Managing winter grass in turf. 9

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

Why is winter grass in turf a problem?

The ability of winter grass to adapt is the key reason why the control of this weed grass is such a problem. Winter grass is a perfect weed competitor. It thrives in shade and full sun, over a wide range of soil moisture levels 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, if it’s too wet, or compacted soil. The seed, is brought in by various means (wind, footwear, unclean machinery, contamination, etc.). In addition, it forms seed banks at the soil surface containing over 2000 seeds/m2, and these seeds can remain viable for years.

How to ID winter grass.

Winter grass (Poa annua) 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 control 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. Low soil pH helps 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 5.0.¹Sprague, NB. and Burton, GW. 1937. Annual Blue Grass (Poa annua L.) and its Requirements for Growth. New Jersey Agricultural Experiment Station Bull. 630. 24 p. 
•  Winter grass does not germinate at pH 3.6, and at a pH of 5.2, only 66% of seed germinates.²Ferguson, NL. 1936. A greenkeeper’s guide to the grasses: The meadow grasses. Journal of the Board of Greenkeeping Research. 15: 274–279. 
•  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 4.5.³Juska, FV., and Hanson. AA, 1969. Nutritional requirements of Poa Annua L. Agron. J. 61: 466–468. doi:https://dx.doi.org/10.2134/agronj1969.00021962006100030041x,
• Elemental sulphur reduced winter grass to only 7% in contrast to 30% with no sulphur.
• Varco and Sartain (1986) ⁴Varco, JJ and Sartain. JB. 1986. Effects of phosphorus, sulfur, calcium hydroxide, and pH on growth of annual bluegrass. Soil Sci. Soc. Am. J. 50: 128–132. doi:https://dx.doi.org/10.2136/sssaj1986.03615995005000010025x, found that sulphur, acidifies the top 2.5cm of the soil, and reduces winter grass emergence and establishment.

Soil Phosphorus –

Maintain low soil phosphorus levels.

• Applying P increases winter grass levels in a mixed Poa/Agrostis sward.⁵Goss, RL., Brauen, SE., Orton, SP. 1975. The effects of N, P, K and S on Poa annua L. in bentgrass putting green turf. J. Sports Turf Res. Inst. 51: 74–82..

What winter grass killers are on the market?

There are several winter grass killers to manage winter grass on turf, and if you use these properly, they will all help kill winter grass. 

You can class these into two groups of winter grass killer: Winter grass pre-emergent herbicides and post-emergent winter grass herbicides.

Examples of pre-emergent winter grass herbicides are Onset 10GR® (prodiamine), Echelon® (oxadiazon), and Specticle® (indaziflam). These can be liquids or granular pre-emergents. 

Examples of post-emergent winter grass 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.

All of the following stop winter grass before it becomes a problem. Our other blog on factors that can cause pre emergent herbicide failure may also be of interest.

Prodiamine (Barricade®/Onset 10GR®)

• The graph below shows how prodiamine works well for long-term control of winter grass. This length of control is for the liquid (Barricade Herbicide) and granule formulations (Onset 10GR).
• Do not apply to newly seeded, sodded, or sprigged turf. You only use prodiamine when you have a full turf cover, and the roots are greater than 3 cm depth. Prodiamine can stunt or prune root growth and cause porpoising or bridging of turf over the surface. 
• One application gives up to 6 months control.
• Apply in at least 500 L water/Ha.
• You cannot seed for at least six months after use, so bare this in mind if you are inending to reseed.

Pendimethalin (Battalia®/Various)

• See graph below to see pendimethalin performance.
• Water in within one day of use, aiming to use 10-15 mm of irrigation or before rainfall.
• With thatchy or soils high in organic matter it gives poor control. So avoid using on soils having greater than 6% organic matter.
• It has a 12-hour re-entry period;
• Pendimethalin will permanently stain (yellow) any objects it contacts;
• Provides up to 2 months of control.

Post emergent winter grass killers in turf.

Post emergent winter grass killers kill winter grass that is 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 can be a bit “fiddly” to use.

• Use lower rates on sandier soils;
• On overseeded couch, if you use this in teh spring you will get better results. Also  perennial ryegrass has greater tolerance at this time;
• 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 get turf injury on cool-season turf.

Endothal (Poachek®).

Work carried out by Neylan and Nickson (2015)⁶Neylan, J.; Nickson, D. Poa Annua Control in Warm-Season Grass Sportsfield 2016. Available online: https://www.turfmate.com.au/uploads/STA%20Poa%20annua%20control%20in%20warm%20season%20grasses%20Final%20Report%20May%202016.pdf. 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 winter grass 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 
• When you use this at high rates, it causes temporary discolouration in Kentucky bluegrass and bentgrass turf.

Methiozolin (Poacure®).

• Poacure is a safe to use winter grass 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 Poacure mainly moves upwards in the plant. 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-treated⁷Askew, S.D., and B.M.S. McNulty. 2014. Methiozolin and cumyluron for preemergence annual bluegrass (Poa annua) control on creeping bentgrass (Agrostis stolonifera) putting greens. Weed Technology 28(3):535-542. doi:10.1614/WT-D-14-00018.1 
• 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 was from 57 to 64% ⁸Brosnan, J.T., G.M. Henry, G.K. Breeden, T. Cooper, and T.J. Serensits. 2013. Methiozolin efficacy for annual bluegrass (Poa annua) control on sand- and soilbased creeping bentgrass putting greens. Weed Technology. 27(2): 310-316. ⁹Flessner, M.L., R.W. Glenn, and J.S. McElroy. 2013. Methiozolin absorption and translocation in annual bluegrass (Poa annua). Weed Science. 61(2): 201-208.

The following may give poor results when using Poacure®.

• If you increase the rate or frequency because you think it is working too slowly.
• Poorly calibrated equipement result in the wrong use rate.
• Too many treatments are made in the autumn or winter.
• After use there is a period of heavy or prolonged rainfall.
• Mechanical treatments such as aeration which stress the turf.
• The use of ethephon (Incognito 480SC)  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) like trinexapac (Amigo 120).

Propyzamide (Checkpoint®, Various).

Neyland and Nickson in 2015¹⁰Neylan, J.; Nickson, D. Poa Annua Control in Warm-Season Grass Sportsfield 2016., found that after 5 weeks that there was no change in winter grass levels. Unlike other winter grass herbicides, products like Checkpoint are very slow-acting. However, it does tend to work well in cool weather. However, it has variable performance, especially in soils of greater than >4% organic matter, and is very mobile in the soil.

Sulfonylureas (Various).

In the USA, these are used as “late transition aids”. They are common tools as winter grass 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, so don’t only rely on these as to control winter grass in warm-season turfgrass¹¹McElroy JS, Flessner ML, Wang Z, Dane F, Walker RH, Wehtje G (2013) A Trp574 to Leu amino acid substitution in the ALS gene of annual bluegrass (Poa annua) is associated with resistance to ALS-inhibiting herbicides. Weed Sci 61:21–25[/mfn 

This winter grass killer group is safe to use on couch, and Duke 100WG 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 winter grass killers.

• Firstly, never apply these within seven days of organophosphate insecticides to avoid turf 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 irrigate on the morning after application and leave a 5-metre buffer area around cool-season areas. 
• 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 then rains heavily, these winter grass herbicides may cause turf damage and, finally
• Turf damage may also occur in soils with pH >8.5.

Bispyribac-sodium (Nominee®).

• Nominee is the only SU winter grass herbicide, that is 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.¹²Mccullough, P. E., & S. E. Hart. 2006. Temperature influences creeping bentgrass (Agrostis stolonifera) and annual bluegrass (Poa annua) Response to BispyribacSodium. Weed Technology, 20(3): 728-732. doi: 10.1614/WT-05-010R2.1 

Foramsulfuron (Tribute® herbicide).

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

Iodosulfuron (Duke®)

• Proforce Duke 100WG herbicide is a Destiny herbicide alternative.
• Duke herbicide 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 to suppress winter grass in Kikuyu grass.

Rimsulfuron (Coliseum herbicide®)

• Coliseum is another great winter grass killer.
• Uptake is via roots and leaves. 
• You should irrigate one hour after you apply to move this winter grass herbicide into the soil.
• Delay seeding with cool-season turfgrasses for 10 to 14 days after use.
• Coliseum is not safe to use for winter grass in Kikuyu.

Trifloxysulfuron (Recondo 100 WG®)

• Recondo 100WG herbicide is a Monument herbicide alternative.
• You can sprig or seed couch or zoysia grass 4-weeks after use. 
• You should delay seeding with cool-season grass for 6 weeks after use.
• Recondo 100WG is not safe to use as a winter grass killer in Kikuyu.

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 winter grass killers or winter grass herbicides but can safely be used for winter grass in Kikuyu grass.

Ethephon (Incognito®/Various).

• Incognito is a great tool to reduce winter grass seed head 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;
• If you use ethephon with Poacure you can get creeping bentgrass loss. So, don’t mix these if the greens are going to be under stress;
• Incognito uptake is via the leaf, so apply in the correct amount of water.
• It is important you keep the water pH below 4.5 to optimise ethephon efficacy.

Paclobutrazol (Regulate®/Various).

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

Trinexapac-ethyl (Amigo®/Various).

• Amigo 120 is a great tool for winter grass in turf but you should not use it to remove winter grass;
• Incognito, in a tank mix with trinexapac, is the only product that stops winter grass seed head;
• 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.