By Jay McCurdy, PhD, and Michael Richard
Winter weeds may seem like the least of your concerns during peak summer growing conditions, but with a little preparation, things will go a lot smoother later in the year.
Annual bluegrass (Poa annua) is a problematic winter annual weed in most maintained turfgrass. It has a bunch type growth habit with light green leaves having a boat-shaped tip. Annual bluegrass is a prolific seed producer and is commonly identified by its unsightly greenish white seed heads. Each plant produces hundreds of seed that can lay dormant in the soil for years before germinating. Annual bluegrass can withstand low heights of cut and frequent mowing. It grows just about anywhere but prefers areas with moist or compacted soils. Contrary to its name, both annual and perennial biotypes exist.
Annual bluegrass seed germinate in late summer and early fall when soil temperatures drop below approximately 70o F. During the fall, seedlings grow vegetatively, and the plant produces seed from late winter through early summer. For practical reasons, this is important, mainly because herbicidal control of plants in reproductive stage (having flowered or produced seed) is far less successful.
Several flushes of seed germination are common throughout the fall, winter, and spring. Heat and dry conditions during the summer usually lead to plant death; however, perennial biotypes may persist in certain climates.
Controlling annual bluegrass requires an integrated chemical and non-chemical strategy. Cultural practices such as proper mowing, fertility, and irrigation, are important, but rarely provide complete control. Proper herbicide selection and application timing are important for commercially acceptable control.
As is the case with most weeds, a healthy turf is the best defense against annual bluegrass infestations. Sound agronomic practices, such as maintaining the correct soil pH and fertilization, deep infrequent watering, appropriate insect and disease management, and proper mowing, will go a long way towards reducing annual bluegrass pressure.
Whatever you can do to improve drainage and root health will aid in turf competition. Weeds like annual bluegrass thrive in areas that are overwatered or are poorly drained. Improving soil drainage can aid in turf competition.
Unless you overseed, eliminate nitrogen fertilization during the fall and winter dormancy. This practice will unnecessarily promote annual bluegrass growth and potentially expose warm-season grasses to winter injury.
Raising mowing height in the fall can increase competitiveness of warm-season grasses. Scalping turf opens the canopy that can lead to infestation by winter annual weeds. Although scalping and verticutting are useful for ryegrass establishment, they also promote annual bluegrass.
Early seeding dates may suppress annual bluegrass emergence. However, perennial ryegrass may suffer due to heat, drought, or disease stress. On the other hand, it’s common to delay overseeding so that annual bluegrass can be controlled using post-emergence herbicides a week or two prior to overseeding.
In regards to cultural control of annual bluegrass, experimentation at your facility will be key.
Where cultural practices fall short, herbicides are often needed. There are several herbicide options and strategies for controlling annual bluegrass.
Use characteristics. For most practitioners, it is easiest to discuss herbicides based upon similar use characteristics (as an example: pre-emergence vs. post-emergence). Remember, pre-emergence herbicides are applied prior to germination. Post-emergence herbicides kill weeds already emerged from the seed. Some pre-emergence herbicides have slight post-emergence control. Some post-emergence herbicides have a little pre-emergence control. The consensus amongst weed scientists is that pre-emergence applications should almost always be applied with a post-emergence herbicide or should be followed up with a post-emergence herbicide treatment in order to eliminate escapes. Neither strategy should be relied upon solely.
You will notice that many pre-emergence herbicides are available in various forms of liquids, dry flowables, and granular materials. In general, the granular material that is broadcast through a fertilizer spreader does not have the same effectiveness as a liquid broadcast applied uniformly through a sprayer. This is due to poor uniformity of distribution.
Modes of action. A fuller, more meaningful conversation about chemical control of annual bluegrass must also mention herbicide mode of action. Best management practices include the use of multiple modes of action within season and across years. Relying upon a single mode of action leads to resistant biotypes. Alternatively, just because two herbicides have the same mode of action, does not mean they provide equal annual bluegrass control, thus the importance of regional University trials and demonstrations. Attend Field Days and Conferences!
Pre-emergence weed control
Pre-emergence herbicides are applied before annual bluegrass germination. Germination varies from year to year. Applying herbicides early rather than late is generally recommended; however, if you apply too early, there will not be enough herbicide remaining to prevent weed seed germination. This leads many managers to split fall pre-emergence applications. For instance, one might apply 60% of the recommended rate in mid-August and then the remaining amount roughly 8 weeks later in mid-October.
Some pre-emergence herbicides inhibit root growth of desired turfgrass; this can be bad if you are trying to recover from fall wear and tear, or if you’re growing-in from seed, sod, or sprigs. Some pre-emergence herbicides are safer than others, but safety is rate and timing dependent. Always read and follow label directions.
Pre-emergence herbicides need to be incorporated into the soil after application via irrigation or rainfall. Once again these herbicides lack post-emergence activity so timing is critical to proper efficacy.
Root growth inhibitors. Fall applications of commonly used pre-emergence herbicides such as Pendulum (pendimethalin), Barricade (prodiamine), and Dimension (dithiopyr), may effectively control annual bluegrass. These mitotic inhibiting herbicides and others inhibit cell division in roots of young plants and are available in a variety of other trade names and formulations. Exclusive use of mitotic inhibiting herbicides has led to biotypes resistant to these herbicides. Rotating herbicide mode of action, as well as tank mixing with post-emergence options, is recommended to reduce resistance and allow for continued use of these pre-emergence herbicides.
Each of these herbicides has a unique application interval before overseeding with perennial ryegrass (approximately 2 to 4 months, depending upon rate and region). These herbicides are tolerated by established ryegrass when applied according to label directions.
Protox/PPO inhibitor. Ronstar (oxadiazon), a protoporphyrinogen oxidase inhibitor, is available in two basic formulations, liquid or granular. Its use as a liquid requires foliar broadcast application in a water carrier. Foliar applications of oxadiazon cause injury to non-dormant green tissue. Liquid applications have their place, but only in dormant bermudagrass or when injury can be tolerated. The granular formulation is typically applied on a fertilizer or inert carrier, like calcined clay. The granular formulation allows the herbicide to bypass the plant leaf blade and avoid foliar injury. Granular application is frequently used for high-end sports fields due to its relatively benign effects upon bermudagrass roots and pegging.
Pre-emergence activity of oxadiazon has been shown to vary, likely due to application timing and incorporation into the soil. Granular formulations on small prills help to evenly distribute the herbicide. Ronstar G is labeled for application at least 60 days prior to ryegrass overseeding but is labeled neither for nor against application to established ryegrass.
For all the positive attributes of oxadiazon, it does have one draw-back: it typically lacks control of winter broadleaf weeds, such as henbit, deadnettle, fireweed, and chickweed – thus requiring a follow up application of broadleaf herbicide (Trimec or the like).
Cell wall formation disruptor. Specticle (indaziflam), a cellulose biosynthesis inhibitor, is an important option for rotation where resistance to mitotic inhibiting herbicides is confirmed. Only use indaziflam in established sports fields free of pest, disease, or stress. This herbicide has longer soil residual than many pre-emergence herbicides (up to a 12-month overseeding interval).
Pre/post weed control
Herbicides with pre and post-emergence activity control young seedlings and prevent success of yet to germinate seeds. These herbicides work well as a late fall/early winter clean-up application.
PS II inhibitors. Two of the most common pre/post herbicides in warm-season turf are Aatrex (atrazine) and Princep (simazine). These photosystem II inhibiting herbicides also control broadleaf winter annual weeds. Atrazine should only be applied to dormant bermudagrass unless injury can be tolerated. Simazine is a bit safer but can also cause injury and stunting. Continuous use of these two products has led to annual bluegrass that is resistant to PSII inhibitors, so effectiveness is sometimes limited when these herbicides are applied alone. Their price makes them a good addition to the tank during winter dormancy. Neither is applied to overseeded ryegrass.
Root growth inhibitors. Kerb (pronamide) provides both pre-emergence and post-emergence control of annual bluegrass but is typically thought of for its early post-emergence activity. Pronamide is frequently used in instances where PSII and ALS inhibitor resistance exists. Like Barricade, Pendulum, and Dithiopyr, it must be applied well in advance (approximately 90 days) before overseeding. It is not safe on established ryegrass.
Protox/PPO inhibitor. Sureguard (flumioxazin), a protoporphyrinogen oxidase inhibiting herbicide, can be applied to dormant bermudagrass to control annual bluegrass. Applications to actively growing turfgrass, including ryegrass, will cause injury.
Post-emergence weed control
Post-emergence herbicides are more effective if applied to young seedling plants rather than mature stands. Applications in late spring, when seed heads are highly visible, are only modestly effective. This means proper post-emergence timing is late fall through winter, when weather cooperates.
Post-emergence herbicides are separated into two categories: selective and non-selective.
Non-selective herbicides should only be applied when turf is fully dormant in winter or when injury can be tolerated.
EPSP inhibitor. Round-up (glyphosate) is the most commonly used non-selective herbicide in turf. It inhibits amino acid production by interrupting the EPSP enzyme. Resistance to glyphosate is on the rise, thus it may be necessary to tank-mix with another post-emergence herbicide, such as simazine. While there are “glyphosate tolerant” ryegrass varieties on the market, it is no longer advisable to rely upon glyphosate as a sole strategy for annual bluegrass control.
Glutamine synthetase inhibitor. Finale (glufosinate) inhibits nitrogen metabolism and is an option where glyphosate resistance is common. Control can be inconsistent due to weed size and maturity. There are no tolerant ryegrass varieties.
PS I inhibitor. Reward (diquat) diverts electron energy in a way that leads to fast acting burn down of most green plant material. It too is an option where glyphosate resistance is common; however, control can be inconsistent due to weed size and maturity. It injures/kills ryegrass.
Selective herbicides control annual bluegrass with limited injury to the desired turfgrass species.
ALS inhibitors. There are several acetolactate synthase (ALS) inhibiting herbicides that control annual bluegrass, including Revolver (foramsulfuron), Katana (flazasulfuron), Monument (trifloxysulfuron), TranXit (rimsulfuron), and Certainty (sulfosulfuron). They are frequently used as transition aids during bermudagrass green-up.
Velocity (bispyribac-sodium) is an ALS inhibitor that is safe on established perennial ryegrass and controls immature annual bluegrass and some broadleaf weeds. Apply no less than 30 days after ryegrass emergence, which limits use to early- to mid-winter and spring application in mid-south and gulf-coast, respectively.
The herbicides mentioned can be applied before overseeding but with various timing, rate, and regional restrictions. Apply according to label instructions.
PS II inhibitor. Xonerate (amicarbazone), a photosystem II inhibitor, post-emergently controls annual bluegrass and has considerable safety on well-established perennial ryegrass. However, amicarbazone will not control annual bluegrass resistant to simazine and atrazine, as these herbicides share the same mode of action.
Lipid synthesis inhibitor. Prograss (ethofumesate) can be applied in established perennial ryegrass stands. Only apply to dormant bermudagrass. Ethofumesate has some soil residual and can limit spring bermudagrass emergence from dormancy if applied too late in the winter.
- Cultural practices affect turf health and density; these are basic tenets of any preventative weed control program. Increased mowing heights and decreased fall fertility are major components of this strategy.
- Effective chemical control incorporates a pre and post-emergence strategy.
- The first line of defense is a properly timed pre-emergence application. However, very rarely does pre-emergence alone completely control annual bluegrass.
- A post-emergence application is applied in fall or winter to control late germinating bluegrass. Products with pre and post-emergence activity are ideal options at this delayed follow-up timing.
- Selective herbicides can be applied to non-dormant turf to control annual bluegrass. Proper timing for effective control is late fall through winter. If you wait until March, you are probably too late.
- Non-selective herbicides may be applied when turf is truly dormant. Extreme caution should be used in high traffic sports surfaces due to potential for prolonged injury. Due to the potential for slowed recovery, use caution when applying non-selective herbicides in combination with pre-emergence herbicides.
- In overseeded turf, have a plan that either relies upon early application of a pre-emergence herbicide, or applies postemergence herbicide at the appropriate time prior to overseeding. In either case, a delayed pre-emergence herbicide applied to established ryegrass is a good idea. Post-emergence herbicides that are tolerated by perennial ryegrass (such as Prograss, Velocity, and Xonerate) work well when timed appropriately.
Jay McCurdy, PhD, is assistant professor and turfgrass extension specialist at Mississippi State University; Michael Richard is turfgrass extension associate at Mississippi State. The information given here is for educational purposes only. References to commercial products, trade names, or suppliers, are made with the understanding that no endorsement is implied and that no discrimination against other products or suppliers is intended.