Integrated Pest Management Manual
Weeds are usually described as any plants growing where they are not wanted. Any undesirable grass or broad-leafed plant species, from a small herbaceous plant to a woody shrub, vine, or tree, may be considered a weed if it is growing in a turf planting. Dicotyledonous (broad-leafed) plants are those that have two cotyledons in each seed. They are characterized by broad leaves and often have woody stems. Some species (e.g., sunflower) only become woody in old parts of stems and roots; these are referred to as semi- herbaceous dicots. Most weeds in turf have little or no woody tissue and are herbaceous (e.g., plantains, dandelion).
Grasses are members of the plant family Gramineae. All grasses are monocotyledonous, have long, narrow leaves with parallel veins, and fibrous root systems. Some grasses produce underground stems called rhizomes (e.g., Kentucky bluegrass, quackgrass) or above-ground runners called stolons (e.g., creeping bentgrass), while others (e.g., bermudagrass) produce both.
It is important to understand the distinction between monocots and dicots since turfgrass herbicides kill only dicots. (Desirable turf species are monocots.)
Why Weeds Invade Turf
Undesirable ("weed") plants will invade man-made environments such as turfgrass plantings wherever they are established. Weeds growing in turf are found where soil has been exposed or disturbed by compaction, planting activities, or maintenance activities such as sidewalk edging. For example, goosegrass and knotweed readily colonize heat-stressed and compacted soil sites along sidewalks or on athletic fields. They also occur where the turf is weakened by adverse environmental conditions (such as drought), thatch accumulation, diseases, or pests to the extent that the turf cannot compete for nutrients, water, or light with weed species. Exposure to de-icing salts, fertilizer or other chemical spills, and dog urine can also leave bare spots in which weeds will grow. Weeds are very common where the grass species being grown is not well-adapted to its environment. Many weed species possess efficient methods of seed dispersal such as wind dissemination of winged or hairy seeds or the ability to spread rapidly by rhizomes, runners, or tubers.
The description of each potential weed species is beyond the scope of this report. Contact the Cooperative Extension Service of the agricultural university in your state or your Park Service regional Integrated Pest Management coordinator for specific information on the most important weeds in your region. A list of useful pictorial weed guides is included in the reference section.
The life cycles of turf weeds can be grouped into the following major types.
Summer annual weeds. These are weeds that grow each spring or summer from seed. Examples include prostrate spurge, ragweed, large crabgrass, and goosegrass. They mature, produce seeds, and die in one growing season. Seeds generally overwinter before germinating the following spring. The majority of annual weeds are of this type. Some annuals, such as crabgrass, can root from leaf-stem junctions, forming dense colonies.
Winter annual weeds. These are weeds that germinate in the fall or late winter from seed, mature and produce seed during the following spring, and die in early summer (e.g., henbit, shepherdspurse, annual bluegrass). Seeds of most of these species are dormant during the spring.
Indeterminate annual weeds. These are weeds such as chickweed and annual bluegrass, that germinate and grow during most seasons in certain regions.
Biennial weeds. These weeds may germinate at any time during the growing season. Examples include wild carrot, bull thistle, and mullein. They usually produce a radial cluster (rosette) of leaves lying close to the soil during the first season. In the second year they produce flower stalks (using food stored from the first season's growth), produce seeds, and die.
Perennial weeds. These plants live for three or more years. Some species may not flower the first year, while others may produce mostly nonviable seeds. Many perennials (e.g., curly dock, dandelion, and common milkweed) spread primarily by producing seeds, while others (e.g., field bindweed, white clover, silverleaf nightshade, bentgrass, and quackgrass) spread both by seed and vegetatively. Vegetative spread can occur by rhizomes, stolons, tubers, or rooting of stem nodes that touch the soil.
The seasonal abundance of weeds is related to their specific life cycles. Summer annuals grow from spring until fall and are killed by low fall or winter temperatures. Winter annuals are present from fall to late spring, so they are usually not found during the summer. Biennials grow during the spring, summer, and fall of their first year, survive over the following winter, and flower during the next growing season. Therefore, some biennial stages are likely to be present at any time of the year. Perennials grow during each growing season. Their aboveground structures may die over the winter (e.g., Dallisgrass and yellow nutsedge) or may remain viable but dormant.
Knowledge of the life cycle of a particular weed species is an important part of its management. For example, mowing a patch of annual weeds to remove the flowers can prevent seed set. Refrain from cultivating areas where there are high populations of weeds which reproduce by rhizomes.
Impact of Weeds
The most obvious impact of weeds on turf areas is the competition with and replacement of turf by weed species. In the case of weeds that over grow an area and then die, such as crabgrass and knotweed, unsightly dead areas can be created. This often leads to the necessity for increased expenditures for turf renovation.
Toxicity to humans and animals is also a consideration. Some common weeds are poisonous if consumed (e.g., black nightshade, pokeweed, poison hemlock, and Johnsongrass); cause inflammation when touched (e.g., stinging nettle, and poison ivy, oak, and sumac); or cause allergic reactions (e.g., common ragweed and goldenrod). Visitor injury or annoyance can result from bees or wasps seeking nectar from some weeds. Furthermore, many weed plants or their seeds have spines, thorns, or burs which can create similar allergic effects.
Weedy areas provide habitat for desirable wildlife and beneficial insects but they can also harbor rodents and arthropods such as rats, ticks, mites, and fleas that might attack humans and domestic animals, or carry diseases that may infect humans and domestic animals. Weeds can also serve as secondary hosts for some fungal pathogens and insects that might attack desirable turfgrasses.
MONITORING AND THRESHOLDS FOR TURF WEEDS
Monitoring For Weeds
Monitoring for actively growing weeds should be done periodically throughout the growing season. Less frequent inspections should be made during winter and early spring to identify sites of soil disturbance or other adverse effects, which may give rise to future weed problems.
It is essential that all monitoring results be reported completely and accurately by site and date so that future surveys will cover the same areas. Recorded weed information allows the site manager to develop a weed history of an area. This will result in a more accurate prediction of future weed management needs.
Regular visual inspections of turfgrass areas should be conducted to look for actively growing weeds as well as newly germinated weed seedlings. Weeds are most likely to be found in areas where some type of disturbance has taken place, such as areas of soil compaction, areas of open soil, worn areas on athletic fields, or areas of soil moisture extremes. Weeds also are likely to grow in turf that has been stressed. This could be the result of being mowed too short, heavy thatch accumulation, competition from trees, or insect or disease attack. Turf can be stressed from extremes in soil pH or the accumulation of road salts along roadways as well. "Dog blight" from animal urine, fertilizer or pesticide spills or misuse, localized wet or dry spots, or accumulations of debris can create open areas where weed infestations begin. Edging along walkways may also open up areas of bare soil where weed seeds can germinate.
Certain weed species tend to be found in certain habitats, so monitoring for a particular weed should be based on a knowledge of its biology. For example, crabgrass is a spring annual that needs light to germinate. Therefore, crabgrass seedlings are most likely to be found in bare or thinning areas in the spring. If they are not found in areas such as this, it is unlikely that they will be found in a shaded area of denser turf. Also, since crabgrass is a spring annual, it may be a waste of time to look for seedlings in mid-summer. This would be an excellent time to look for mature plants, however, to identify seed sources for the following year. Late fall would be the best time to look for seedlings of winter annuals such as henbit or annual bluegrass.
Monitoring large areas of turf for weeds can be very time-consuming, so certain techniques should be employed to make the monitoring process more efficient. One commonly used technique in weed monitoring is to randomly walk 50' long lines of turf, identifying and counting all weeds which touch the line. Lines should be randomly placed in areas that represent all turf species, habitats (e.g., sun v. shade), or different use areas that may be present.
Action Thresholds for Turf Weeds
It is extremely difficult to set specific threshold population levels for weeds in turf since the problems caused by weeds are largely aesthetic, rather than medical or economic. Each park manager should establish threshold and action levels for his/her own area by maintaining records and scouting of weed populations in all turf areas. Action levels will be lower in high-use areas such as lawns around buildings and picnic or rest areas than they will be in large, unused turf areas or parking areas. Vigorous weed competitors such as crabgrass, white clover, and quackgrass should have a lower action threshold than other weeds.
NON-CHEMICAL CONTROL OF TURF WEEDS
Employ sound cultural practices including regular soil testing, proper fertilization at the correct time, mowing at the correct height and frequency, and deep irrigation when needed. Frequent shallow watering discourages root growth and can encourage weed seed germination and some turfgrass diseases. Mow no shorter than 2.5" for cool-season grasses such as Kentucky bluegrass, tall fescue, fine-leaf fescues, and perennial ryegrass to prevent weakening of grass and encouragement of weed seed germination. Always remove debris and heavy thatch from turf and alleviate soil compaction through core aeration.
Renovate chronically poor turf sites to regionally adapted species and cultivars. Fall is the best time to do this for cool-season grasses, while spring is best for warm- season grasses.
Finally, develop a regular monitoring program for weeds and disturbed areas. Set a tolerance level for weeds and remove them mechanically or with proper application of a registered herbicide when that level is exceeded.
Plant turfgrass species and cultivars that are adapted to the growing area and, if possible, resistant to diseases and insects. Even though it is more expensive, use certified seed which is free of noxious weed seeds. Renovation and new plantings should always be done at the times of year that are most appropriate for the particular species; i.e., fall for cool-season grasses and late spring for warm-season grasses. When preparing the area for planting, allow weed seeds to germinate and then cultivate or apply a non-selective herbicide to kill young plants. Cultivation without use of a non-selective herbicide is generally not recommended for weeds that produce rhizomes, stolons, or bulblets because it breaks these structures into smaller pieces and may therefore result in dispersal rather than control of the weed.
During establishment of turf, inspect regularly for weeds. Mechanically remove weeds found in small populations or spot treat areas with a concentrated weed population with a registered herbicides. Always check the herbicide label for information on seedling tolerance.
Cultural Methods of Turf Weed Management
Turf management practices that increase the health, density, and general vigor of grass will discourage weeds through competition. It is essential to use turfgrasses that are adapted for the specific planting area (i.e., region, climate, light intensity) and type of use (e.g., heavy traffic). This will promote the best possible sod development. When turf is established or renovated, grass seed, sod, topsoil, and mulches that are free of weed seeds should be used.
Turf maintenance practices should stress proper fertilization and liming based on the results of soil tests. The amount of nitrogen and timing of its application are extremely important factors for maintaining turf density and discouraging weed encroachment. Consult your Park Service regional Integrated Pest Management coordinator for the correct information for your area. Deep watering (to wet soil to a depth of 5"-8") when grass begins to show signs of wilting will prevent the development of shallow root systems and weak turf, and will help to reduce weed, disease, and insect problems. Frequent, shallow watering encourages the germination of some weed seeds and should be avoided.
It is also important to remove leaves or other accumulated debris from turf, since this can smother or shade the grass, allowing weeds to grow in its place. Heavy thatch is reduced by a combination of core aerification, maintenance of soil Ph between 6.0 and 7.0, and use of balanced fertilizers with slow-release nitrogen. Thatch also can be avoided through the use of tall fescue or other bunch-type grasses (where adapted) and by avoiding excessive nitrogen fertilization.
Mechanical Methods of Turf Weed Management
Frequent mowing will prevent or reduce seed production in some weed species. A few weed species such as Johnsongrass or poison ivy can generally be removed from turf by scheduled mowing. Cool-season grasses should be cut no shorter than 2.5" in height to prevent weakening of the grass plant, and high mowing will promote a dense turf that can more effectively compete with weeds. Despite proper mowing, weeds may still become a problem in turfgrasses. Lower mowing is desirable for some grasses such as bermudagrass and zoysiagrass.
Biological Control of Turf Weeds
No biological control agents for weed control have been approved by APHIS for use in turf plantings.
CHEMICAL CONTROL OF TURF WEEDS
When chemical weed management is necessary, start by using spot applications only in areas where weeds are starting to dominate a stand. When needed, apply broadleaf herbicides primarily during fall or spring to mature turfgrass stands. Annual grass weeds are best controlled with preemergence herbicides applied in early spring, prior to weed seed germination. Keep in mind that proper herbicide selection and use can be complicated. Your regional Park Service Integrated Pest Management coordinator will have more information relevant to your particular situation.
An in-depth discussion on chemical weed control is beyond the scope of this module. However, information on selection of turf herbicides can be obtained from your regional Integrated Pest Management Coordinator, your local Cooperative Extension Service Agent, or your local agricultural state university turf specialist. Some basics of chemical weed control are as follows.
Weed control prior to establishment or renovation. In sites where extremely persistent, perennial weeds exist (e.g., quackgrass, bermudagrass, and red sorrel), a non-selective herbicide such as glyphosate should be applied twice on a 30-day interval. This is ideally done prior to tilling and a few weeks after tilling. The first application will kill existing weeds, while the second will kill weed seedlings which have germinated from seeds that were dormant in the soil. Being non-selective, glyphosate also will injure or kill desirable turfgrasses, flowers, and other herbaceous plants. Glyphosate, however, has no soil residual and treated areas can be reseeded within 24 hours of use.
When new areas of turf are being established, shallow cultivation will bring many buried weed seeds to the surface and allow them to germinate. This should be followed by an application of a non-selective herbicide such as glyphosate to control these weeds.
Perennial broadleaf weed control. These weeds are generally controlled or reduced to below threshold levels with a single fall or spring application of a selective herbicide. Do not use these herbicides unless there is sufficient soil moisture to support active growth of weeds. Air temperatures should range from 65 to 85
Annual grass weed control. This group of weeds includes crabgrass, goosegrass, foxtails, sandburs, etc. These weeds are considered major weeds because they can effectively compete with grasses and can significantly reduce turf stand density in a single season. High mowing greatly retards annual grass weed populations, but when monitoring indicates increasing populations of annual grasses, an application of pre- emergent herbicide should be planned for the next spring. To be effective, these herbicides must be applied in early spring prior to weed seed germination and they must be watered-in by rain or irrigation within three to five days of application. Watering-in is critical and in the absence of irrigation these herbicides are best applied on a rainy day. Consult a turfgrass specialist or your county cooperative extension service for information about appropriate application times and herbicide selection in your area.
Postemergence annual grass weed control. The postemergence approach to annual grass weed control requires more knowledge and professional skill to be effective. Consult an extension turfgrass specialist in your region for more detailed information on herbicide selection and use.
An example of a turf weed management program which has been implemented by the National Capital Region of the Park Service follows.
Turfgrass Weed Management in the National Capitol Region
The National Park Service manages turfgrass in three of its management zones: developed, special use, and historic. Within each management zone the visual objective for turfgrass quality will depend on the management objectives established for the park and management resources available.
Generally, turfgrass can be classified into three classes: ornamental turfgrass, recreational turfgrass, and greenspace turfgrass. These classes will determine the type and level of maintenance each should receive.
Classes can be used in integrated pest management to determine thresholds and management strategies for specific pests. The following guidelines have been developed specifically for weed management in turfgrass to assist in determining where and when the use of herbicides would be appropriate.
The intent of these guidelines is to sustain the quality of turfgrass appropriate for specific sites, while minimizing dependency upon herbicides to achieve this quality. This can best be done by using a fully integrated turf management program which defines the site objectives, assures objective monitoring, and encourages cultural management practices.
Class A. Ornamental turfgrass
Lawns classified as ornamental have the highest visual quality objective. Turfgrass must appear uniform in color and texture with weeds and bare spots unnoticeable to the general public. Ornamental lawns are exposed to minor foot traffic and receive the highest level of maintenance. Ornamental lawns provide the setting for memorials and other significant sites and features. Considering the intensive maintenance these areas require and that visual quality necessitates limited visitor use, managers should restrict the Class A designation to the minimum area necessary to achieve the visual management objective.
Class B. Recreational turfgrass
Turfgrass which provides the setting for certain passive recreational and athletic activities can be classified as recreational turfgrass. Class B areas would include small urban parks and some playing fields for organized sports, as well as turfgrass surrounding offices, parking lots, and other support facilities. Although such areas may have ornamental significance, the visual quality and level of maintenance is less demanding than ornamental turfgrass. Visitor use is common and some weed infestation is tolerable. The uniformity in color and texture is not as critical as in ornamental areas.
Class C. Greenspace turfgrass
Greenspace turfgrass encompasses large areas that receive minimal maintenance other than mowing. The aesthetic objective for the site is achieved simply by the presence of turf and not by its quality. Greenspaces would include large picnic and informal recreation areas, parkway medians, and roadsides.
Managers should classify all turfgrass within a park into these three classes. In some parks all three classes may occur contiguously. The classification scheme allows for managers to exercise their judgement and experience in classifying turfgrass. Classification, however, should be based on the parks statement for management and other mandates and factors establishing the visual objective for the site. In historic areas, class designation should be supported by documentation describing the appearance of the site during the period portrayed. All site classifications should be defined in the resource management plans and reviewed routinely to assure their proper designation.
The application of herbicides in conjunction with other weed management tactics may be necessary to achieve the visual quality objective. Proposals to apply herbicides (Pesticide Proposal Form 10-21A) should be supported by the site classification and quantitative monitoring data showing that the action threshold for the class has been reached. The action threshold is expressed as the maximum percentage of weed cover allowable before herbicide applications can be made. Turfgrass should be monitored routinely and systematically, recording quantitative measures of weed cover.
Class A -- Ornamental Turfgrass
Spot treatment. Spot treatment may be considered when an action threshold of 15%- 29% weed cover is reached. Treatment must be limited to those areas that have reached the action threshold and can be sprayed with a backpack or other similar single nozzle, small capacity sprayer.
Recovery treatment. An action threshold of 30%-49% weed cover indicates a possible need for broadcast application of herbicides by backpack or tractor-mounted, multi-nozzle boom sprayer. Treatment must be preceded by a complete review of the turf management program to determine why the weed level reached the Recovery action threshold. The review will examine the level of use, compaction, turf variety, mowing height, moisture management, fertility, Ph, and other factors considered pertinent to maintain Class A turf.
Herbicide applications must be used in conjunction with other tactics to remedy the management deficiencies or site factors responsible for weed infestation. Recovery treatments will not generally be approved for consecutive years.
Renovation. When the action threshold reaches 50% or greater weed cover, complete renovation is warranted. Renovation will be preceded by broadcast application of glyphosate or other similar broad-spectrum, post-emergent, short-residual herbicide that kills all vegetation. The treatment must be preceded by review of the overall turf management program as described for recovery treatments. The site will be seeded or sodded as appropriate. All other management or site deficiencies determined in the review must be corrected.
Class B -- Recreational Turfgrass
Spot treatment. Not permitted in recreational turfgrass.
Recovery and renovation. As described for Class A turfgrass.
Class C -- Greenspace Turfgrass
Herbicide applications are not permitted on Class C turfgrass.
Monitoring is the most important component of any integrated pest management program. Only through routine monitoring can a manager be aware of changes in turf status. A proposal to apply herbicides must be supported by objective monitoring data demonstrating that the action threshold has been reached. Visual estimates are often inaccurate and therefore unacceptable as justification for a herbicide request. In addition to determining the percent weed cover, monitoring also determines the percent bare ground which is susceptible to weed invasion. Monitoring facilitates the identification of the weed species present. Species identification is critical in developing the appropriate management strategy. Post-treatment monitoring provides a measure of the efficacy of the overall management program.
The procedure and intensity in which monitoring is applied will depend on the size and uniformity of the area being examined. If the area of interest is not uniform in cover, it should be subdivided and monitoring performed in each subdivision. A minimum of five monitoring points should be used in any size area and no less than ten per acre. The more monitoring points used, the more definitive the assessment will be. However, monitoring must be simple, quantitative, and efficient. Before monitoring, there must be a consensus as to what constitutes a weed, since some species such as bermudagrass, annual blue grass, and clover may or may not be considered weeds depending on the site and its management.
Monitoring in small areas can be performed by randomly selecting monitoring points by the toss of a marker (rock, ball, etc.). The marker can be thrown at five random points in an "M" pattern spread over the expanse of the area to be examined. In areas of more than one acre, mark off a series of straight or zigzag lines and randomly select points along those lines. Measurements at the monitoring points can be made in a variety of ways. At each monitoring point a specific area of turf should be examined and the percentage of turf, weed, or bare ground determined. For example, a one foot square block of turf might be 33% covered by turf, 33% by weeds, and 33% bare. If possible, determine what weed species are present. This is important because the life cycle of a weed is a consideration in determining how to manage it. For example, it is not a good idea to cultivate an area with perennial weeds which produce rhizomes, since this will cause more plants to be produced. Winter annuals will set seed in early spring and could be mowed before this to prevent seed formation.
All herbicide applications must be made in the early morning or evening before or after visitors are present. Treatments must be made in a safe and responsible manner either by a licensed applicator or individual working under the "line of sight supervisor" of a licensed applicator. Treated areas must be posted.
Pesticide Approval Request (Form 10-21A)
In addition to the information already required, requests for herbicide use on turfgrass must include:
1) turfgrass classification, class A or B.
2) the percent weed infestation and weed species present,
3) the method used to determine the level of infestation, pre- and post-treatment, and
4) a statement of other management measures that will be taken.
Table 1 summarizes the herbicide treatment options based in turfgrass class and percent of weed cover. If herbicides are going to be part of your turf weed management strategy at a particular site, this will help you to decide the most appropriate way in which to ue them.
Table 1. Herbicide treatment options based on turfgrass class and weed cover.
|% Weed Cover||Class A||Class B||Class C|
|0% to 14%||CULTURAL||CULTURAL||CULTURAL|
|15% to 29%||SPOT||CULTURAL||CULTURAL|
|30% to 49%||RECOVERY||RECOVERY||RECOVERY|
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2. Baldwin, F.L., and E.B. Smith. Weeds of Arkansas Lawns, Turf, Roadsides, and Recreation Areas: A Guide to Identification. MP 169 of the Cooperative Extension Service of Arkansas, Little Rock. AR.
3. Dennis, L.J. 1980. Gilkey's Weeds of the Northwest. Oregon State University Press, Corvallis.
4. Nelson, E.W. 1979. Nebraska Weeds. Nebraska Dept. of Agriculture, Lincoln.
5. Regional Technical Committee of Project NC-10. 1960. Weeds of the North Central States. University of Illinois Agric. Exp. Sta., Urbana.
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2. Aldrich, R.J. 1984. Weed-Crop Ecology Principles in Weed Management. Breton Publishers, MA.
3. Bohmont, B.L. 1990. The Standard Pesticide User's Guide. Prentice-Hall, Inc., NJ.
4. Herbicide Handbook. 1989. Weed Science Society of America, Champaign, IL.
5. Shurtleff, M.C., T.W. Fermanian, and R. Randell. 1987. Controlling Turfgrass Pests. Prentice Hall, Inc., NJ.
6. Weed Control Manual and Herbicide Guide. 1991. Meister Publishing Co., OH. (Updated each year. Lists currently available herbicides by common and trade names).
7. Weed Control and Plant Growth Regulation. 1989. Air Force Manual, 91-19. Washington, D.C.