for several years and then almost exclusively perennial types after 15 to 20 years.

The key to high quality annual bluegrass putting surfaces in the spring is effective seedhead suppression. Specific plant growth regulators are able to suppress the seedhead and not kill the plant. In fact, studies have found that by not producing seedheads the plants are able to divert energy that would be used to form a seed and use it for overall plant health.

The traditional method of seedhead suppression in the spring in northern climates is performed with mefluidide and ethephon. Independent of the product used for suppression, application timing is the critical factor that determines the intensity and duration of seedhead suppression.

Research has shown the ideal timing for seedhead suppression with ethephon appears to be between 400 and 600 base 32 growing degree days and ideal timing for mefluidide is 500 to 650. The inclusion of trinexapac in a tank mix combination seems to extend the application window as well as the duration of suppression with both products. In most areas of New York, only two applications will be necessary for adequate suppression. Ideal timing for seedhead suppression programs can be monitored during the growing season at the ForeCast website

Mefluidide treated putting greens often sustain some injury, mostly to creeping bentgrass. Tank mixing mefluidide with iron generally reduces bentgrass injury; however, mefluidide rates often need to be increased to compensate for slight antagonism. Also, tank mixing mefluidide or ethephon with trinexapac has been shown to reduce the incidence of basal rot anthracnose. However, trinexapac alone does not suppress annual bluegrass seedheads.

Ethephon does not appear to produce the same amount of injury as mefluidide. However, there have been reports of “false crowning” of plants treated more than twice that can increase the likelihood of scalping. Again, tank mixing with trinexapac seems to minimize this problem.

Enhancing Annual Bluegrass

The predominance of annual bluegrass on golf and sports turf has led many to simply accept its invasion and develop cultural management programs to enhance performance. Annual bluegrass is a shallow-rooted, cool season grass that grows well in spring and fall, but suffers severe summer stress and winter injury. In addition, it is susceptible to a number of fungal diseases that require regular fungicide applications to maintain a quality turf.

Ideal management programs include regular fertilization, especially in cool months of the year, as well as light and frequent watering practices. Annual bluegrass will often thrive under normal mowing programs; however, mowing heights below 1/8 inch have been shown to increase the likelihood of basal rot anthracnose.

Trinexapac has been shown in many studies to improve the health and performance of annual bluegrass stands. Programs that make multiple low rate applications throughout the season have been shown to be most effective. In fact, it is vital to sustain an application protocol during the season as studies have shown that ceasing applications results in a “growth rebound” and compromises annual bluegrass health.

Current research indicates that trinexapac is rapidly metabolized under high temperatures when applied to the plant during summer months. This has often required increased rates or decreased application intervals. Research at the University of Wisconsin-Madison indicates application of Primo should be spaced no more than 200 base 32 growing degree days apart. Increasing the interval more than 200 GDD allows for the plant to release from regulation, thereby altering performance and plant health.

Selective Annual Bluegrass Suppression

Plant growth regulators have been shown to reduce annual bluegrass on golf turf without any significant disruption of play or reduction in turf quality. The benefits of less annual bluegrass include reduced winterkill, no unsightly seedheads, reduced N requirement, and a reduced severe disease spectrum.

The Type II PGR’s paclobutrazol and flurprimidol have been shown to be the most effective in reducing annual bluegrass populations over a period of time. Higher cut creeping bentgrass turf on fairways tends to be a more conducive environment for reducing annual bluegrass compared to putting greens and tees with more chronic and focused surface disruption.

The most effective programs include multiple applications throughout the season that provide a cumulative reduction. Flurprimidol and paclobutrazol programs have been shown to reduce fairway populations as much as 70 percent in two years. This type of success is usually achieved when a comprehensive cultural management program of reduced fertility and irrigation plus overseeding programs to favor the more hardy and desirable creeping bentgrass turf are used.

Use Growing Degree Days to Improve Growth Regulator Performance

The most commonly applied PGRs used on putting greens are *NYPrimo Maxx (trinexapac-ethyl), *NYTrimmit (paclobutrazol), and Cutless (flurprimidol). These products alter growth rate in
two distinct phases. Following PGR application clipping yield becomes suppressed relative to non-treated turfgrass; the suppression phase. After a period of time the suppression phase ends and clipping yield increases to a level greater than non- treated turfgrass; the rebound phase. Researchers have found that the duration of the suppression phase is dependent upon air temperature. 
As air temperatures increase into the summer the length of
the suppression phase decreases. This occurs because turfgrass plants breakdown PGRs, such as *NYPrimo Maxx, faster as air temperatures increase. This means that calendar based PGR re-application intervals are not efficient at maintaining yield suppression because the ideal re-application interval changes during the course of a growing season.

Growing degree day models are used extensively in traditional agriculture to estimate crop growth and development in relation to air temperature and recently have been used to estimate weed growth and development in turfgrass, i.e. Poa annua seed head formation ( To calculate GDD he high and low air temperature are averaged together, subtracted from a base temperature where metabolism is minimal, and added to values from the previous days.

A recent study measured daily relative clipping yield from a creeping bentgrass putting green treated with *NYPrimo Maxx every 100, 200, 400, and 800 GDD as well as every four weeks. The GDD was calculated in degrees Celsius with a base temperature of 0°C and began after the previous *NYPrimo Maxx application. After the GDD threshold had been surpassed
 (i.e. 200 GDD after *NYPrimo Maxx application), Primo was re-applied and the model was reset to zero. A spreadsheet program such as MS Excel can be used to track the progression of GDD
after PGR application and convert temperatures to Celsius. Temperature °C=Temp °F-321.8

It was determined that the 400 GDD, 800 GDD, and four week re-application frequency did not maintain season-long yield suppression. Rather the 100 and 200 GDD re-application frequencies maintained season-long yield suppression. The 100 GDD re-application interval resulted in a greater level
of yield suppression than the other treatments. The 200 GDD re-application interval is the furthest *NYPrimo Maxx re-application interval to maintain yield suppression because the yield begins to transition into the rebound phase after 200 GDD. During a heat wave with high temperatures of 100°F and lows around 75°F (average day temp. 89°F) 200 GDD occurs in 7 days or less. This illustrates how *NYPrimo Maxx re-application interval needs to be adjusted depending upon air temperatures to avoid the rebound phase. As temperatures warm into the summer, Primo needs
to be re-applied more frequently than it does in spring and fall to avoid the rebound.

The 200 GDD re-application
interval is only meant for *NYPrimo Maxx applications to creeping
bentgrass golf putting greens. Taller
 mowed turfgrass such as Kentucky bluegrass athletic fields are 
more sensitive to *NYPrimo Maxx and would have a different Primo
GDD threshold. Some preliminary research on Poa annuaputting greens found that the 200 GDD re-application interval
is effective at maintaining yield suppression of Poa. We also have found that 200 GDD applications to mixed bent/Poa green
decreased the Poa annua population from 23% to 16% of the surface.

The GDD threshold for *NYTrimmit application to creeping bentgrass and Poa annua golf putting greens is 300 GDD re-applications (base °C) maintained yield suppression during the growing season for both grass species. After approximately 350 GDD the turf entered the rebound phase. Use these new models to determine best application frequency for PGR use.

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