Managing
Weeds in Turfgrass: Introduction
Excerpt from 2010-2011 Chapter 17, Publication 75, Guide to Weed Control, Order
this publication
Table
of Contents
- Introduction
- Managing
Weeds in Turfgrass
- Cultural Practices to Manage
Weeds in Turfgrass
- Chemical Weed Control
- Related
Links
Introduction
- In April
of 2009, the Ministry of Environment ammended the Pesticides Act with the Cosmetic
Pesticides Ban Act, 2008 and Ontario Regulation 63/09. Pesticides are now classified
for sale and use under 11 different classes. It should be noted that agriculture
(e.g. nursery production) is exempted. For more information on the legislation
see the Ministry of the Environment’s website at www.ontario.ca/pesticideban.
- 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)
Managing
Weeds in Turfgrass
The major species of broadleaf weeds infesting turf
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.
Cultural
Practices to Manage Weeds in Turfgrass
The easiest, cheapest and most effortless
way to keep turf fields 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 mowing,
fertilizing, watering and drainage, controlling thatch, avoiding compaction, selecting
the appropriate turfgrass and removing weeds mechanically.
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 stand. Regular, timely fertilizer applications
can help turf out-compete broadleaf and grassy weeds. Too few nutrients applied
to the turf leads to increased susceptibility to weed infestation and some disease.
Too much fertilizer may lead to soft, weak grass that is also prone to disease
and will not stand up to traffic.
Watering is critically
important when normal rainfall does not provide enough moisture. Irrigate weekly,
using 2.0–2.5 cm of water per application to help produce thick, deep-rooted
turf.
Frequent light irrigation will have the opposite effect on the grass
roots. It encourages the germination and growth of shallow rooted species such
as crabgrass and creeping bentgrass. Too much irrigation water will lead to infestation
by yellow nutsedge and annual bluegrass. Provide adequate watering near trees
and hedges because they compete for available moisture.
Drainage
is important to ensure roots are healthy and functioning. Waterlogged soils do
not provide roots with the air they need to function properly. 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 discourages weed infestations.
Select turfgrass species to match the growing environment
and the intended use. For example, fescues are tolerant to low-light intensity
in shaded areas under trees and also tolerate low maintenance. 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 if weed populations are not too high. Weeds can be
removed by hand or pulling with a knife or V-shaped spudding tool. There are many
new non-selective weed control tools on the market that use steam, hot water,
super-heated water, propane flamers, etcetera. There is limited research on the
level of control of these new non-selective weed control tools. Try in a small
area to determine their effectiveness. Be aware of applicator safety.
Chemical
Weed Control
The following herbicide information is for use on
sod farms, golf courses, turf used for lawn bowling, cricket, lawn tennis and
croquet, and for sports fields hosting national or international tournaments.Chemical
weed control is often the easiest, most effective and cheapest way to control
annual and perennial weeds when populations are very high.
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.
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 half the usual concentration
(0.4–0.6 kg in 800 L water/ha) or mecoprop-P (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. Weeds
not normally controlled by 2,4-D may be controlled by mecoprop, or dicamba, or
with combinations of one of these with 2,4.D. Mecoprop 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, page 335, for information
on the response of common weeds to various herbicides. Thorough wetting of the
weed foliage is important.
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 turf weed species, including knotweed, which is not controlled
by most other herbicides except when very young.
Broadleaf Herbicides:
2,4-D
controls many broadleaf weeds including dandelion, plantains, shepherd’s
purse, smartweeds and vetch.
Mecoprop controls 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 controls broadleaf weeds
including 2,4-D tolerant weeds such as chickweeds, clover and young knotweed but
does not adequately control plantains.
MCPA controls dandelion, plantains
and shepherd’s purse. 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 compared to only one of these herbicides.
Grass
Herbicides:
BETASAN, DIMENSION and corn gluten meal based products
can be applied before crabgrass emerges in the spring, and ACCLAIM SUPER and DIMENSION
can be applied after emergence. Corn gluten meal based products can also be applied
in the fall.
Herbicide Application Timings
Site
Preparation or Preplant (PP) Treatments
Preplant treatments are applied
before the turf is sown or planted. Also see Preplant Weed Control, Preplant Site
Preparation Prior To Any Crop on page 81 for details of
products, rates and
remarks.
Preemergence (PRE)
Rainfall at 15–20
mm within 7–10 days after application is necessary to activate preemergence
treatments. These materials prevent emergence of many weed seedlings without reducing
turf stand. Apply immediately after seeding or at least before the emergence of
the turf.
Postemergence (POST)
Herbicidess are
applied after the turf seeds have sprouted. Applied as directed, the weeds can
be killed without injury to the turf. It is important to apply herbicides to weeds
at their most susceptible stage. Smaller weeds are generally easier to kill but
enough leaf surface is needed to intercept the required amount of herbicide.
Related Links
