Before we discuss what is required to achieve success in managing Winter Grass in turf we must first define what a weed is. A weed isn’t a weed unless it’s where you don’t want it. Winter grass is also known as Poa annua is regarded as a weed. There are however many examples where golf course greens are 100% winter grass and it provides an excellent putting surface.

The key to managing winter grass in turf is by a combination of cultural and chemical means. With a healthy full cover of turf, there is little opportunity for winter grass to become established.

Where it does become a problem there are several options available for winter grass removal but first, you need to correctly identify it. The schematic above is the best I could find showing the life cycle of winter grass so thanks to Barenbrug for this. If anyone knows any better ones please feel free to let me know?

Identification.

Winter grass (Poa annua) is an annual having both perennial and biennial biotypes. It has a light green, generally tufted growth habit being easily identified by its seed head and having a boat-shaped leaf tip that curls up at the ends.

Why is winter grass a problem?

Its adaptability is one of the key issues affecting why managing this can be such a problem.  Depending on soil moisture levels it can thrive in both shade and full sun is very tolerant of low mowing heights and can persist all year round. Plants reach maturity quickly and seed profusely making them an aggressive weed that can be difficult to control. This weed grass can dominate turf that is weakened by wear (leaving thin or bare areas), deficient in nutrients, excessively wet, and growing in compacted soil. It is likely that the seed is brought in by various means (wind, footwear, unclean machinery, contamination, etc.) and has a preference for a soil pH of 6 to 7.

The key to getting rid of winter grass is cultural control. 

Certain factors favour this weed grass so managing these can help prevent the spread of this weed.

Soil pH – acidification.

•  Winter grass grown in sand with a pH of 6.5 has a much stronger root growth than if grown at pH 5.0. (Sprague and Burton,1937)
•  Winter grass does not germinate at pH 3.6 whilst 66% of the seed germinates at pH 5.2. (Ferguson, 1936)
• found winter grass produces twice as many roots at pH 6.5 compared to pH 4.5 while also producing four times as many seed heads at pH 6.5. (Juska and Hanson, 1969)

Soil Phosphorus – reduce the levels.

• Goss et al (1975) found that P applications significantly increase winter grass populations in a mixed Poa/Agrostis sward.

Soil Sulphur.

• Applications of elemental sulphur reduce winter grass to only 7% in comparison to 30% in the treatments with no sulphur (Sprague and Burton, 1937).
• Varco and Sartain (1986) found that sulphur applications only acidify the top 2.5cm of the soil resulting in decreased winter grass emergence and establishment.

What is on the market for managing winter grass in turf?

There are now several products on the market for winter grass control that are classified as either pre-or post-emergent herbicides. The former is not effective for controlling the perennial types as these do not come back from seed.

Barricade® (prodiamine), Ronstar® (oxadiazon), and Specticle® (indaziflam) are examples of pre-emergent herbicides while post-emergent herbicides include Nominee® (bispyribac), Coliseum® (rimsulfuron), and Poacure® (methiozolin).

The following is an overview of the options currently on the market. If you require more information check out the turf product labels and Technical Notes or please feel free to contact us directly. At the end of this article is a comprehensive table showing all currently Australian registered winter grass pre and post-emergent herbicides.

Pre-emergent options for managing winter grass in turf

These act by creating a barrier at the soil surface which prevents root growth with the exception to this being oxadiazon which acts only on shoots. They should only be applied to established turf and assuming no resistance issues exist, most will provide a similar level of control if applied before germination and are sufficiently watered in. The major difference will be the longevity of the control which can vary dramatically.

Dithiopyr (Dimension®)

• Applications can be made 10 days after emergence with no injury to creeping bentgrass seedlings and
• Do not overseed, reseed or sprig treated areas within 3 months (6 months if 3.5 L/ha is used)

Ethofumesate (Tramat®/various)

• This is a selective herbicide that controls winter grass in cool-season grasses and overseeded couch. It provides both pre-and post-emergent control and
• Overseeding should not take place within 1 month of spraying.

Indaziflam (Specticle®)

• Aim to irrigate with 3-6mm water as soon as possible after application as adequate soil moisture is needed to activate the chemical and provide maximum control;
• As a pre-emergent herbicide, Specticle® should be applied before weed seeds germinate and
• The minimum interval between application of Specticle® and overseeding is 12 months.

Metalochlor (Pennmag®/Meteor®)

• Soil texture and moisture play a major role in the longevity achieved although shorter periods of herbicide activity may be observed in sandy soils with regular irrigation or rainfall;
• The product must be watered in with 2-4mm of irrigation and
• It gives up to 8 weeks control.

Oryzalin (Prolan®)

• Do not use this on newly seeded turf areas;
• Allow at least 10 weeks between the last application and overseeding and
• Avoid applying to stressed turf.

Oxadiazon (Ronstar®/various)

• This controls winter grass by killing the young seedlings as they come in contact with the herbicide;
• If any winter grass seed has germinated, oxadiazon will not provide control;
• To get the best results, water as soon as possible after application;
• It gives up to 10 weeks control and
• Allow 4-5 months before overseeding.

Prodiamine (Barricade®/Various)

• See graph below
• Do not apply to newly seeded, sodded, or sprigged turf. Delay application until turf is at 100% cover and
  the root system is developed beyond a 3 cm depth.
• One application gives up to 6 months of control.
• Aim to apply in at least 500 L water/Ha.
• Do not reseed for at least 6 months.

Pendimethalin (Battalia®/Various)

• See graph below
• Aim to incorporate within one day of the application using 10-15 mm of irrigation or when rainfall is expected.
• Soils containing a high percentage of organic matter can result in poor control and consequently application to soils having greater than 6% organic content should be avoided.
• It has a 12 hour re-entry period.
• Pendimethalin will stain (yellow) any objects it contacts.
• One application provides up to 60 days of pre-emergent activity.

Post emergent options for managing winter grass in turf.

Amicarbazone

Like simazine and atrazine this is a triazine herbicide with trials in the USA showing it to be very effective against winter grass in bentgrass. It is marketed in Australia as Xtron.

• Use lower rates on lighter textured soils;
• On overseeded couch, amicarbazone applications in the spring result in greater winter grass control and greater perennial ryegrass tolerance;
• Factors such as placement, temperature, and application timing may affect the results gained in getting rid of winter grass;
• Tall fescue is more tolerant to amicarbazone than winter grass at moderate temperatures of 20-25°C as a consequence of less absorption and an increase in plant metabolism;
• Do not apply if the soil pH is > 7.4 and
• If the air temperature is going to be above 27 ºC your are likely to get unacceptable turf injury on cool-season turf.

Endothal (Poachek®).

In work carried out by Neylan and Nickson (2015) in Couch and Kikuyu, they determined that effects occur in the first day or so after application.

Ethofumesate (Tramat®, various).

This gives effective pre-and early post-emergent control of winter grass in new seedings of perennial ryegrass and is readily absorbed by emerging shoots and roots and translocated to the foliage.

• The germination of cool-season grasses other than perennial ryegrass is inhibited;
• It is highly toxic to seedling fine fescues;
• Surface organic matter decreases its effectiveness so remove thatch before applying;
• It isn’t recommended for turf with more than 70% winter grass;
• Do not use during periods of extreme stress such as heat, cold, and drought and
• When used at high rates, it may cause temporary discolouration in Kentucky bluegrass and bentgrass turf .

Methiozolin (Poacure®).

• This is safe to use on all common turfgrass;
• It is very slow-acting, giving a gradual transition from winter grass to bentgrass;
• Both foliage and roots absorb the chemical but it mainly shows acropetal movement. When made as a root application, methiozolin translocates from the roots to the crown;
• After two years of single treatments in spring and autumn, winter grass cover was 34% in treated plots and 56% in the non-treated (Askew and McNulty 2014);
• Both soil-plus-foliar and foliar-only treatments are more effective than soil-only treatments;
• Control on a soil-based root zone ranged from 72 to 80% in contrast to on a sand-based root zone where control ranged from 57 to 64% (Brosnan et al. 2013a, Flessner et al. 2014).

The following may lead to an acceleration whem managing winter grass in turf using methiozolin.

• Increasing the rate or frequency because of you think its working too slowly;
• Calibration errors or spray mistakes that increase the application rate;
• Too many treatments in the autumn or winter;
• Heavy or prolonged rainfall after application;
• Mechanical treatments such as aeration which stress the turf;
• Using ethephon followed by mild to moderate stress;
• Extreme environmental stress such as drought or heat and
• Using it along with other herbicides/plant growth regulators (PGRs) such as paclobutrazol.

Propyzamide (Revere®, Various).

The Neyland and Nickson trials in 2015, found that after 5 weeks that there was no significant change in the winter grass population. One of the idiosyncrasies of this chemical is it is very slow acting although it does tend to work well in cooler weather in contrast to the sulfonylureas. However, propyzamide has inconsistent performance especially in soils of greater than  >4% organic matter, and exhibits a high degree of soil mobility.

Sulfonylureas (Various)

These are used as “late transition aids” in the USA and have become important tools in managing winter grass in turf and also as ryegrass transition aids. Sulfonylureas all work in a similar way and being in the same chemical grouping means that switching from one active to the other will not prevent chemical resistance from developing. Consequently, relying entirely on these for winter grass control in warm-season turfgrass is not recommended (McElroy et al. 2013).

They are all safe to use on couch but only iodosulfuron is safe to use on Kikuyu. This group includes  bispiribac-sodium (Nominee®​), rimsulfuron (Coliseum®), foramsulfuron (Tribute), iodosulfuron (Duke®), and trifloxysulfuron (Monument®).

General usage rules:

• Never apply these within seven days of organophosphate insecticides to avoid excessive herbicide injury (Ferrell et al, 2004).
• Foramsulfuron, iodosulfuron, rimsulfuron, and trifloxysulfuron will be tracked and damage susceptible turfgrass if not allowed to dry before traffic is permitted. Additionally, tracking can occur from morning dew the day after treatment so irrigating on the morning after application and leaving a 5-metre buffer area around cool-season areas are recommended to prevent tracking problems.
• If the tank water pH is less than 5.5 use a buffer solution to raise the pH to 7.0.
• Ensure that complete and even spray coverage is achieved with no overlap.
• Apply with the boom no higher than 50 cm above the turf with flat fan nozzles and a medium droplet size.
• Application to very dry sandy soils followed by excessive rainfall may cause turf damage.
• Turf damage may also be increased in highly alkaline soils having a pH >8.5.

Bispyribac-sodium (Nominee®).

• This is the only SU registered for use on bentgrass golf greens and is marketed in the USA as “Velocity”.
• Bispyribac-sodium can cause excessive injury when applied to creeping bentgrass mowed at 3 mm. (Teuton et al 2007).
• Severe injure to turfgrass can occur if applied at temperatures less than 13 ºC and above 29 ºC (McCullough and Hart 2005).

Foramsulfuron (Tribute®).

• This works best if rainfall or irrigation does not occur within two hours of treatment.
• Ryegrass can be overseeded in treated areas 2 weeks after application.

Iodosulfuron (Duke®)

• This is predominantly a foliar herbicide with less soil uptake and
• Aim to allow at least six weeks between the last application and overseeding with cool-season grasses.

Rimsulfuron (Coliseum®)

• This is root and foliarly absorbed and irrigating one hour after application will move the herbicide into the soil and increase herbicidal effectiveness and
• Overseeding with cool-season turfgrasses should be delayed for 10 to 14 days after application.

Trifloxysulfuron (Monument®)

• Couch and zoysiagrass can be sprigged or seeded 4 weeks after application and
• You should delay overseeding for 6 weeks after application.

Plant Growth Regulators (PGRs) for managing winter grass in turf.

A Plant growth regulator is any compound that alters plant growth and includes plant hormones, herbicides, growth inhibitors, and even bio-stimulants.

Ethephon (Incognito®/Various).

• This is a good tool for significantly reducing seedhead populations of winter grass;
• Timing can be difficult and one method is to make the first application at the first sign of the “boot” stage. This is when the stem is swollen, indicating that it contains a seedhead;
• In a trial conducted on four Virginia putting greens, ethephon when mixed with methiozolin, had an interaction with post coring stress causing creeping bentgrass loss. Therefore, don’t mix methiozolin with ethephon, especially if you expect mechanical or environmental stress on greens; and finally
• It is foliarly absorbed.

Paclobutrazol (Regulate®/Various).

• This inhibits winter grass growth making desirable turf grass more competitive consequently allowing it to crowd out the less desirable winter grass and
• Paclobutrazol is root absorbed.

Trinexapac-ethyl (Amigo®/Various).

• Although an excellent tool for managing winter grass in turf you should not consider using trinexapac-ethyl for getting rid of winter grass;
• Ethephon, when it is tank-mixed with trinexapac-ethyl, is the only effective treatment available for seed head prevention of winter grass;
• Repeated applications of trinexapac-ethyl will enhance the summer tolerance of winter grass;
• This suppresses winter grass growth and allows the desirable grass species to outgrow the weed and lastly
• It increases leaf etiolation caused by Acidovorax avenae subsp. avenae and Xanthomonas translucens.