Weed Control Options in Landscape Beds and Groundcovers
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Cooperative Extension Service Landscape
Feb. 2001
L-12
Weed Control Options
in Landscape Beds and Groundcovers
Jay Deputy and Joe DeFrank, Department of Tropical Plant and Soil Sciences
W eed management in landscape plantings can be
complex. Challenges arise from differences in the
combinations of ornamental plants grown, the type of
ner” stems either underground (rhizomes) or over the
surface of the soil (stolons), or by both methods. These
grasses are especially hard to control because of the dif
weeds that occur, and the use of mulches. Herbicide op ficulty in completely killing all of the rhizomes and sto
tions are often limited by mixtures of woody and herba lons. Popular turfgrasses like bermudagrass, centipede
ceous ornamentals in the landscape, as well as health grass, and zoysiagrass are warm-season perennials that
and environmental concerns. Usually, various control often invade neighboring landscaped garden areas by
strategies must be combined to keep weeds from de extending runners. These grasses are most likely to be a
tracting from the beauty and quality of a landscape. continual problem during the warm summer months
Weeds can be grouped in three general categories: when they are actively growing. Repeated herbicide
broadleaf, grasses, and sedges. Each type presents its treatments may be necessary to maintain a killed strip
own special problems and often requires different con between landscape beds and the turf.
trol measures. With Hawaii’s year-round growing sea Annual grasses are easier to control than perenni
son, all of these weed types can constantly become es als. They are classified as “summer” or “winter” types
tablished in a landscape, and a continuing year-round based on their emergence pattern, although both types
program is necessary to keep weeds under control. tend to persist throughout the year in Hawaii. Summer
Many of the common broadleaf weeds that infest annuals germinate more prolifically in the spring, grow
Hawaii’s landscape are annuals; that is, they grow from during the summer months, and flower in late summer
a seed, flower and produce seeds, and die within one or early fall, after which they gradually begin to die.
growing season—a period of weeks or months. Annual Common summer annuals include large crabgrass in
weeds are most likely to enter the cultivated landscape landscape beds and smooth crabgrass, goosegrass, and
area through seed dispersal by wind, birds, or soil ero smutgrass in turf areas. Winter annuals tend to emerge
sion. In Hawaii, many annuals do not appear seasonally more in the fall in Hawaii, persist through the winter
but rather occur throughout the year, with no winter dor into spring, and die in the hot, dry summer months.
mancy period. They therefore have a continuous life Certain sedges can be troublesome weeds. They
cycle and present a constant problem. persist in the soil by producing underground tubers as
Both annual and perennial grassy weeds occur in propagules, in addition to spreading by seed and rhi
Hawaii’s landscapes. Perennial grasses live for many zomes. Sedges are often mistakenly identified as grasses,
years. They can spread by seed dispersal as do annuals, but herbicides that control grassy or broadleaf weeds
but creeping perennials also spread by extending “run are usually ineffective on them. Among the most com-
Caution: Pesticide registrations frequently change. Mention of a pesticide in this publication is not a guarantee that the product
will be registered for the same use in the future. Read the pesticide label to be sure that the intended use is included on it, and
follow all label directions. Consult an agrichemical sales representative, the Hawaii Department of Agriculture, or the University of
Hawaii Cooperative Extension Service for up-to-date information on available pesticide formulations.
Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June
30, 1914, in cooperation with the U.S. Department of Agriculture. Andrew G. Hashimoto, Director/Dean, Cooperative Extension Service/CTAHR, University
of Hawaii at Manoa, Honolulu, Hawaii 96822. An Equal Opportunity / Affirmative Action Institution providing programs and services to the people of Hawaii without
regard to race, sex, age, religion, color, national origin, ancestry, disability, marital status, arrest and court record, sexual orientation, or veteran status.
CTAHR publications can be found on the Web site or ordered by calling 808-956-7046 or sending e-mail to ctahrpub@hawaii.edu.L- 12 Weed Control Options in Landscape Beds and Groundcovers CTAHR — Feb. 2001
mon weedy sedges in Hawaii are purple and yellow nut stone are generally more expensive than organic
sedge (nutgrass), green and white kyllinga, and McCoy mulches, they are stable over time, allow good water
grass. (See also CTAHR publication L-9, Nutgrass con drainage and air flow, and can make a very attractive
trol in the lawn, landscape, and garden.) addition to the landscape.
Coarse-textured organic mulches such as wood chips
Design and prepare the ornamental planting can be applied up to 4 inches deep and provide long
bed with weed control in mind term weed control. Fine-textured mulches pack more
Grouping similar plant types together allows for more tightly and should be limited to a depth of 2 inches. They
weed control options in landscape beds. Herbaceous degrade more quickly and consequently provide weed
plants, especially annual flowers, are more sensitive to suppression for a shorter period of time. The optimum
herbicides than ornamental shrubs or trees. In such mulch is relatively coarse-textured with a low water
flower beds, the preferred weed control options are use holding capacity.
of preemergence herbicides, mulching, and hand pull Perennial weeds such as bindweed, oxalis, and plan
ing. More herbicides are registered for use around woody tain often have sufficient root reserves to penetrate even
ornamentals, which also help shade out weeds that might thick mulch layers. Some annual weeds can grow
otherwise germinate. through mulches or germinate on top of a mulch as it
A rich variety of plant types and colors is desirable decomposes. Weeds with wind-borne seeds, such as
and attractive in a landscape, but designers should care horseweed, fireweed, and dandelion, are most likely to
fully consider the requirements for maintaining the land become established in a mulch.
scape over time. Before establishing a planting, evalu
ate the site’s soil type and slope and identify existing Weed barriers can improve the weed control
weeds so suitable plants, mulches, and herbicide options obtained with mulches
can be selected. Before the beds are planted, control Solid black plastic film under a mulch dramatically im
existing weeds with a nonselective herbicide such as proves weed control compared to mulch alone. Black
RoundUp® or Finale™. plastic has been used for years and provides excellent
Avoid introducing weeds or their propagules (seeds, control of annual weeds and suppression of perennials.
rhizomes, stolons) into the landscape with soil or plant However, nonporous black plastic restricts water pen
ing materials, or on equipment. Eliminate emerging etration and air exchange; thus, it is not recommended
weeds before they begin to form seed. Soil of container for long-term use in landscape plantings.
grown stock and root balls of field-grown nursery stock Porous, black landscape fabrics (geotextiles) have
may contain weed seeds, tubers, or fragments of rhi been developed to replace black plastic film in land
zomes, which are also transplanted into the landscape. scapes. Landscape fabrics form a barrier and block sun
light from reaching weed seeds but allow the water and
Mulches are an effective tool gas exchange necessary for plant health. However, weed
Mulches are commonly used in landscape beds to im shoots can penetrate up through any openings in the fab
prove appearance, conserve moisture, and control weeds. ric, and grass roots can penetrate down through open
Mulches work well in suppressing annual weeds but gen ings in the fabric if they are allowed to germinate on the
erally will not suppress perennials as effectively. Mulches surface of the mulch. Therefore, the fabric should be
can be classified as organic (bark, wood chips, compost, free of rips and other openings for maximum weed con
leaves), inorganic (crushed rock, crushed coral, gravel), trol. A key to the successful use of fabrics is maintain
and synthetic (black plastic, landscape fabric). Mulches ing the mulch layer free of weeds, either by hand-weed
limit light and physically block seedling growth. ing or the use of herbicides. Some brands of fabric, such
As organic mulches break down into finer particles, as Biobarrier II®, contain a preemergence herbicide. Al
the mulch layer becomes a good growing medium for though relatively expensive and labor intensive to in
weeds. Inorganic and synthetic mulches therefore usu stall, landscape fabrics are cost effective if the planting
ally provide better weed control than organic mulches. is to remain in place for several years.
Although natural inorganic mulches such as gravel or Landscape fabrics are most useful for long-term
2L- 12 Weed Control Options in Landscape Beds and Groundcovers CTAHR —Feb. 2001
weed control around trees, shrubs, and woody ornamen Herbicides are classified according to their mode of
tal plantings. They are less suitable for annual flower action and the kinds of weeds they control. Some classes
beds that are replanted periodically, or in plantings where of herbicides are effective either on grasses or broadleaf
a fabric could inhibit rooting and spread of groundcover weeds but not both. Many herbicides that are effective
plants. Landscape fabrics can eventually be damaged on grasses or broadleafs are not at all effective on sedges.
by tree and shrub roots, and pulling up a fabric may be Therefore, management strategies can be very differ
difficult due to root growth within the material. ent, depending on the weeds present and their location
When installing a fabric, first remove existing weeds within the landscape bed.
and stones. Cut the fabric to fit loosely around tree trunks
and shrubs, leaving enough room for future growth and Preemergence herbicides
also to inspect the root crown for injury. For unplanted Preemergence herbicides affect germinating seeds and
beds, cut an “H” in the fabric for each planting hole. are generally used to control annual grasses and broad
Avoid leaving soil from the planting hole on top of the leaf weeds that spread primarily by seed dispersal. To
fabric, because this will serve as a source of weed seeds. be effective, this type of herbicide should be applied to
After planting, fold the fabric back down to keep the soil several weeks before weed seeds would nor
the sheet as continuous as possible, and secure it with mally germinate. In Hawaii, seeds are continually being
U-shaped pegs. Apply a thin layer of organic or rock disbursed and germinating, which necessitates periodic
mulch on top of the fabric to prevent its deterioration applications of preemegence herbicide throughout the
from exposure to UV light (photo-degradation). If weeds year.
grow into or through the fabric, remove them when they Granular preemergence herbicide formulations may
are small to prevent holes from forming in the fabric. be applied with a fertilizer spreader. The spreader must
be calibrated to apply the rates specified on the product
Mulches in combination with herbicides label. Liquid formulations can be applied with com
improves weed control pressed-air or hose-end sprayers. Both granular and liq
Because mulches rarely provide complete weed control, uid preemergence herbicide formulations can be applied
preemergence and postemergence herbicides can be ap over the top of existing ornamentals and groundcovers,
plied to improve the level of control. Some factors to if the label allows. Preemergence herbicides must cover
consider before selecting an herbicide are the area uniformly and come in contact with the soil,
• Which weeds are present and which weeds are ex and most of them need to be activated by irrigation or
pected to emerge? Are the grassy weeds annual or rainfall within 1–2 weeks of application. Best results
perennial? Choose an herbicide or combination of her are obtained when at least 1⁄2 inch of water is applied to
bicides that will be effective on these weeds. the area immediately after applying the herbicide. This
• Which ornamental species are present in the plant also helps to wash the herbicide off the foliage of any
ing? Is it safe to over-spray a grass herbicide? Be sure existing plants and onto the soil surface.
the herbicide is registered for use on these species. Preemergence herbicides differ in the length of their
• How close are susceptible ornamentals and turf, and what residual effect and should be reapplied accordingly.
is the risk that they will be injured by the herbicide? Some preemergence formulations are volatile and must
• What is the potential for residual effects of the herbi be cultivated into the soil for maximum effectiveness;
cide on subsequent plantings, especially those con others form a thin film over the surface of the soil and
taining annual flowers? are ineffective if the soil surface is disturbed after appli
• What precautions need to be taken to protect the ap cation. Generally, herbicides degrade faster under wet,
plicator and property owners? warm conditions than under dry, cool conditions.
• What method will be used to apply the herbicide Dinitroanilines are a group of preemergence herbi
(granular or spray formulation)? cides that are most effectively used on groundcovers or
• How much will the treatment cost? plantings that contain only herbaceous plants. This group
• How long will it take for the treatment to be effec includes Treflan® (trifluralin), Surflan® (oryzalin), Pen
tive, and how long will it last? dulum ™ and PreM ™ (pendimethalin), Barricade ™
3L- 12 Weed Control Options in Landscape Beds and Groundcovers CTAHR — Feb. 2001
(prodiamine), and TeamPro® (trifluralin + benefin). They Mulches can interact with herbicides
inhibit root development in germinating seedlings and The placement of an herbicide in relation to an organic
are most active on annual grasses but also prevent the mulch can affect the herbicide’s performance, especially
emergence of some broadleaf weeds. Other preemer the preemergence formulations. In addition, specific
gence herbicides for landscapes are Devrinol ® characteristics of organic mulches can affect how her
(napropamide), Pennant™ (metolachlor), and Dacthal® bicides work. A mulch that is primarily fine particles
(DCPA), all of which prevent the emergence of annual can reduce the availability of some herbicides. The finer
grasses and some broadleaf weeds. Pennant, in addition, the organic material (peat moss or manure, compared to
has preemergence activity on yellow nutsedge. Gallery® bark), the greater the binding of the herbicide. Most her
(isoxaben) controls a wide spectrum of broadleaf weeds, bicides are tightly bound by soil organic matter, less with
and Goal® (oxyfluorfen) has greater preemergence ac mulches, and although the binding may minimize leach
tivity on broadleaf weeds than grasses. ing, it can also minimize the activity of the herbicide.
Additional choices are available for annual grass and Mulch that is made up of larger, coarse particles will
broadleaf control in woody ornamental plantings. In have little effect on reducing herbicide activity.
addition to the other preemergence formulations, con A more important factor is the depth of the mulch.
trol of annual grasses and some broadleaf weeds is also A preemergence herbicide applied on top of a thin mulch
obtained with Casoron™ (dichlobenil), Ronstar™ (oxadia may be able to leach through to where the weed seeds
zon), and the combination products Regal O-O® (oxy are germinating, but when applied to the top of a thick
fluorfen + oxadiazon), Rout® (oxyfluorfen + oryzalin), layer of mulch it may not get down to the zone where
Ornamental Herbicide2® (oxyfluorfen + pendimethalin), the seeds are germinating. Products like Ronstar and
Snapshot TG® (isoxaben + trifluralin), and XL™ (oryzalin Goal that require a continuous surface layer must be
+ benefin). These products are best for woody ornamen placed on the soil surface under the mulch. Most
tal plantings because of their potential for injuring her preemergence herbicides work best when applied un
baceous plants. Ronstar has greater preemergence ac derneath the mulch layer. Such placement is possible
tivity on broadleaf weeds than grasses. Casoron con only if the herbicide is applied before the mulch is de
trols annual weeds and also certain perennial grasses posited or if additional mulch is spread after herbicide
and tough-to-control weeds like thistle, horsetail, oxa application. Another reason to apply herbicides under
lis, wood sorrel, and plantain. It can be used around es mulch is to reduce volatilization losses.
tablished woody ornamentals such as yews, arborvitae, In general, the preemergence herbicides listed above,
and juniper, but not around firs, spruces, or hemlocks. with the exception of Casoron, do not control established
Casoron should be applied during cooler weather and perennial grasses. Preemergence herbicides for crabgrass
covered with mulch or watered in soon after applica do control perennial grasses germinating from seed, and
tion. Of the herbicides registered for landscapes, Casoron some inhibit pegging down of creeping perennials. How
is one of the best for preemergence control of biennial ever, to effectively control perennial grasses in ground
and perennial weeds. Because Casoron affects some turf covers and ornamental landscape beds, postemergence
grasses, care must be taken to prevent the granules from herbicides should be used.
reaching turf areas when applying it to ornamental beds.
Also, Casoron and Ronstar should not be used in herba Postemergence herbicides
ceous ornamental beds. Postemergence herbicides are used to kill weeds after
Use the information contained in herbicide labels they have germinated and are growing. To be effective,
to determine tolerance of ornamental plants to a given most postemergence herbicides must be absorbed
herbicide. Match herbicides with weeds present and through the leaves; therefore, liquid sprays generally
consider using herbicide combinations. Combinations work better than dry, granular materials. Postemergence
of herbicides increase the number of species of weeds herbicides are effective against grasses, sedges, and broa
controlled and provide effective control of grasses and dleaf weeds. They are most effective when applied to
many broadleaf weeds. young, vigorously growing weeds. Postemergence her
bicides generally provide little residual weed control.
4L- 12 Weed Control Options in Landscape Beds and Groundcovers CTAHR —Feb. 2001
They are either inactivated by binding to soil particles compound in this group; it controls both annual and pe
or rapidly degraded by soil microorganism. The rennial grasses, including bermudagrass, in a wide range
postemergence products that are available for control of of ornamentals.
perennial grasses and broadleaf weeds include both se Sedges such as nutgrass (purple and yellow nut
lective and nonselective materials. Nonselective herbi sedge), green and white kyllinga, and McCoy grass are
cides kill all plants, including desirable ornamentals and perennials that present their own special problems. They
turfgrasses, so extreme care should be used when ap are not affected by most of the grass or broadleaf herbi
plying them. Selective postemergence products carry less cides, although the preemergence herbicide Pennant™
risk of injury to desirable plants, and many can be ap will control yellow nutsedge. The most effective con
plied directly over the top of ornamentals and turf. trol for most sedges is with the postemergence products
Image (imazaquin) and Manage™ (halosulfuron). Both
Selective postemergence herbicides can be applied over many broadleaf ornamentals and
The selective herbicides for grass control, generally re turfgrasses. Always read the label to be sure it is safe to
ferred to as postemergence grass herbicides, are the com over-spray the existing plants.
pounds of choice in broadleaf groundcovers and orna Very few if any postemergence herbicides can safely
mental beds due to their high degree of safety to desir be over-sprayed against broadleaf weeds in groundcovers
able plants. This group of selective herbicides includes or ornamental beds. This is because most plants used
Poast™ and Vantage™ (sethoxydim), Fusilade ™ and for groundcover or ornamentals are also broadleaf plants
Ornamec™ (fluazifop), Acclaim™ (fenoxaprop), and that will be affected by the herbicide. In these cases,
Envoy™ (clethodim). These products are systemic, mean selective and nonselective post-emergence herbicides
ing that they are taken up into the plant’s system and may be used as a spot spray on existing broadleaf weeds
affect the roots as well as rhizomes and stolons of pe if they are located away from the desirable plants, such
rennial grasses. Repeat treatments are usually necessary as on top of mulch, and there is little danger of over
for long-term control of established grasses. Members spray. Because most broadleaf weeds in Hawaii are an
of this class of herbicide only control grasses and have nuals, the best control is obtained with one of the
practically no effect on most broadleaf weeds, sedges preemergence products mentioned above.
such as nutgrass and kyllinga, and other nongrass mono Herbicide combinations increase the weed control
cots such as wild onion and wild garlic. These herbi spectrum. When allowed by the labels, some of the grass
cides must not be applied over the top of ornamental and broadleaf herbicides discussed previously may be
grass or to many of the turfgrasses, and care should be mixed. Product labels must be checked carefully for lists
taken to prevent drift into areas planted with these when of ornamental species that the product has been regis
applying the products to ornamental beds. tered for use in.
Sethoxydim (Poast, Vantage) and fluazifop
(Fusilade, Ornamec) have been used extensively for Nonselective postemergence herbicides
annual and perennial grass control in a wide range of In some situations it is desirable to eliminate all vegeta
herbaceous and woody broadleaf ornamentals. These two tion for renovations, vegetation management, or simple
types of herbicides are commonly used to control estab spot spraying. Herbicides used for this purpose fall into
lished Johnsongrass, bermudagrass, and quackgrass, te the general category of nonselective postemergence
nacious weeds with few control options when growing products. These herbicides can further be separated into
among ornamentals. Sethoxydim does not affect several different types based on their mode of action.
centipedegrass if used at recommended rates, and is Some are soil-applied and enter plants through root
therefore very effective in controlling most grassy weeds uptake, providing immediate as well as residual con
in centipedegrass turf. Fluazifop is effective on all an trol. Examples of this type are Pramitol® (prometon) and
nual and perennial grasses including all warm-season Spike® (tebuthiuron). Others are applied to the foliage
turfgrasses used in Hawaii. Acclaim effectively controls and primarily used to eliminate existing vegetation.
annual grasses, with suppression of certain perennial RoundUp Pro™(glyphosate) and Finale™ (glufosinate
grasses including bermudagrass. Envoy is the newest ammonium) both use this mode of uptake. Some herbi
5L- 12 Weed Control Options in Landscape Beds and Groundcovers CTAHR — Feb. 2001
cides have both types of activity; examples are Hyvar® jury to nontarget plants with any of these products, and
(bromacil), Krovar® (bromacil + diuron), Arsenal® especially those that translocate, extreme care must be
(imazapyr), and Sahara® (imazapyr + diuron). taken when applying them around desired plants. Shield
Some of these herbicides are systemic and are more ing nontarget plants to prevent spray contact can mini
effective in killing woody and perennial plants. Sys mize the potential for systemic injury.
temics tend to be slow acting but result in total kill of It is important to identify by name the grassy and
the entire plant. Others are contact herbicides and tend broadleaf weeds and sedges that infest the landscape beds
to work very quickly, earning them the nickname of and groundcovers you maintain. Determine if they are
“burndown” herbicides. These rapidly “top-kill” all veg annual or perennial, and then develop a control strategy
etation contacted by their spray, but perennial weeds can that accommodates the ornamentals being maintained
regrow. Common contact herbicides include Reward™ and the weeds that need to be controlled. Depending on
and Aquatrim II (diquat), Scythe® and Quick® (pelar the situation, you may need methods that are cultural,
gonic acid), and Montar® and Weed Ender® (cacodylic chemical, or a combination of these to control weed prob
acid). Finale also produces a rapid burndown. lems.
Fumigants are a valuable type of nonselective herbi
cide used for treating soil prior to planting. They may Herbicide injury to nontarget plants
control nematodes, insects, and plant diseases in addition Soil-applied herbicide damage to established ornamen
to weeds. Methyl bromide has been the mainstay soil fu tal plants is often temporary, but serious growth inhibi
migant for many years. However, its use has become re tion can occur with newly planted plants. Preemergence
stricted in recent years and will be completely banned by herbicides that contain oryzalin or isoxaben may be more
the year 2005. Two newer preplant fumigants are metham likely to cause injury to nontarget plants. Some meth
sodium, sold under the brand names Metham CLR ®, ods are available to remove unwanted herbicide toxic
Vapam HL®, Soil Prep®, and Sectasory®, and dazomet, ity from the soil, but usually it just takes time for the
sold under the brand name Basamid™ Granular. herbicide to completely degrade. To speed degradation,
It is important to understand that all nonselective supplement soil organic matter by incorporating com
herbicides kill any plant they contact and can cause un post, and keep the soil moist but not wet during periods
wanted injury to desirable plants upon contact with green of hot weather. Herbicide injury symptoms vary with
stems, roots, or foliage. All must be applied only as di the plant species and the herbicide and can include yel
rected or used as spot treatments in which ornamentals lowing (chlorosis), bleaching, root stunting, distorted
and turf are not contacted. Due to the potential for in growth, and death of leaves or the entire plant.
Mention of a trademark, company, or proprietary name does not constitute an endorsement, guarantee, or warranty by the
University of Hawaii Cooperative Extension Service or its employees and does not imply recommendation to the exclusion of
other suitable products or companies.
Caution: Pesticide use is governed by state and federal regulations. Read the pesticide label to be sure that the intended use
is included on it, and follow all label directions.
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