7.1 Environmentally Compatible Weed Management
7.3.3 Physical Disturbance, Soil Temperature, and Crabgrass Invasion
7.3.4 Do Fertilizer or Organic-based Approaches Work?
7.3.5 Preemergence Herbicides and Crabgrass Control
7.3.6 Postemergence Crabgrass Control
7.3.7 An Integrated Approach to Crabgrass Control.
7.4 Cultural Control of Broadleaf Weeds
7.5 Chemical Control of Broadleaf Weeds
7.5.2 Using Broadleaf Herbicides
7.1 Environmentally
Compatible Weed Management
Weeds are plants that disrupt the visual and functional appearance of turfgrass areas. Environmentally compatible weed management begins with an understanding of turfgrass visual and functional performance expectations in an effort to develop an integrated weed management program. In the absence of this understanding, weed management programs do not have a clear objective (threshold) beyond complete weed eradication without regard for economic and environmental concerns.
Weeds in lawn turf are often considered to solely detract from visual quality expectations. Yet, there is growing evidence that high populations of summer annual weeds could lead to increased runoff from turf areas reducing surface water quality. Increased runoff is associated with low density, weak turf. A high percentage of bare area infested with annual weeds is indicative of weak turf conditions.
Functionally, weeds can reduce the playability and safety of golf and sports turf. Weeds will disrupt the footing of an athletic sports field and alter the lie and roll of a golf ball, thereby disrupting the fairness of a golf game. These disruptions can have health as well as economic effects on turfgrass facilities.
Severe weed infestations requiring some type of control measure are often associated with less than adequate growing conditions (light, water, air movement), excessive traffic, improper culture, and failure to manage other pests such as diseases and insects. As a rule, weeds will invade when turfgrass is not maintained in a healthy and vigorous condition, maximizing its competitive ability.
7.2 Starting Out Right
Maintaining a competitive turf begins with proper plant selection and establishment. Improper site preparation, poor timing, and improper after care all leave the turf area lacking from the beginning.
Recently, the growing use of turfgrass sod has increased the amount of annual bluegrass and creeping bentgrass infestations due to their contaminating sod fields. Be sure to specify annual bluegrass-free sod or bentgrass-free sod if using Kentucky bluegrass or other mixtures.
In addition to contaminated sod, contaminated seed could be a source of weeds such as Poa trivialis. There is evidence that Poa trivialis grows naturally and seeds or stolons can germinate after lying dormant. It is entirely possible that it is introduced as a contaminant in turf seed. Seed producers have since self-imposed Poa trivialis growing and shipping restrictions to help prevent this.
Another important weed management practice during establishment is timing your seeding to avoid specific weed problems. For most cool season turfgrass stands, this is the reason late summer to early fall is an ideal time to seed as most weeds are less competitive at this time. However, if annual bluegrass is your primary concern, as it is on most golf courses, then it is best to seed in late June and July when annual bluegrass germination is significantly reduced.
For more information on maintaining a competitive turf stand, see section 2 on Turfgrass Culture.
A few chemical herbicides can be used at or during establishment to reduce weed competition such as preemergence siduron and postemergence fenoxaprop and bromoxynil applications. There are significant species restrictions for the postemergence products, so be sure to read the label carefully.
7.3 Annual Grass IPM
A well maintained turfgrass area provides many aesthetic and functional benefits. Decades of scientific research have been conducted to aid with management that maximizes plant health and minimizes environmental impact. Still, significant concern for environmental quality and human health has raised public awareness and led to increased scrutiny of management practices, especially pest management.
For many years, the turfgrass industry has been implementing a broad based decision-making management system known as Integrated Pest Management (IPM). IPM has evolved since its inception to more completely embrace the importance of turf culture that maximizes plant health. Still, misconceptions persist regarding the more traditional aspects of IPM such as “using only biological control” and “no use of pesticides”.
The misconceptions of IPM pose a unique challenge from a weed management perspective -- visual thresholds are subjective (some like the look of weeds, some don’t), functional thresholds are exceptionally low or not known (how many weeds can an athletic field have before the game is disrupted?) and there are no effective biological controls once the weed is established. Therefore, the most effective IPM program for weed management is prevention by maintaining turfgrass density.
The role of turfgrass density is critical for IPM, as well as for maximizing the environmental benefits of turf. For example, studies from university research have indicated that a thin, unfertilized turf resulted in greater nutrient runoff that could contaminate surface water bodies. Basically, as weeds invade a thin turf, initially, the density of weeds and turf is adequate to cover the soil. Annual weeds, such as crabgrass, die off in the fall and leave bare soil exposed to the spring rains, allowing runoff to occur. It is these scenarios where weed control can be justified to preserve surface water quality.
Decreased turf density resulting from insect and disease damage, excessive traffic, poor drainage, etc. will allow weeds from weed seeds stored in the soil (soil seedbank) to germinate. A primary weed arising from seed in turf stands is the annual grass weed, crabgrass (Digitaria spp.).
7.3.1 Crabgrass Management
Among the 60 species in the genus Digitaria, 13 weedy species infest crops in the US. Of these 13, there are three major crabgrass species distributed in the United States: large, smooth, and southern. Smooth crabgrass is prominent in the northern climates, especially the north eastern US. Large crabgrass is found throughout the US. Southern crabgrass is primarily found in Florida and mid-southern states.
Crabgrass management has evolved over the last 60 to 70 years. Years ago, the preferred management method was to alter the growing environment to limit crabgrass. Typically, this was achieved by drastically lowering the soil pH. The principle of pH manipulation was to alter the environment so that the organism (crabgrass) could not be successful. Unfortunately, the turf usually suffered as well.
Today, we rely on selective herbicides applied prior to crabgrass germination (preemergence) or after crabgrass emergence (postemergence). Again, concern over pesticide use has increased the need for understanding turfgrass and crabgrass ecology, innovative herbicide application programs that minimize exposure, and use of fertilizers and organic-based approaches.
7.3.2 Crabgrass Ecology
In spite of the available technology for managing crabgrass, it remains one of the most troublesome weeds in the US. Fidanza and Dernoeden (1996) have provided some useful information regarding crabgrass emergence patterns as influenced by growing degree days. In addition, studies from the 1950s and 1970s suggest that crabgrass could have up to a four-month period where seeds could continually emerge from the top 2” of soil. Of course, most managers are familiar with phenological indicators (such as forsythia and lilac flowering) as a tool to predict timing of emergence. Still, many ecological questions remain unanswered.
To more thoroughly understand the distribution and adaptation of crabgrass to regions and cropping systems, several weed science specialists in the US were surveyed on their view of crabgrass distribution. Approximately 90% of the respondents indicated that the three major species (smooth, large, southern), are regional problems. Large crabgrass was the most prominent species in all cropping systems from orchards to forage crops to golf and other turf areas. Smooth crabgrass was identified as a more significant problem in turf than any other system. When asked what factors limit crabgrass distribution and adaptation, the respondents believed that temperature, light, and seedbank were the most important, with moisture, cultivation, and soil pH to be of lesser importance.
A few respondents (4%) indicated that within a crabgrass species, the plants looked very different. Some suspected these plants responded to different environmental factors. Does this mean that smooth crabgrass from Rhode Island looks different than smooth crabgrass from Long Island, NY or from State College, PA? The results of several field and growth chamber experiments at Cornell indicated that, in fact, plants of the same species from different areas in the same region look different. However, when evaluating characteristics important for crabgrass control programs (such as emergence date, growth rate, and flower initiation) there were no significant differences between smooth and large crabgrass and within each species. Simply put, while species may be different and the plant may look different, in general they respond similarly to environmental factorss.
A difference between the crabgrass species in the above study worth noting regards flowering (seed production). The study found that smooth crabgrass plants germinating after mid-July did not produce seed. These late germinating plants serve only to deplete the seed bank, in that the plants contribute less to the seedbank than they withdraw. This observation has been noted in other field studies.
7.3.3 Physical Disturbance,
Soil Temperature, and Crabgrass Invasion
To more thoroughly understand the influence of soil temperature and seedbank factors, a comprehensive field study was initiated to investigate various types of physical disturbance on crabgrass emergence, development, and seed production.
Two study sites were established on mature stands of tall fescue and Kentucky bluegrass with different histories of crabgrass infestation. At both sites, four openings were created (1 inch, 2 inches, 4 inches, and 8 inches). Each site also had an undisturbed area. Each opening was maintained throughout the season by weekly clipping the encroaching leaf blades. The study area was maintained at 2.5” clipping height with no supplemental fertilization. One site had the thatch layer removed on half the plots to investigate the influence on crabgrass invasion. (The thatch layer was measured to be 0.5” thick.) Soil temperatures were monitored in each opening and in the undisturbed turf at 1” and 2” depths.
As expected, the undisturbed turf had significantly less crabgrass plants than any of the openings but was not able to completely exclude the crabgrass seedlings. The thatch layer reduced the crabgrass emergence in the disturbed plots but not in the undisturbed plots. In general, the undisturbed turf had 10 to 25% of the seedlings as the disturbed turf. In fact, crabgrass emergence varied little among the openings greater than 2 inches suggesting that any disturbance will result in crabgrass infestation if a seedbank is available.
Timing of emergence (seedlings emergence date) was not different relative to the of the amount of disturbance, however, smooth crabgrass germinated 1 week earlier than large crabgrass in disturbed versus undisturbed turf. In fact, initial crabgrass emergence began when soil temperatures in the undisturbed turf were between 54°F and 58°F for 3 consecutive days at the 1 inch depth. This is within the range of temperatures reported by Fidanza and Dernoden. Interestingly, the length of emergence (number of weeks that new seedlings emerged) was greater in undisturbed turf than in disturbed turf. This was possibly related to soil temperature which was significantly moderated by turf cover.
From a weed management perspective, based on these studies the window for successful preemergence control of smooth crabgrass in a disturbed turf is earlier and narrower as compared to undisturbed turf. However, an undisturbed turf that may become disturbed will need sustained protection from crabgrass infestations. Yet, as previously observed, late germinating smooth crabgrass plants will not produce seed and while short term visual quality is reduced, long term seedbank management is enhanced.
7.3.4 Do Fertilizer or
Organic-based Approaches Work?
The growth curve of a cool-season grass is marked by two significant periods of top growth. These are periods when temperatures are cool and daylight is long. In fact, research has shown that cool season turfgrasses produce about 60% of the entire shoot growth for the season the first 6 weeks in the spring (roughly about 25% of the growing season). This marked increase in turf density at the time of crabgrass emergence might have some ability to reduce crabgrass infestations.
Former Cornell Team Members Hummel and Neal conducted a demonstration in seven locations throughout New York State comparing fertilizing to not fertilizing with or without preemergence herbicides. The results indicated that when turf quality was acceptable prior to treatments, the fertilized plots reduced crabgrass populations by an average of 84% over the unfertilized plots. However, where turf quality began poor, fertility alone provided only 31% control as compared to unfertilized plots.
Recently, the natural organic product corn gluten meal (CGM) has been demonstrated to show herbicidal activity. The specific chemical responsible for the activity has not been isolated and characterized. Still, CGM is 10% nitrogen and when applied according to label directions, it supplies 2 lb. of actual N per 1000 square feet. . The nitrogen applied at this time extends top growth at the expense of root growth. However, when the turf is thin, the spring nitrogen will increase density that might provide some weed control. Nitrogen from the CGM is not consistent with recommendations for a fall-based fertility program.
Several studies have concluded that CGM was able to provide about 30 to 60% crabgrass control in the first year, with greater than 80% control reported in subsequent years. To overcome this reduced control in the first year, several researchers have suggested applying a preemergence herbicide at the low rate in conjunction with the CGM.
Cornell Turf Team Members Andy Senesac, Ph.D. (Suffolk County Extension Weed Scientist) and Frank Rossi (Cornell Department of Horticulture) began an experiment comparing the use of corn gluten meal (Weed-Z-Stop, With Out Weeds, Safe and Simple) at two rates (both with and without herbicide applications) to treatments using organic fertilizer and synthetic fertilizers. The study, initiated in 1997, is being conducted in Ithaca and in Riverhead, NY on thin turf stands with a history of crabgrass invasion.
Results from the studies have been consistent with regard to the level of crabgrass control achieved with the CGM. After two years of applications, season-long control with CGM does not exceed 60%. In addition, control from the CGM is not significantly different from the synthetic fertilizer applications or, in some cases, from the organic fertilizers. Both sites have demonstrated a substantial increase in turfgrass density in response to the nitrogen from the various sources that likely has an influence on crabgrass invasion. Interestingly, the CGM plus herbicide treatment has maintained above 90% crabgrass control, suggesting that this transitional program might be effective.
7.3.5 Preemergence Herbicides
and Crabgrass Control
IPM and Water Quality Perspective. The indiscriminate use of preemergence herbicides runs counter to a well-implemented IPM program. Clearly, by inhibiting the successful emergence of crabgrass plants, there is little information available on the population that might develop. As a result, there is limited ability to develop historical records that lead to reasonable aesthetic and functional thresholds, the cornerstone of an IPM program. Still, preemergence herbicides are widely used.
Most preemergence herbicides have a great attraction for soil particles (adsorption coefficient; Koc). In addition, these herbicides tend to be largely insoluble. Therefore, it is rare (except on extremely sandy soils) that preemergence herbicides used for turf in the north are found in the groundwater. However, they can pose a risk for surface runoff with most of the active ingredients rated as medium to large potential for surface runoff by the Natural Resources Conservation Service. Still, environmental fate studies conducted by turfgrass researchers in the last decade have concluded that a dense turf will significantly reduce runoff loss to surface water. Why then, if the turf is dense, do we need to apply a preemergence herbicide each year, even if there is little risk to water quality?
How do preemergence herbicides work? Preemergence herbicides that reduce the emergence of weed seedlings primarily act by inhibiting cell division. Cell division is one of the first steps in plant growth, as one cell divides into two cells, and then both cells elongate. Following a preemergent herbicide application, the chemical must be activated by moisture in the soil. The herbicide then becomes active at the soil thatch interface where many weed seeds are present. As weed seeds germinate under optimal environmental conditions, a small seedling protrudes from the seed and begins to grow towards the soil surface. The seedling has enough energy stored in the seed to reach the surface, at which time it is then able to begin using light energy in photosynthesis. It is important to note that preemergence herbicides do not affect un-germinated (dormant) seeds. The seed must germinate to encounter the herbicide in the soil-thatch interface.
Once the seedling encounters the herbicide, cells in the seedling continue to expand, but not divide. This expansion (not growth) depletes the energy stored in the seed before the seedling can emerge and become “self-sufficient”. The result is that the plant does not survive. Over time, there is question as to how many years of preemergence herbicide applications are needed to reduce the crabgrass seedbank below the threshold level. Are preemergence herbicide applications needed every year to every area of turf, or just on areas where the turf is always thin (along paved surfaces)?
How long do preemergence herbicides work? The duration of herbicide activity (also called residual) depends on environmental conditions such as moisture, temperature, light, and the amount of organic matter in the soil. Once applied and activated, the herbicide remains at a critical concentration at the soil-thatch interface for periods ranging from 6 to 16 weeks, depending on the product.
Herbicide degradation also affects how long a peremergence herbicide lasts. Preemergence herbicides degrade through chemical or microbial processes in the soil until the concentration falls below the critical level where activity is reduced. This can be accelerated when the soil remains warm for extended periods of time. Warm, moist soils encourage microbial degradation of the herbicides carbon structure; the microbes use it as a food source. This is why in years of early and extended soil warming, preemergence herbicides fail to provide season-long control. Simply, the crabgrass germination period exceeds the residual activity of the herbicide.
Do Preemergence Herbicides Affect Turf Growth? The effect of preemergence herbicides on rooting has been investigated during sod establishment where new roots must penetrate the preemergence herbicide barrier. Hummel found that annual applications of prodiamine applied at 2 lb. ai/A (4 times the high use rate) reduced rooting of established Kentucky bluegrass by about 8%. However, in general, preemergence herbicides are thought to be less injurious to root development in established turf.
Turfgrass ecology and physiology could explain this further. Grass root tips are regions of active cell division (called meristems). The root meristems could be affected if it contacts a preemergence herbicide that inhibits cell division. As mentioned previously, turfgrass rooting is most active in the early spring when the soil is cool and top growth has yet to be initiated. It follows, then, that a preemergence herbicide which inhibits cell-division could affect root production during a critical development stage. Accordingly, delaying a preemergence application until soil temperatures warm and roots are through their active stage would avoid the herbicide injuring the new roots. Yet, if crabgrass has already emerged, most preemergence products will not provide control, hence, proper timing remains critical.
For many years, turfgrass pathologists have speculated that the use of preemergence herbicides can contribute to reduced disease tolerance. There are several anecdotal reports of increased bluegrass susceptibility to leafspot, but few documented studies. Researchers at Clemson University identified several preemergence herbicides that can increase the incidence of brown patch on tall fescue, however, the class of herbicides investigated are not widely used on cool-season turf. In addition, Hummel found an increase in severity of Necrotic ringspot with prodiamine applied above the labeled rates. Still, preemergence herbicide influence on cell division may have physiological side effects that are not well understood.
Core Cultivation and Preemergence Herbicide Activity. The role of physical disturbance on crabgrass emergence and development has been discussed. However, many questions have been asked regarding the influence of core cultivation on preemergence herbicide performance. One might think that by disrupting the herbicide barrier, crabgrass control would be reduced. However, in two separate studies where preemergence herbicides were applied and then the area core cultivated, no reduction in crabgrass control was observed. This was true even if the cores were processed or removed.
Preemergence Herbicides Applied in the Fall. In an effort to reduce the amount of activity required on a turf stand in the spring, many managers have experimented with preemergence herbicide applications in the fall or late season. Researchers over the years have concluded that the effectiveness of this practice is highly product, rate, and environmentally related. Bhowmik at the University of Massachusetts found that prodiamine (*Barricade) applied at 0.5 lb. ai/A in October 1997 provided 65% control when rated in August 1998. In fact, this was not significantly different from the April 1998 application of prodiamine at 0.65 lb. ai/A. The best prodiamine program (92% control) was 0.65 lb ai/A applied in October, followed by 0.38 lb. ai/A applied in April. Comparatively, in 1998, dithiopyr (*Dimension) applied at rates of 0.25 to 0.38 lb. ai/A did not provide even 80% control, regardless of application strategy. These results confirm previous reports that the dinitroaniline family of herbicides (pendimethalin, prodiamine, trifluralin+benefin) can provide season long control when applied in the previous fall, while materials such as bensulide, dithiopyr, oxadiazon (*Ronstar) and siduron (Tupersan) are not as effective.
A significant limitation to the use of preemergence herbicides in the fall is the potential to restrict overseeding or other turf establishment procedures the following spring. As discussed earlier, the effect of the herbicides on cell division is rarely selective in that all germinating grass seeds can be inhibited (except in the case of siduron, which is selective for warm season grass seed and can be used at the time of turf establishment). Consequently, if there is turf loss over the winter, the ability to recover the area from seed might be affected.
Researchers at Penn State University applied several preemergence herbicides in October and then overseeded the areas with creeping bentgrass (CB), Kentucky bluegrass (KBG) or perennial ryegrass (PR) in the spring. The plots were rated for density in June. All preemergence herbicides delayed seed germination and seedling development of all grass species. Overall, PR seedings were the most successful in establishing on oxadiazon and dithiopyr treated plots. Of the three species tested, bentgrass was the most sensitive to herbicide residual with no plot reaching 50% density by June.
Clearly, the fall strategy has a trade-off in that dithiopyr and oxadiazon will allow turf establishment in the spring following preemergence herbicide application in the fall but they do not provide acceptable season-long crabgrass control. In contrast, the dinitroaniline materials provide acceptable season-long control but severely limit the success of spring seedings.
7.3.6 Postemergence Crabgrass
Control
Crabgrass Growth and Development. Studies have indicated that crabgrass plants in more highly disturbed turf with low density reach a size more difficult to manage (greater than two tillers) more rapidly. In contrast, the plants that emerge in undisturbed turf need almost 7 weeks to reach the two tiller size. This would permit the turf manager to observe crabgrass pressure following germination then determine the appropriate postemergence strategy over a longer period.
When reviewing the ecological aspects of summer annual weed infestations exclusively from seed, an annual measure of contributions to the seedbank is vital. Undisturbed turf reduces crabgrass seed production in the surviving plants as compared to disturbed turf. For example, slightly disturbed turf produces 5 times the amount of seed as undisturbed turf. Undistrubed turf is a significant long term management strategy. If crabgrass thresholds could be increased as part of an IPM program, there would be a net depletion of the seedbank in dense turf stands.
IPM Approach. Monitoring weed populations is not widely practiced in the turfgrass industry mostly because adequate turfgrass density restricts weed invasion and also due to the widespread use of preemergence herbicides. Additionally, aesthetic thresholds on high value turf areas and functional thresholds on golf putting greens and sports fields are essentially zero; by the time crabgrass is visible it has exceeded threshold levels or it might be too large to effectively control. Therefore, crabgrass skeletons left over from the previous fall, will provide insight into where infestations might occur or, as mentioned previously, areas where turf is consistently thin could be more closely monitored for crabgrass infiltration.
The time required for the level of monitoring needed to successfully reduce pesticide use may be prohibitive to traditional lawn care companies that visit a site 4 to 5 times per year. However, golf course superintendents and sports field and grounds managers who are at the site each day could implement a population-based approach by monitoring at appropriate times.
Postemergence Herbicides. Effective control of emerged crabgrass plants is highly dependent on growth stage and environmental conditions, independent of the herbicide used.
MSMA is a contact-action herbicide meaning that it is absorbed, but not transported, by the plant’s vascular system. The vegetation contacted by the herbicide is killed, but not the entire plant. As a result, several applications are required to control plants having more than 1 tiller.
Fenoxaprop (Acclaim Extra) is effective on crabgrass plants from emergence to the 3 tiller stage. Larger plants may need several applications and it may take 14 to 21 days before the crabgrass is eliminated. Additionally, the effectiveness of fenoxaprop is reduced when plants are drought stressed. Research has indicated that moisture stress must be alleviated within 48 hours of applying fenoxaprop for effective control.
7.3.7 An Integrated Approach
to Crabgrass Control.
The successful implementation of IPM programs based on reasonable thresholds poses a unique challenge for turf managers, lawn care providers, and sod producers. The widespread use of preemergence herbicides in most instances ensures a weed-free turf, regardless of whether or not the herbicide application is needed to provide a weed-free turf.
|
Table 7.3.1 Registered chemical control products for grassy weeds in New York State |
||
|
Weed |
Strategy1 |
Common Name |
|
Crabgrass, goosegrass and other annual grassy weeds |
PRE |
pendimethalin |
|
PRE |
benefin |
|
|
|
PRE |
trifluralin |
|
|
PRE |
bensulide |
|
|
PRE |
benefin plus trifluralin |
|
|
PRE |
oxadiazon |
|
|
PRE |
prodiamine |
|
|
PRE |
oxadiazon plus bensulide |
|
|
PRE |
siduron |
|
|
PRE |
dithiopyr2 |
|
|
EPO |
dithiopyr2 |
|
|
POST |
fenoxaprop |
|
|
POST |
MSMA |
|
Annual bluegrass |
PRE |
pendimethalin |
|
|
PRE |
benefin |
|
|
PRE |
oxadiazon |
|
|
PRE |
prodiamine |
|
|
PRE |
siduron |
|
|
PRE |
dithiopyr2 |
|
|
PRE |
oxadiazon plus bensulide |
|
|
PRE |
bensulide |
|
|
POST |
ethofumesate |
|
|
POST |
sulfosulfuron |
|
Poa trivialis |
POST |
sulfosulfuron |
|
Tall fescue and perennial ryegrass |
POST |
chlorsulfuron |
|
Yellow Nutsedge |
POST |
bentazon |
|
|
POST |
halosulfuron |
|
|
POST |
MSMA |
|
|
POST |
sulfosulfuron |
|
Wild onion, garlic |
POST |
chlorsulfuron |
|
Perennial grassy weeds Quackgrass, Tall fescue, Orchardgrass, bentgrass, etc. |
NSPOST |
glyphosate |
|
1 Strategy code: PRE= preemergence control; EPO= early
postemergence (3-5 leaf stage); POST= postemergence; NSPOST= nonselective
postemergence control 2 Product use restrictions when used on Long Island.
See Table 7.3.3. |
||
While herbicide use has environmental concerns, research indicates
that, when used properly, applications do not pose water quality concerns and
have low environmental toxicity. However, a more integrated approach that sets
reasonable thresholds, utilizes ecological information as a base for management
decisions, monitors weed populations, and implements effective control
strategies is likely to reduce pesticide use.
The first and foremost strategy is to maintain turfgrass density. If the turf is thin, implement a spring based fertilizer program or begin applying CGM. Additionally, introduce rapidly germinating turfgrass species (such as ryegrass) to compete with crabgrass seedlings for resources. Improved density alone in the first year can provide 30 to 80% control, depending on how thin the turf was to start.
In areas where crabgrass infestation is likely, particularly along paved surfaces, a preemergence strategy might be warranted where competition from turf might be reduced. However, one could argue that crabgrass and other annual weeds invade these areas and stabilize the soil; a key aspect of urban water quality. Nevertheless, the visual quality expectations of most turf areas will not allow this level of infestation.
An integrated approach would be to observe the emergent weed population then use a timely postemergence herbicide to control existing plants in combination with a preemergence herbicide to prevent further infestation. This strategy will reduce the effect preemergence herbicides have on turfgrass rooting (which will have slowed in response to the herbicide application) and reduce the amount of preemergence herbicide applied by targeting areas known to be infested. However, if fall seeding is planned, consider using a preemergence herbicide with a shorter residual to reduce the influence on turf seedling development.
Managing annual weeds, such as crabgrass, that infest exclusively from the seedbank can be challenging on highly disturbed turf areas. Annual weeds provide an opportunity to utilize ecological information to the advantage of the turf. Turf density reduces crabgrass infestations, however, not always below threshold levels. If turf density can be maintained until emerged weed seedlings are not able to produce viable seed, the seedbank will be depleted. This will require adjustments in weed threshold levels. Furthermore, the impact of the annual use of preemergence herbicides on the crabgrass seedbank must be better understood to justify continuing this practice. A crabgrass management program must be viewed in the larger context of environmental quality and realistic expectations of turfgrass quality. As such, society will more completely understand the role of a well-maintained turf in an urban environment and demand a more integrated approach.
7.4 Cultural Control of
Broadleaf Weeds
Broadleaf weed populations in turfgrass are influenced by cultural practices. For example, low mowing and inadequate nitrogen favor white clover. Low mowing also weakens turf and favors weeds such as carpetweed, spurge, plantains, and dandelion. Poorly drained areas can favor weeds such as ground ivy, while compacted sites favor knotweed and plaintains.
Proper management practices to maintain a dense, vigorous turf is the best and most lasting method for broadleaf weed control. Of particular importance are proper fertilization, mowing, and watering. Several broadleaf weed species cannot be satisfactorily controlled with herbicides, further increasing the importance of proper cultural management to reduce their establishment and spread. Herbicides should be considered an aid, but not a cure, for broadleaf weed problems in landscaped turf.