Managing Weeds in Turfgrass: Introduction

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Managing Weeds in Turfgrass: Introduction

Author:	OMAFRA Staff
Creation Date:	25 November 2002
Last Reviewed:	20 June 2008

Pub 75: Guide to Weed Control > Turfgrass > Managing Weeds in Turfgrass: Introduction

Excerpt from 2008 Chapter 17, Publication 75, Guide to Weed Control, Order this publication

Introduction
Managing Weeds in Turfgrass
Cultural Practices to Manage Weeds in Turfgrass
Chemical Weed Control
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Introduction

The best weed management system for turfgrass uses a combination of chemical weed control with cultivation and other techniques. See Chapter 1, Principles of Integrated Weed Management.
Site Preparation Before Planting - see Chapter 6, Special Methods of Weed Control.
To convert kg/ha or L/ha to g/100m² or mL/100m² - Multiply by 10 and change units.
For example - 11 kg/ha becomes 110 g/100m² (28.4 L/ha becomes 284 mL/100m²)
All treatments should be applied in 150-300 L/ha (60-120 L/ac) water unless otherwise noted.
Recommended rates /ha or /ac refer to area actually treated with herbicide.
Herbicide Application Timings:
- Soil Applied Treatments include:
  - Preplant (PP) - Also see Chapter 6, Special Methods of Weed Control for details of products, rates and remarks.
  - Preplant Incorporated (PPI) - Two incorporations at right angles operating at a depth of 10 cm using a double disk (7-10 km/hr) or vibrating shank S-tine cultivator (10-13 km/hr) are required unless otherwise stated. Cultivation-type equipment used for herbicide incorporation is known to spread perennial weeds to previously uninfested areas. Special attention should be directed toward machinery cleanliness, and/or treating fields with perennial weeds last.
  - Preemergence (PRE) - Rainfall of 15-20 mm within 7-10 days after application is necessary to activate preemergence treatments. Shallow cultivation, rotary hoeing or harrowing will control weed escapes and improves herbicide activity in the absence of rainfall.
- Postemergence (POST) - Leaf stage of the weeds is critical for good weed control. Smaller weeds are generally easier to kill but there needs to be enough leaf surface to intercept the herbicide. Apply according to recommended leaf stages. Crop stage is important to optimize crop safety. Adjuvants will frequently improve the weed control when used as directed. Weather or other conditions may influence the optimum rate of adjuvant, see the product label for more details. Always use appropriate drift management technology.

Managing Weeds in Turfgrass

The major species of broadleaf weeds infesting lawns in Ontario are dandelion, plantain, black medick, chickweed, prostrate knotweed, mallow, henbit, ground-ivy, and white clover. The major grassy weeds are crabgrass, annual bluegrass, quackgrass, orchard grass and bentgrass.

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Cultural Practices to Manage Weeds in Turfgrass

The easiest, cheapest and most effortless way to keep lawns free of weeds is to encourage vigorous growth of turfgrass. Most weeds cannot compete in a dense, healthy turf. Growth practices that encourage vigorous grass growth discourage weed infestations. Such practices include proper irrigation and drainage, use of fertilizers, insect and disease control, and the use of the correct type of turfgrass for the situation.

Mowing stimulates bud development and tillering, inducing the sod to become thick and dense. Cut commonly grown grasses such as the fescues, bluegrasses and turf-type perennial ryegrasses to a height of 4-6 cm. Invasion by weeds may occur, if grasses are cut shorter than 4 cm.
Avoid scalping the turf when cutting around trees and flowerbeds. Do not remove more than one-third of the leaf area when mowing. Mowing too frequently may reduce the carbohydrate reserves of the turf, thereby reducing its competitiveness. Mowing can also be used to remove annual weeds and eliminate seed production, reducing or preventing the spread of weeds.
Fertilizer is particularly important in establishing a thick, dense, and healthy turf sward. Too few nutrients applied to the turf will lead to increased susceptibility to disease and insects. If this happens, the root system of the turf will not develop and the grass will not be able to withstand traffic or recover from injury. Too much fertilizer may lead to soft, weak grass that is prone to disease damage.
Watering is critical during periods of drought that may injure, kill or induce dormancy in the turf, thereby allowing weeds to become established. Irrigate weekly, using 3 cm of water per application to help produce deep-rooted turf. Frequent light sprinkling will have the opposite effect on the grass roots. Light water applications encourage the germination and growth of shallow rooted species such as crabgrass and creeping bentgrass. Too much irrigation water will lead to infestation by yellow nut sedge and annual bluegrass. Remember to provide adequate watering near trees and hedges because they compete for available moisture.
Drainage is important to ensure that waterlogging does not occur, so install adequate drainage if needed.
Compaction should be avoided to prevent invasion by weeds that thrive in compacted soil. Knotweed and annual bluegrass often invade turfgrass where compaction, caused by excessive traffic, is a problem. While aeration practices do help, the best answer is to modify the area to reduce traffic. Immediate resodding or reseeding of damaged areas discourage weed infestations.
Select turfgrass species to match the growth conditions. For example, fescues are tolerant to low-light intensity in shaded areas under trees, whereas turf-type perennial ryegrass varieties are quick to establish in newly seeded areas and will crowd out germinating weeds. Kentucky bluegrass, although slow to establish, is very competitive once established.
Mechanical removal of weeds can be very effective, provided weed populations are not too high. Weeds can be removed by hand pulling, with a knife or V-shaped spudding tool.

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Chemical Weed Control

Chemical weed control is often the easiest, most effective and cheapest way to control annual and perennial weeds when populations are very high.

New Turf - After seeding new turf, many annual weeds may emerge before the grass seedlings are up. If these young weeds are not controlled, they will shade and eventually crowd out much of the grass population. Mowing at a height of 6-8 cm will eliminate many such weeds. If they are extremely thick, 2,4-D at 1/2 the usual concentration (0.4-0.6 kg in 800 L water/ha) or mecoprop (0.4-0.6 kg in 400 L water/ha) can be used - but not before the grass has been up at least 4 weeks.

Established Turf - Applications of 2,4-D are most effective in the spring, or in September when the weeds are growing well. This treatment generally thins clover in the lawns. Weeds not normally controlled by 2,4-D may be controlled by mecoprop-P, and dicamba, or with combinations of these 3 herbicides. Mecoprop-P and dicamba are also sold in commercial mixtures with 2,4-D. See Table 18-2 Classification of Weeds According to Response to Various Foliage Sprays for information on the response of common weeds to various herbicides. Thorough wetting of the weed foliage is emphasized.

Do NOT use mixtures containing dicamba close to shrubs, or other susceptible ornamentals, at rates above 0.425 kg/ha (active). Prepared mixtures of this chemical and 2,4-D are effective on a wide range of lawn weed species, including knotweed, which is not controlled by most other herbicides except when very young.

Broadleaf Herbicides:

2,4-D will control many broadleaf weeds including dandelion, lamb's-quarters, mustards, peppergrass, pigweeds, plantains, shepherd's purse, smartweeds, vetch.
Mecoprop-P will control many 2,4-D and MCPA tolerant weeds such as chickweeds, clovers, ground ivy and black medick, and is generally safer to use on bentgrass than 2,4-D.
Dicamba will control broadleaf weeds including 2,4-D tolerant weeds such as chickweeds, clover and young knotweed but will not adequately control plantain.
MCPA will control field horsetail, dandelion, lamb's-quarters, mustards, plantains, shepherd's purse, and wild carrot. Weeds tolerant to MCPA include chickweeds, clovers and black medick. Two and three way mixes of these herbicides extend the spectrum of weeds that are controlled by using only one of these herbicides.

Grass Herbicides:

Betasan and Dimension can be applied before crabgrass emerges in the spring, and Acclaim Super and Dimension can be applied after emergence.

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