Managing winter grass in turf. 9

Why is managing winter grass in turf a problem?

Without a doubt, its ability to adapt is the key reason why managing winter grass in turf is such a problem. The ability of winter grass to grow in a wide range of conditions means that it is a perfect weed competitor. It can thrive in both shade and full sun, in a variety of soil moisture conditions and it is very tolerant of low mowing heights. It also persists all year round as a result of being a profuse seeder.

The fact it matures quickly makes it even more difficult to control. Worst of all, this weed grass will dominate turf that is weakened by wear (leaving thin or bare areas), nutrient deficient, too wet, or is in compacted soil. The seed is likely brought in by various means (wind, footwear, unclean machinery, contamination, etc.). In addition, it forms seed banks at the soil surface that contain over 2000 seeds/m2, and can remain viable for years.

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.

How to get rid of winter grass?

Cultual control is the first step to control winter grass in a lawn or a sports ground. Because certain factors favour this weed, it means that by “getting ahead of the curve”, it helps prevent its spread and future establishment.

Soil pH –

Aim to keep the soil pH below 7 because this can play a major role in limiting winter grass establishment and growth.

•  In sand with a pH of 6.5, winter grass has 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)
•  At a soil pH of 6.5 it produces twice as many roots and four times as many seed heads in contrast to pH 4.5 (Juska and Hanson, 1969)
• Applications of elemental sulphur reduce winter grass to only 7% in contrast to 30% in the treatments with no sulphur (Sprague and Burton, 1937).
• Varco and Sartain (1986) found that although sulphur applications only acidify the top 2.5cm of the soil, this resulted in decreases in winter grass emergence and establishment.

Soil Phosphorus –

Maintain low soil phosphorus levels.

• Applying P increases winter grass levels in a mixed Poa/Agrostis sward (Goss et al (1975).

What is on the market for getting rid of winter grass in turf?

Fortunately, there are now several lawn care products available for managing winter weeds on turf. If you use them properly, they will all help control winter grass. They can be classified into two categories: Winter grass pre-emergent herbicides and post-emergent herbicides.

You can use Onset 10GR® (prodiamine), Ronstar® (oxadiazon), and Specticle®  (indaziflam) as examples of pre-emergent herbicides. These are available as liquids or granular pre emergent products. As an alternative, post-emergent herbicides include Poacure® (methiozolin), Nominee® (bispyribac), and Coliseum® (rimsulfuron).

An overview of the available options is provided below. Check out the turf chemical labels or please feel free to contact us directly if you need more information.

Wintergrass pre-emergent herbicides in turf

As a rule these act by creating an invisible barrier at the soil surface which prevents root growth. Winter grass seeds germinate and root down but once they reach this barrier they die. However, the exception to this is oxadiazon which acts only on shoots. Generally speaking these provide a similar level of control if applied before germination and are watered in properly. The major difference will be the longevity of the control. For more information checkout our blog on pre-emergent herbicides.

Dithiopyr (Dimension®)

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

Ethofumesate (Tramat®/various)

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

Indaziflam (Specticle®)

• As adequate soil moisture is needed to activate the chemical and provide maximum control, aim to irrigate with 3-6mm water as soon as possible after application. ;
• Specticle® should be applied before weed seeds germinate although it has been noted that it does have some degree of post emergent control 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 how long this will last. Longevity will be reduced in sandy soils with regular irrigation or rainfall;
• To begin working this needs to be watered in with 2-4mm of irrigation;
• One application can give up to 8 weeks control.

Oxadiazon (Ronstar®/Echelon Duo®/various)

• This acts by killing the young seedling shoots as they come in contact with the herbicide;
• It has no post emergent control of winter grass;
• To get the best results, water as soon as possible after application;
• One application gives up to 10 weeks control and
• Allow 4-5 months before overseeding.
• Echelon Duo® contains both oxadiazon and a residual insecticide in one combination product.

Prodiamine (Barricade®/Onset 10GR®)

• See graph below;
• Do not apply to newly seeded, sodded, or sprigged turf and delay its use until turf is at 100% cover and the root system is beyond a depth of 3 cm;
• Gives up to 6 months of control;
• Apply in at least 500 L water/Ha;
• Don’t seed for at least 6 months.

Pendimethalin (Battalia®/Various)

• See graph below;
• Water in within one day of the application using 10-15 mm of irrigation or when rainfall is expected;
• In soils with a high percentage of organic matter, it can give poor control and so avoid applying to soils having greater than 6% organic content;
• It has a 12 hour re-entry period;
• Pendimethalin will stain (yellow) any objects it contacts;
• Provides up to 60 days of control.

Pre-emergent herbicides for managing winter grass in turf

Post emergent winter grass killers in turf.

Amicarbazone (Xtron®)

Like simazine and atrazine this is a triazine herbicide. Trials in the USA show it to work well against winter grass in bentgrass, although it is actually a bit “fiddly” to use.

• Use lower rates on lighter textured soils;
• On overseeded couch, spring applications give greater winter grass control and the perennial ryegrass has greater tolerance;
• Placement, temperature, and timing may affect the results gained;
• You can use amicarbazone on tall fescue at temperatures of 20-25°C, as it is more tolerant than winter grass;
• Do not apply if the soil pH is > 7.4 and
• If the air temperature is going to be above 27 ºC you 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, found that it begins working in the first day or so after application.

Ethofumesate (Tramat®, various).

This gives pre-and early post-emergent control of winter grass in new seedings of perennial ryegrass and is absorbed by emerging shoots and roots and translocated to the foliage. Other points related to using this turf chemical are:

• It inhibits seed germination of cool-season grasses other than perennial ryegrass;
• Before applying remove any surface organic matter as this has a negative effect on the results;
• It is very toxic to seedling fine fescues;
• Also do not use on turf with more than 70% winter grass;
• During periods of extreme stress such as heat, cold, and drought do not use and finally
• When used at high rates, it causes 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);
• 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 undesirable acceleration in the results gained when using Poacure®.

• Increasing the rate or frequency because of think its working too slowly;
• Avoid any calibration or spray errors 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 finally
• Using it along with other herbicides/plant growth regulators (PGRs) such as paclobutrazol.

Propyzamide (Checkpoint®, Various).

The Neyland and Nickson trials in 2015, found that after 5 weeks that there was no change in winter grass levels. Unlike many winter grass herbicides, this 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.

Sulfonylurea herbicides (Various)

These are used as “late transition aids” in the USA and have become important tools in the control of winter grass in turf and also as ryegrass transition aids. Sulfonylureas herbicides all work in a similar way. Being in the same chemical grouping means that switching from one active to the other will not prevent chemical resistance from developing. So, 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® herbicide), and trifloxysulfuron (Monument® Liquid Herbicide).

General usage rules for managing winter grass in turf using the sulfonylurea herbicides :

• Firstly, never apply these within seven days of organophosphate insecticides to avoid excessive injury (Ferrell et al, 2004);
• Secondly, foramsulfuron, iodosulfuron, rimsulfuron, and trifloxysulfuron track and damage susceptible turfgrass if not allowed to dry before traffic. 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;
• Thirdly, if the tank water pH is less than 5.5 raise the pH to 7.0;
• Ensure that you get complete and even spray coverage with no overlap;
• Apply with the boom no higher than 50 cm above the turf with flat fan nozzles and a medium droplet size;
• If you apply these to very dry sandy soils and it is followed by excessive rainfall it may cause turf damage and finally
• Turf damage may also occur in soils with pH >8.5.

Bispyribac-sodium (Nominee®).

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

Foramsulfuron (Tribute®).

• This works best if rainfall or irrigation does not occur within two hours of treatment.
• You can seed ryegrass 2 weeks after application.

Iodosulfuron (Duke®)

• This is mainly a foliar herbicide with less soil uptake;
• Allow at least six weeks between the last application and seeding with cool-season grasses.

Rimsulfuron (Coliseum®)

• As this is root and foliarly absorbed, irrigating one hour after application will move the herbicide into the soil and increase herbicidal effectiveness;
• Delay seeding with cool-season turfgrasses 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 seeding for 6 weeks after application.

Plant Growth Regulators (PGR’s) for winter grass in turf.

A PGR is any chemical that alters plant growth and includes plant hormones, herbicides, growth inhibitors, and even bio-stimulants.

Ethephon (Incognito®/Various).

• This is a good tool for lowering winter grass seedhead numbers;
• The trick to the use of this is to make the first application at the first sign of the “boot” stage. This is when the stem is swollen, which indicates that it contains a seedhead;
• In a putting green trial, ethephon when mixed with methiozolin, had a negative relationship with post coring stress. The result was creeping bentgrass loss. So, don’t mix methiozolin with ethephon, if the greens are going to be under stress;
• It is taken up via leaf tissue so apply in the correct amount of water.

Paclobutrazol (Regulate®/Various).

• This inhibits winter grass making desirable turf grass more competitive. These grasses can then crowd out the winter grass;
• Paclobutrazol is root absorbed, so apply as a soil drench.

Trinexapac-ethyl (Amigo®/Various).

• This is a great tool for managing winter grass in turf but you should not consider using it for getting rid of winter grass;
• Ethephon, in a tank-mix with trinexapac, is the only working treatment for stopping winter grass seedhead;
• Repeat applications of trinexapac enhance the tolerance of winter grass to summer stress;
• It suppresses winter grass and allows desirable grass species to outgrow it and lastly
• It increases leaf etiolation caused by Acidovorax avenae subsp. avenae and Xanthomonas translucens.

Post-emergent herbicides for managing winter grass in turf