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Weed management

Introduction

Weedy turf does not afford the density necessary for safe footing for athletes and would not be desirable if high quality was demanded in high profile situations such as signature fields or professional sports fields.

Due to the nature of sports fields, voids left in the turf are soon occupied by aggressive, opportunistic weeds that can thrive in less than ideal soil conditions. The presence of weeds usually signifies a symptom of a problem; for example, knotweed thriving on compacted soils, annual bluegrass thriving in over irrigated sites.

The first step in weed management is to accurately identify each type of weed in the field.  The resource book, Weeds of the Northeast by Uva and Neal, should be on every turf manager’s book shelf.

These plant identification websites can be very useful management tools for identifying both desirable grasses and weeds:

Next assess the severity of the weed problem by determining how many weeds are present then decide if there is enough weed pressure to warrant action.  Make adjustments in your cultural management program to ensure you are mowing, watering, fertilizing and aerifying properly to improve turf health and density as well as overseeding on a routine basis. These practices will reduce weed and other pest pressure.  Still some weed situations may require the use of herbicides.   

Managing Weeds on School Grounds and Day Care Centers

Under NYS Law Chapter 85, Laws of 2010 (Child Safe Playing Fields Law), all schools and day care centers must comply with restrictions on pesticide use. The law restricts use of pesticides to products containing active ingredients listed as exempt under 25(b) of the Federal Insecticide, Fungicide, & Rodenticide Act (FIFRA). The pesticides must also include inert ingredients that are eligible under 4a of the 25(b) FIFRA exemption. Pesticides containing any active or inert ingredients not listed under the exemption and eligibility criteria of 25(b) FIFRA must receive one-time emergency application approval from appropriate channels.

  • Allowable herbicides which do not need emergency application approval
  • Reduced risk herbicides which do require emergency application approval
  • Exempt pesticides according to the EPA 25(B) FIFRA

Allowable herbicides

The materials listed in Table 1 are allowable and do not need emergency application approval.

The only pre-emergent herbicide with EPA exemption status is corn gluten meal (CGM). CGM must be applied in early spring before weed seeds emerge. In the spring, CGM should be applied when the crocus flowers are in bloom and before forsythia shrub blossoms emerge. Many annual weed seeds germinate when soil temperatures reach 55°F—the same temperature that triggers flowering of forsythia shrubs. Be aware that adding the recommended rate of 20 lbs./1,000 sq. ft. of CGM will result in a substantial nitrogen addition of 2 lbs./1,000 sq. ft. CGM is 10% nitrogen by weight.

The post-emergence herbicides that qualify under the EPA exemption of 25(b) FIFRA work primarily as defoliants that knock down the leaf and stem tissues of plants in contact with the herbicide. For perennials, roots and rhizomes are generally able to regrow in several weeks, requiring two to three applications in a season.

Table 1: EPA 25(b) FIFRA exempt herbicides that do not require “emergency application” approval

Product Name Active Ingredient(s) Parent company
Ecosmart Weed & Grass Killer 2-Phenethyl Propionate, eugenol EcoSMART
Burnout II Weed & Grass Killer Citric acid & clove oil St. Gabriel Organics
Weed Zap Cinnamon oil, clove oil JH Biotech Inc.
C-cide Citric acid Biological Solutions
Brush, Weeds, and Grass Herbicide Citric acid Greenergy
Matratec Clove oil Brandt
Matran EC Clove oil EcoSmart Technologies
Phydura Citric acid, malic acid, clove oil Soil Technologies Corporation
Several brands available Corn gluten meal Several vendors
ADIOS Sodium chloride Herbanature

More information on these products, see: Allowable Herbicides for Schools & Day Care Centers

Reduced risk herbicides

The following reduced risk herbicides listed in Table 2 require "emergency application approval" from the school board, county or state Department of Health, or NYS Department of Environmental Conservation. Details of the approval process can be found at the DEC website.

Approval of a one-time emergency application of a pesticide not determined as EPA exempt under 25(b) FIFRA is determined by the following:

  1. All public schools seek approval from their local school board
  2. All private schools and day care centers receive approval from the county health department (DOH), State DOH district office, or the state DOH’s Bureau of Toxic Substance Assessment
  3. Non-public schools and day care centers on environment-related determinations are approved by the DEC.

Keep in mind that the NYSDEC Guidance advises that the NYSDEC, DOH and SED, in consultation with OCFS, identified the following situations that these agencies generally would not consider to warrant an emergency pesticide application determination:

  • Routine or repetitive problems
  • Pests which can be managed with allowed pesticides or alternative practices
  • When the pesticide application would be for purely aesthetic purposes

Most herbicides listed are non-selective, and will defoliate all vegetation in contact with the product. However, herbicides containing the active ingredient iron HEDTA are selective against broadleaf weeds. Fiesta Turf Weed Killer contains a higher concentration of iron HEDTA compared to Ortho Elementals Lawn Weed Killer. Fiesta application is by licensed professionals, whereas the Ortho Elementals herbicide can be purchased and applied without licensing requirements.

Table 2:  Reduced risk herbicides that require “emergency application” approval

Product Name Active Ingredient(s) Parent company
Weed Pharm Acetic acid (20%) Pharm Solutions, Inc.
Ground Force Organic Herbicide Citric acid Quality Blending Inc.
Avenger Weed Killer Citrus oil (d-limonene) Cutting Edge Formulations
Lilly Miller® Worry Free® Weed & Grass Killer Citrus oil (d-limonene)  
Ortho Elementals Lawn Weed Killer Iron HEDTA The Ortho Group
Fiesta Turf Weed Killer Iron HEDTA Neudorff
Scythe Pelargonic acid, nonanoic Dow AgroSciences
Safer® Brand Fast Acting Weed & Grass Killer Potassium salts of fatty acids Woodstream Corporation

Exempt pesticides

Table 3 lists pesticides containing active and inert ingredients that are considered exempted from 25(b) FIFRA are not subject to federal registration requirements because their ingredients are considered by the EPA to be demonstrably safe for the intended use. To qualify for use under the NYS Chapter 85, Laws of 2010, a pesticide must contain active ingredients listed as exempt under 25(b) FIFRA and inert ingredients listed as eligible under 4a “Inert Ingredients of Minimal Concern.” All inerts listed under the pesticide label must be eligible under the exemption. See the EPA list of eligible 4a inerts.

Table 3: Active Ingredients Exempted Under 25(b) FIFRA

Castor oil (U.S.P. or equivalent)* Linseed oil
Cedar oil Malic acid
Cinnamon and cinnamon oil* Mint and mint oil
Citric acid* Peppermint and peppermint oil*
Citronella and Citronella oil 2-Phenethyl propionate (2-phenylethyl propionate)
Cloves and clove oil* Potassium sorbate*
Corn gluten meal* Putrescent whole egg solids
Corn oil* Rosemary and rosemary oil*
Cottonseed oil* Sesame (includes ground sesame plant) and sesame oil*
Dried Blood Sodium chloride (common salt) *
Eugenol Sodium lauryl sulfate
Garlic and garlic oil* Soybean oil
Geraniol* Thyme and thyme oil*
Geranium oil White pepper
Lauryl sulfate Zinc metal strips (consisting solely of zinc metal and impurities)
Lemongrass oil  

Disclaimer: Please read pesticide labels prior to use. The information contained here is not a substitute for a pesticide label. Trade names used herein are for convenience only; no endorsement of products is intended, nor is criticism of unnamed products implied. Laws and labels change.  It is your responsibility to use pesticides legally.

Managing Weeds with Herbicides

Using Pesticides Properly

Safety is always a priority when using any pesticide including herbicides. If you have the option to use herbicides as a management tool and conditions warrant the use of a herbicide, keep in mind that you must be a NYS certified pesticide applicator. Only recommended materials should be used and applied according to label directions.  Always use the most current Cornell Turf Pest Management Guidelines for Insect, Disease and Weed Management when dealing with managing pest problems.  

Weed situations on sports fields

There are 2 major weed situations that impact field quality and function: annual weeds like crabgrass, annual bluegrass (Poa annua) and knotweed and perennial broadleaf weeds such as plantain, clover, dandelion, ground ivy and chickweed. Control tactics include preventing weeds from germinating or dealing with those that have germinated and become established.

Preemergence herbicides must be applied 10-14 days before the weeds germinate and are absorbed by the root or shoot to inhibited growth. These materials do not control weeds that have germinated already. So timing of the application is important and can be tricky since weeds germinate at different times. Often a repeat application is necessary within 2 months.

Some important point facts to keep in mind:

  • Some preemergence herbicides are safe on some turfgrasses and not others. 
  • Most preemergence herbicides have a long lasting (residual) effect in the soil and will affect overseeding plans.  A 2-4 month waiting period may be required before overseeding can resume. Turpersan was the only herbicide used safely at the time of seeding or shortly thereafter. Now the herbicide Tenacity has become a very strong competitor with Tupersan. Tenacity can be used the day of seeding Kentucky bluegrass for preemergence control of crabgrass and some broadleaf weeds. There are some rate restrictions for use on fine fescue seedling turf.
  • Preemergence herbicides can be applied to 1 year old established turf.
  • Postemergence herbicides are applied to established weeds and should be applied to actively growing weeds that are not under stressful conditions such as high temperatures and droughty conditions.

Managing Tough Annual Weeds

Weed Time of Control Material Comments
Crabgrass in spring before germination Preemergence herbicide Most materials need to be used on mature turf only. Be sure to select a herbicide that can be used on seedling turf.
after germination, but best control on small plants Postemergence herbicide Apply to actively growing weeds. Rate will depend on size of weeds. Some materials not labeled for all parts of NYS.
Goosegrass in spring before germination Preemergence herbicide Goosegrass germinates 2-4 weeks after crabgrass. Treatment is done later than for crabgrass. A second application may be necessary for late germinating seeds.
after germination, but best control on small plants Postemergence herbicide Apply to actively growing weeds. Rate will depend on size of weeds. Some materials not labeled for all parts of NYS.
Annual bluegrass (Poa annua) in summer   Avoid over watering. Allow field to go to permanent wilting point (the Poa will die). Begin overseeding.
Knotweed in spring before germination Preemergence herbicide Late fall applications are more effective. A second application in early-mid June may be necessary.
after germination Postemergence herbicide Apply to actively growing weeds. Rate will depend on size of weeds.

Always use the most current edition of Cornell Pest Management Guidelines for Commercial Turfgrass

Managing Perennial Broadleaf Weeds

Walk your fields to scout and monitor broadleaf weed (BLW) population to determine if control is necessary.  Set reasonable expectations: can you live with 20-30 weeds? Why not just spot treat small areas of concern? Your fields may only need 1 application every 2-3 years to keep BLWs in check.  Herbicide combinations known as 3-ways are generally used to control a wider range of common broadleaf weeds. Does your field have dandelions, clover, chickweed, etc.? The kinds of BLW you have will determine the best 3-way product to use.

Fall applications, when herbicides can be translocated to underground storage organs (tap roots, rhizomes, tubers), are more effective in controlling BLW than spring applications. In fact, recent research has found that applying herbicides after the first frost provided better control of some difficult to control weeds than applying herbicides in early fall.  The October to November application window appears to be the better time to make an application than the September application period. The earlier application time (September) may miss some later germinating winter annuals.

Spring is not the best time to control broadleaf weeds but sometimes certain weeds, such as dandelions, are abundant and need to be controlled. Research has found that a single application of 2,4-D plus 2,4-DP ester formulation provided acceptable control when the GDD exceeded 145.  The amine formulation was best after 180 GDD.  The ester formulation has a strong odor and should not be used when air temperature exceeds 80°, when the risk of volatility (evaporation) increases and ornamentals can be injured by the vapor drift.

If you have to make applications in the summer do not apply herbicides on hot, windy days or when turf is stressed.

Using Growing Degree Days (GDD) as a management tool

GDD information is helpful when determining when and how to deal with tough to control weeds. Crabgrass germinates over a long period of time in the spring. Researchers have found when using the 50°GDD model that 25% of the crabgrass emergence would occur by 560 GDD, 50% would occur by 800, 75% by 1100 and 100% by 1700 GDD. So applying a preemergence herbicide when the GDD are above 1000 would not be effective.  If the GDD were at 800 a combination product containing both pre and post activity would work well.  The post emergence component would take care of the recently emerged crabgrass and the preemergence component would prevent the remaining seeds from germinating.

Annual bluegrass (Poa) is a low growing winter annual grass that thrives in moist well-fertilized fields and compacted soils and is not controlled by mowing. It is also a prolific seed producer and can produce up to 3,000 seeds per square foot. One tactic used to reduce the spread of this weed is to suppress the production of seedheads with a plant growth regulator, thus reducing the amount of seed produced.

GDD are used to predict when annual bluegrass seedheads would emerge and when the peak production period would occur.  For example, mefluidide (a plant growth regulator) applied between 15 and 30 GDD gave excellent seedhead control but applications beyond 45 GDD were ineffective. Managers need to use all the available management tools and at the right time to effectively manage pests.