Managing winter grass in turf. 9

In this blog what is wintergrass, wintergrass killers, wintergrass herbicides and killing wintergrass in kikuyu and other turfgrass.

Why is managing winter grass in turf a problem?

The ability of wintergrass to adapt is the key reason why managing this weed grass is such a problem. Winter grass can grow in a range of conditions, which means it is a perfect weed competitor. It can thrive in shade and full sun, in a range of soil moisture conditions and is very tolerant of low mowing heights. It also persists all year round as it is a profuse seeder.

The fact it matures quickly makes it difficult to control. Worst of all, this weed grass will dominate weak turf due to wear (leaving thin or bare areas), nutrient deficiency, being too wet, or 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 and has perennial and biennial biotypes. It has a light green, generally tufted growth habit. You can identify it by its seed head, and has a boat-shaped leaf tip that curls up at the ends.

How to stop winter grass.

Cultural methods are the first step to the control of winter grass in lawns, golf courses or sports grounds. Certain factors favour this weed, which means if you “get ahead of the curve”, it helps prevent its spread and future growth.

Soil pH

The aim should be to keep the soil pH below 7. This is important to slow down winter grass establishment and growth.

•  In sand with a pH of 6.5, winter grass has better root growth than if grown at pH 5.0. (Sprague and Burton,1937)
•  Winter grass does not germinate at pH 3.6. At pH 5.2, only 66% of seed germinates. (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 a pH 4.5 (Juska and Hanson, 1969)
• Applications of elemental sulphur reduce winter grass to only 7% in contrast to 30% with no sulphur (Sprague and Burton, 1937).
• Varco and Sartain (1986) found that sulphur applications only acidify the top 2.5cm of the soil but reduce wintergrass 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 wintergrass killers are on the market?

​There are several wintergrass killers to manage winter grass on turf and If you use them properly, they will all help kill wintergrass. You can classify these into two categories of wintergrass killer: Wintergrass pre-emergent herbicides and post-emergent wintergrass herbicides.

You can use Onset 10GR® (prodiamine), Echelon® (oxadiazon), and Specticle® (indaziflam) as examples of pre-emergent winter grass herbicides. These can be liquids or granular pre-emergents. Post-emergent wintergrass killers include Poacure® (methiozolin), Nominee® (bispyribac), and Coliseum® (rimsulfuron).

An overview of winter grass herbicides is below. Check out the turf chemical labels, or contact us directly if you need more information.

Pre-emergents for managing winter grass in turf

Prodiamine (Barricade®/Onset 10GR®)

• See graph below;
• Do not apply to newly seeded, sodded, or sprigged turf. Delay its use until turf cover is at 100%, 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. Aim to use 10-15 mm of irrigation or before rainfall;
• In soils with a high organic matter soils, it gives poor control. So avoid use on soils having greater than 6% organic matter;
• It has a 12-hour re-entry period;
• Pendimethalin will stain (yellow) any objects it contacts;
• Provides up to 2 months of control.

Post emergent wintergrass killers in turf.

Post emergent wintergrass killers kill plants that are already present.

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 sandier soils;
• On overseeded couch, spring use gives greater winter grass control, and the perennial ryegrass also has greater tolerance;
• Water volume, temperature, and timing affect the results;
• 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
• Above an air temperature of 27 ºC, you will likely get bad turf injury on cool-season turf.

Endothal (Poachek®).

Work carried out by Neylan and Nickson (2015) in Couch and Kikuyu grass, found it works in the first day or so after you use it.

Ethofumesate (Tramat®, various).

Ethofumesate is a pre-and early post-emergent wintergrass killer in new seedings of perennial ryegrass. Uptake of Ethofumesate is by emerging shoots and roots, and it then moves to the foliage.

• It inhibits seed germination of cool-season grasses other than perennial ryegrass;
• Remove any surface organic matter before use 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;
• Don’t use during stressful periods like heat, cold, and drought, and finally
• When you use this at high rates, it causes temporary discolouration in Kentucky bluegrass and bentgrass turf.

Methiozolin (Poacure®).

• Methiozolin is a safe to use wintergrass killer on all common turfgrass;
• It is very slow-acting, giving a gradual transition from winter grass to bentgrass;
• Uptake is by foliage and roots, but it mainly shows acropetal movement. When you apply this 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 work better than soil-only treatments;
• Control on a soil-based root zone ranged from 72 to 80%. This contrasts with a sand-based root zone where control ranged from 57 to 64% (Brosnan et al. 2013a, Flessner et al. 2014).

The following may give poor results when using Poacure®.

• If you increase the rate or frequency because you think it works too slowly;
• Calibration errors that increase the use rate;
• Too many treatments in the autumn or winter;
• Heavy or prolonged rainfall after use;
• Mechanical treatments such as aeration which stress the turf;
• The use of 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).

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, products like Checkpoint are very slow-acting. However, it does tend to work well in cooler weather in contrast to the sulfonylureas. However, it has inconsistent performance, especially in soils of greater than >4% organic matter, and exhibits a high degree of soil mobility.

Sulfonylurea herbicides (Various)

In the USA, these are used as “late transition aids”. They have become common tools as wintergrass herbicides, and to remove ryegrass during transition. Sulfonylureas, all work in the same way and are in the same herbicide chemical group. Switching from one active to the other will not prevent chemical resistance. It isn’t recommended to only rely on these as wintergrass herbicides in warm-season turfgrass (McElroy et al. 2013).

This wintergrass killer group is safe to use on couch, and iodosulfuron is also safe to use on Kikuyu grass. This group includes bispiribac-sodium (Nominee​), rimsulfuron (Coliseum), foramsulfuron (Tribute selective herbicide), iodosulfuron (Duke® herbicide), and trifloxysulfuron (Recondo herbicide).

Rules for sulfonylurea wintergrass killers:

• 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 you get complete and even spray coverage with no overlap;
• Use with a spray adjuvant;
• 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 dry, sandy soils and it is followed by heavy rainfall, these wintergrass herbicides may cause turf damage and, finally
• Turf damage may also occur in soils with pH >8.5.

Bispyribac-sodium (Nominee®).

• Bispyribac-sodium is the only SU wintergrass herbicide, that is registered for use on bentgrass golf greens. In the USA. it is called “Velocity”
• If you apply this to creeping bentgrass mowed at 3 mm (Teuton et al. 2007), it can cause injury;
• Severe bentgrass injury can occur at temperatures less than 13ºC and above 29ºC (McCullough and Hart 2005).

Foramsulfuron (Tribute®).

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

Iodosulfuron (Duke®)

• Iodosulfuron is mainly a foliar with less soil uptake;
• Allow at least six weeks between the last application and seeding with cool-season grasses.
• You can safely use Iodosulfuron for wintergrass in Kikuyu grass and its a great wintergrass killer.

Rimsulfuron (Coliseum herbicide®)

• Rimsulfuron is another great wintergrass killer;
• It is root and foliar absorbed;
• You should irrigate one hour after application to move this wintergrass herbicide into the soil and increase the effectiveness;
• Delay seeding with cool-season turfgrasses for 10 to 14 days after application.
• Rimsulfuron is not sake to use for wintergrass in Kikuyu.

Trifloxysulfuron (Recondo 100 WG®)

• Couch and zoysiagrass can be sprigged or seeded 4-weeks after use;
• You should delay seeding with cool-season grasses for 6 weeks after application.
• Trifloxysulfuron is not safe to use for wintergrass in Kikuyu.

Plant Growth Regulators (PGRs) 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. PGRs are not wintergrass killers or wintergrass herbicides but can safely be used for wintergrass in Kikuyu grass.

Ethephon (Incognito®/Various).

• Incognito is a great tool to reduce winter grass seedhead numbers;
• The trick is to use it at the first sign of the “boot” stage. The boot stage is when the stem swells and indicates that it contains a seedhead;
• In a putting green trial, ethephon mixed with methiozolin results in creeping bentgrass loss. So, don’t mix methiozolin with ethephon if the greens are going to be under stress;
• Methiozolin uptake is via the leaf, so apply in the correct amount of water.

Paclobutrazol (Regulate®/Various).

• Paclobutrazol inhibits winter grass and makes desirable turf grass more competitive. Desirable grass can then crowd out the winter grass;
• Regulate is taken up by roots, so apply as a soil drench.

Trinexapac-ethyl (Amigo®/Various).

• Amigo, or similar, is a great tool to manage 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 product that stops winter grass seedhead;
• Repeat applications 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 bacterial leaf etiolation.