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Weed Management Strategies for Ornamental Landscape Beds and Ground Covers

Publication Number: P3279
View as PDF: P3279.pdf

Weed control in ornamental landscapes can be challenging. The diversity of plant material within these systems, and the types of weeds that occur, are much different than those of dense monoculture turfgrass. Frequent soil disturbance and the use of mulches further compound the complexity of weed control.

Design and Preparation

Ornamental landscapes require a large degree of planning and preparation in order to maximize their success. The most crucial step in ornamental bed preparation is planning. Selecting plant material and assuring that the soil and site characteristics accommodate those plants is fundamental to successful landscapes. For instance, not all plants necessitate soil renovation; there are many that can readily handle wet, poorly drained soils or, alternatively, dry and unirrigated conditions. Healthy plants that compete with weeds for resources, such as nutrients, sunlight, and water, will help suppress weeds.

Small mass plantings make cultural practices and herbicide selection much easier during the maintenance of landscapes. Complex combinations of broadleaf, grass, and sedge plants may result in areas of the landscape where no chemical weed control is possible and ideal cultural practices are difficult to conduct.

Cultural weed control is crucial to a weed-free landscape. Dense plantings and year-round ground cover provide more weed suppression than even the best herbicide program. Achieving this density often requires multi-tiered plantings of upper-canopy plants supported by underlying ground covers (Figure 1).

Ground cover with large, hot-pink flowers in a landscape.
Figure 1. Perennial ground covers suppress weeds and are easily maintained with preemergence herbicides.

Water management is an important function of many built landscapes. For example, landscapes can be designed to retain or slow runoff from impervious surfaces, such as roofs and parking lots (Figures 2 and 3). These landscapes function to ensure rainwater infiltrates slowly through the natural soil, rather than being forced to run off quickly into sewers and streams. However, landscapes designed for such purposes must address two key issues:

  1. too much water runoff may disturb mulch and increase weeds, and
  2. too little surface and subsurface drainage may lead to soggy soil and, in turn, unhealthy landscape plants incapable of competing with opportunistic weeds.

Creative hardscapes can help alleviate these issues and also facilitate nonselective weed-control options. These hardscapes should be semi-pervious and accommodate excess runoff without soil erosion. In any case, proper soil amendments to allow drainage and nutrient retention are important principles that should not be overlooked in the design and installation of successful landscapes.

View from beside a covered front porch showing a border area filled with ornamental gravel and stones and edged with pavers. This border separates the porch from a traditional ornamental bed with shrubs, flowering plants, and ornamental grasses.

Landscaping next to a large, brick building. The bed is filled with traditional mulch and has a center lane filled with ornamental rocks. The rock lane goes from below the gutter downspout connected to the building into the center of the bed. Water drains onto the rock lane, where it is directed down a pipe to a basin.

Figures 2 and 3. Creative hardscapes improve moisture management, decrease disturbance, and facilitate the easy use of nonselective herbicides.

Common Weeds

Ornamental landscape weeds fall into two general categories: annuals that spread by seed and perennials that spread by vegetative material, such as rhizomes and stolons. Table 1 lists some of the most troublesome weeds. Annual weeds are generally easier to control because they are vulnerable to both hand removal and preemergence herbicides. Control of perennial weeds, such as tree saplings, smilax, poison ivy, nutsedge, Florida betony, and dollarweed, is largely accomplished by mechanical weed control (hand-pulling or hoeing) and application of postemergence herbicides.

Table 1.1. Troublesome ornamental landscape annual weeds

Annual weed species

Scientific name

annual bluegrass

Poa annua

crabgrass

Digitaria spp.

goosegrass

Eleusine indica

foxtail

Setaria spp.

broadleaf-signalgrass

Brachiaria platyphylla

barnyardgrass

Echinochloa spp.

chamberbitter

Phyllanthus urinaria

annual sedges

Cyperus spp.

annual kyllinga

Kyllinga sesquiflorus

cock’s comb kyillinga

Kyllinga squamulata

doveweed

Murdannia nudiflora

spreading dayflower

Commelina diffusa

pigweed

Amaranthus spp.

Virginia buttonweed

Diodia virginiana

chickweed

Stellaria media

woodsorrel

Oxalis spp.

Table 1.2. Troublesome ornamental landscape perennial weeds.

Perennial weed species

Scientific name

Bermudagrass

Cynodon spp.

Florida betony

Stachys floridana

false-green kyllinga

Kyllinga gracillima

green kyllinga

Kyllinga brevifolia

nutsedge

Cyperus spp.

knotroot foxtail

Setaria parviflora

greenbrier

Smilax spp.

dollarweed

Hydrocotyle spp.

Cultural Weed Control

Improper irrigation is most frequently at fault for weedy ornamental beds. When possible, landscape managers should irrigate based upon plant demand rather than using daily or weekly irrigation timers. In most ornamental beds, there are intense flushes of weed emergence during the warm summer months when moisture is excessive (after rainfall or when landscapes are irrigated too much or too frequently).

Mulching materials may help control weeds in ornamental landscapes. When plant canopy is lacking, mulch may help prevent seedling establishment. However, when overapplied, mulch may compromise the health of desired ornamentals. A mulch layer of 1 to 2 inches, and no more than 3 inches, is generally recommended. Apply mulch evenly around the base of shrubs and trees, and do not create mounds or “mulch volcanoes.”

Cloth weed barriers are relatively cheap (sometimes less than 10 cents per square foot), but they are rarely used during initial installations around home and commercial landscapes. In some instances where new landscape beds are installed, or old beds are being redesigned, this option makes sense, as long as these areas will not contain living ground covers or annuals.

Mechanical Weed Control

There is no substitute for hand-weeding of landscape beds. Complete chemical and cultural control of weeds is not possible. When hand-removing weeds, it’s important to excavate or extract roots because many plants have the ability to regrow from underground roots and stems. Applying mulch afterward may help suppress regrowing weeds.

Chemical Weed Control

Preemergence Herbicides

Preemergence herbicides (Table 2) prevent seedling establishment. They must be applied preventively before major weed emergence. Because preemergence herbicides degrade over time, it is necessary to reapply them for adequate year-round coverage. Typical application dates are based on emergence cycles of common landscape weeds.

Apply in the spring before crabgrass emergence when soil temperatures at a 1-inch depth are below 55°F (typically mid- to late February in Mississippi). Apply a second late-spring/early-summer application roughly 6 to 10 weeks later. Recommendations differ for each product, so always read the label.

Time fall applications before annual bluegrass emergence when soil temperatures fall below roughly 70°F (typically mid- to late October in Mississippi).

Preemergence herbicides are formulated as either liquid or granular products. Each formulation has some distinct advantages. Liquid applications tend to saturate thick mulch layers and penetrate to the soil layer. Granular products may be able to penetrate dense plant canopies; however, these products can lodge in the sensitive whorls of desired plants.

Table 2. Common preemergence herbicides for ornamental landscapes.

Example product

Active ingredient

Hi-Yield Dimension

dithiopyr

Surflan, Balan

oryzaline

Pendulum 2G

pendimethalin

Pendulum Aquacap

pendimethalin

Lebanon Treflan 5G

trifluralin

Harrell’s 4.8G

trifluralin + oxyfluorfen

Freehand

pendimethalin + dimethenamid

Snapshot

trifluralin + isoxaben

Gemini

prodiamine + isoxaben

Showcase

trifluralin + isoxaben + oxyfluorfen

Goal

oxyfluorfen

Ronstar, RegalStar

oxadiazon

SureGuard, BroadStar

flumioxazin

Goose and Crab

oxadiazon + bensulide

Tower

dimethanamid

Devrinol 2G

napropamide

Gallery

isoxaben

Casoron 4G, Barrier

dichlobenil

Specticle G

indaziflam

Barricade 4FL

prodiamine

This is not an exhaustive list and does not imply product recommendations. Always read and follow label directions.

Postemergence Herbicides

Even with preemergence herbicides, some amount of hand-weeding will be required. Selective and nonselective herbicide application may also be necessary. Landscape managers should be properly trained to scout and identify sensitive plant material in order prevent damage to sensitive ornamentals.

When applying any herbicide, make sure to use a sprayer that is thoroughly rinsed and cleaned. The only way to be absolutely sure a sprayer is clean is to avoid using it to apply potentially harmful herbicides.

Grass Control

When applied appropriately, certain herbicides control grasses within broadleaf plantings (Table 3). These herbicides are sometimes referred to as ACCase inhibitors. They target a form of the ACCase enzyme that is unique to grass plants. For instance, sethoxydim and fluazifop control bermudagrass in non-grass plantings (e.g., Lirope, boxwood, and cedar).

Broadleaf Weed Control

Most ornamental plantings contain broadleaf plants. Herbicides that commonly control broadleaf weeds, such as 2,4-D and dicamba, are not safe on broadleaf ornamentals. However, some of these herbicides may have labels that allow selective application as long as there is no contact with foliage, bark, or roots of desired plant material. Table 3 suggests several popular postemergence herbicides with labeling for various weeds within certain ornamental plantings. Always use caution because all of these herbicides have the potential to be volatile and/or taken up by roots.

Non-selective Herbicides

Herbicides such as Round-up (glyphosate), Reward (diquat), and several other similar products cause injury and death of most ornamental weeds and landscape plants. Therefore, they should be used sparingly and only when applied selectively to undesirable weeds.

Table 3. Common postemergence herbicides for ornamental landscapes.

Weeds controlled or suppressed

Trade name

Active ingredient

Labeled on

grasses

Segment, Vantage, Grass Getter

sethoxydim

mondograss, Liriope, monkeygrass, other broadleaf plantings

Fusilade II, Ornamec

fluazifop

Envoy Plus

clethodim

Acclaim Extra

fenoxaprop

broadleaf weeds, yellow nutsedge, some grasses

Basagran

bentazon

ajuga, boxwood, impatiens, English ivy, marigolds, ornamental cabbage, petunias

yellow and purple nutsedge, Kyllinga species

Sedgehammer, ProSedge

halosulfuron

Can be applied over the top of some ground covers and around the base of many woody ornamental trees and shrubs.

yellow and purple nutsedge, Kyllinga species, pigweed, deadnettle, clover, hairy bittercress, chickweed, sicklepod

Image

imazaquin

Most ground covers, including Liriope, Asiatic jasmine, juniper, and hosta, as well as around the base of many woody ornamental trees and shrubs.

This is not an exhaustive list and should not imply product recommendations. Always read and follow label directions.

Conclusion

Proper design and installation, combined with cultural and mechanical weed-control methods, reduce pesticides required to maintain ornamental landscapes. Best practices include using proper site preparation and managing water to increase plant health and decrease soil disturbance. When herbicides are necessary, preemergence applications are the most effective strategy against common landscape weeds. When applied routinely, and before the germination cycle of common weeds, these herbicides are relatively safe and effective. Due to the diversity of plant material within ornamental landscapes, selective postemergence herbicides are scarce, and nonselective postemergence herbicides must be used carefully to prevent injury to desired plants.

For more information, please see MSU Extension Publication 1532 Weed Control Guidelines for Mississippi.

The information given here is for educational purposes only. References to commercial products, trade names, or suppliers are made with the understanding that no endorsement is implied and that no discrimination against other products or suppliers is intended.


Publication 3279 (POD-10-21)

By Jay McCurdy, PhD, Associate Professor, and Geoff Denny, PhD, former Assistant Extension Professor, Plant and Soil Sciences.

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Authors

Portrait of Dr. James Dewey McCurdy
Associate Professor
Turfgrass Extension Specialist/Weed Scientist/Weed Control-Turf and Ornamentals

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