Weed Identification & Management
One researcher estimates that weed management, worldwide, occupies more human time and effort than any other single activity. Certainly it is a major aspect of the management of any turf or landscape site.
The first step in controlling weeds is to know the species with which you are dealing. Weed identification can be tricky, considering the huge number of weed species that exist and the similarity of many closely related types. However, by learning the major weed species, you will be able to recognize most of the weeds you’ll see in landscapes.
Most weeds in turf and landscapes fall into two major groups. The first we call broadleaf weeds, or, as scientists refer to them, dicots. The second major group is the monocots, which includes the grasses. However, other monocots can pose problems for the landscape manager. Sedges, which many mistakenly believe to be grasses (hence the misnomer “nutgrass”), are common weed pests. Algae and mosses, which are neither monocots nor dicots, can be difficult to eradicate as well.
It is important to know how to tell these groups apart because the success or failure of control measures depends on using the right product. For example, broadleaf herbicides usually do not affect grasses and vice versa. And while most of us intuitively recognize a grass plant when we see it, you’ll run into situations where it is difficult to tell.
Once you have decided which major group to which a weed belongs, identifying particular species can become more difficult because related species often appear quite similar. The most convenient identification system is one that uses photos or drawings. However, picture-identification manuals can only include a limited number of species, and this is their main drawback. Even so, good picture manuals cover most of the weeds you’ll encounter and are the kind on which most turf-and-ornamental professionals rely.
More exhaustive manuals use keys that rely on anatomical characteristics. These are typically more comprehensive but also are more difficult to use and may require some expertise. Extension weed specialists are excellent resources when you have trouble identifying a weed species.
Another way to group weeds is by their life cycle. As with ornamentals, some weeds are perennial and live for many years. Others complete their life cycle in just 1 or 2 years—these are annuals and biennials, respectively. Weed-control strategies heavily depend on whether a weed is perennial or annual. Any good identification manual provides this kind of information.
If you do not know the identity of a weed, you often can still determine whether it is perennial. Perennials possess underground, permanent structures that annuals do not. Locating these indicates that the weed is indeed perennial. Look for rhizomes, bulbs, tubers or large, fleshy roots. However, young perennials often are difficult to tell apart from annuals because they have not yet developed these structures.
Germination time is another aspect of weed biology that affects control measures, especially timing. Summer annuals almost always germinate in spring or early summer.
Winter annuals and many biennials germinate in late summer or fall. Perennials can germinate almost any time, depending on the species, but do so predominantly in spring and fall. Obviously, you must apply pre-emergence controls before germination takes place, so this information has great practical impact.
MAJOR WEED GROUPS
Broadleaf weeds (dicots). As the name implies, these usually have relatively wide leaf blades compared to grasses, whose leaves tend to be long, narrow and pointed. However, it is difficult to generalize by this because broadleaf weeds display such a wide variety of leaf shapes and sizes. The key characteristic is leafvenation (see Figure 1, at left). Grasses (and other monocots, such as sedges) have parallel venation, meaning that all of the veins in their leaves run parallel to one another. Broadleaf plants possess either palmate or pinnate venation. Venation usually is an accurate way to distinguish a broadleaf weed from a monocot.
Leaves also are defined as either simple or compound (see Figure 2, left). Simple leaves possess one intact leaf blade, while compound leaves have multiple leaflets. This can be an important identification characteristic for broadleaf weeds. A leaf type shared by several common weeds—for example, clover and many other legumes, poison ivy and Oxalis—is the trifoliate pattern.
Trifoliate leaves possess three leaflets (again, see Figure 2).
A large number of herbicide products are available for broadleaf control. Broadleaf weeds may be either perennial or annual, and both types are susceptible to these products, although perennials often require repeat treatments. In turf, selective post-emergence broadleaf-weed treatments are routine and ordinarily quite effective. In ornamentals, most of which also are broad-leaved, selective control is difficult, and you must rely on other approaches.
Monocots. If you determine you have a monocot, the next step is to decide whether it’s a grass or a sedge (or something else). Remember: “Sedges have edges.” Sedge stems are three-sided with sharp angles. You can easily feel this with your fingers by grasping the plant stem. In addition, sedge leaves arise from all three sides of the stem. Once you become familiar with the appearance of sedges, you’ll have little difficulty spotting them in the future.
If the weed is not a sedge, it’s probably a grass. Grass leaves arise from just two sides of the stem, which is not triangular like sedges. Other common monocot weeds that are neither grasses or sedges include wild garlic, horsetails and rushes—all noted for having round, hollow leaves.
Selectively controlling perennial grassy weeds in turf with post-emergence treatments is not as easy as controlling broadleaf weeds. Much depends on the weed and the type of turf. In some instances, treatment is simple—in others, no good selective control exists.
Pre-emergents are probably the best option for annual grassy weeds, but post-emergence controls are available for many species. Grassy weeds in ornamentals often are relatively easy to control with selective herbicides.
• Sedges. Only a few sedge species are weedy in the United States, but they can be serious problems when conditions favor them. Only a limited number of products perform well on sedges, but they tend to be highly selective and work effectively in turf and, when registrations permit it, in ornamental plantings as well.
• Woody perennials. Woody perennials, often simply referred to as “brushy” species on labels, present significant problems in rights-of-way, fence lines and other non-crop areas. The most common examples are young trees and vines such as grape, honeysuckle or poison ivy. Along fence lines that border turf areas, trees and vines sprout and can grow with physical protection from close mowing, which otherwise would effectively control these plants.
Herbicide products formulated specifically for treating woody plants post-emergently in non-crop sites are available. However, the best approach, when possible, is to use a long-residual soil herbicide that will keep all weeds from becoming established on the site. Mulch also reduces fence-line weed problems.
LIFE HABIT AND CONTROL STRATEGY
Annuals. Annuals live no longer than the current season, and pre-emergents are the best way to control them. By preventing seeds from germinating, you effectively eliminate the weed problem. If you already have an infestation, you can be confident that once the current generation has passed, pre-emergence herbicides will prevent further encroachment.
The point is not that you should simply wait for the weeds to die off—if you must eliminate an existing annual- weed infestation, products exist for this purpose. Rather, take the view that prevention—with pre-emergence herbicides and cultural controls—makes the most sense. Because, for practical purposes, there is no end to the weed seeds waiting to germinate, many grounds-care professionals use pre-emergence herbicides regularly. Turf managers caring for high-quality turf often consider this a “must.”
Perennials. Perennial weeds present a somewhat different problem. It certainly is true that pre-emergents are an important part of controlling perennial weeds. However, if you inherit an existing perennial-weed problem, you must have some means of eliminating them—as perennials, they will not die off at the end of the season. Further, no pre-emergence control is perfect, and a few weeds always manage to break through. This is not a large problem with annuals but creates a situation you must correct if the weeds are perennial. Fortunately, numerous options exist for removing most perennial broadleaf weeds and some perennial grasses from turf.
Seedlings. Control measures are most effective when taken against younger weeds. Thus, it is advantageous to be able to identify seedlings in many cases. Entire manuals have been written covering seedling identification, and these are useful to have on hand because the first leaves of seedlings—the seed leaves, or cotyledons—look very different from the plant’s subsequent leaves. However, by being observant you soon will learn the species to which a seedling belongs. The first true leaves appear quickly, and these usually resemble the typical form of the plant.
Herbicides are grouped as systemic or contact, pre-emergence or post-emergence, and selective or non-selective. Each type is suitable for some uses and not for others, depending on the site and the type of weed. For example, a non-selective herbicide is not suitable for use on desirable turf or ornamentals, except perhaps for directed spot spraying in beds. Contact herbicides are ineffective against perennial weeds, which require systemic products. Pre-emergents are not useful against established weeds. For more information on pesticides, see Chapter 12.
Once you identify a weed, you must find a product registered to control it. References are available that cross-reference according to species controlled, and many suppliers provide similar guides. Extension agents also make control recommendations, and chemical suppliers should have staff members with such expertise. Once you have identified products that provide control, you can narrow your choices according to site and crop registrations, cost, availability, preferred formulation and other factors.
TIMING HERBICIDE APPLICATIONS
Timing of applications depends on numerous factors: Pre-emergence herbicides. Obviously, these need to be present in the soil before the target weed seeds begin to germinate. Turf managers often try to apply pre-emergents close to germination dates, believing this gives herbicides maximum potency when the seeds began to germinate. However, recent research suggests that many pre-emergence herbicides are effective in the spring even when applied the previous fall. Therefore, application windows for many products may be wider than previously thought.
Your particular weed problem should dictate the actual time of application, which must precede weed-seed germination. Generally, you must treat for spring- and summer-germinating weeds in early spring or the previous fall. Many perennials and biennials, as well as winter annuals, germinate in the fall as well as spring. You should apply pre-emergents in late summer or early fall to control these weeds.
Post-emergence herbicides. Because these products control weeds after they’ve started growth, germination times are not so relevant for post-emergence treatments. However, some products require cool-weather conditions to avoid injuring desirable turf and ornamentals, so spring or fall are the preferred application times. In addition, the age and physiological status of the weeds also affects herbicide activity, especially systemic products. Product labels indicate the best application times.
The No. 1 rule for cultural weed control in turf is to maintain a dense, vigorous stand. Dense turf effectively competes against most weeds. Aside from this, it is beneficial to correct soil problems that promote the growth of certain weeds. For example, dry, compacted soil tends to favor certain weeds such as knotweed. Low, wet areas promote the growth of moisture-loving weeds such as nutsedge. Correcting these conditions reduces these weed problems. Areas that are thin or bare from disease are quickly filled in by weeds.
In ornamental beds, a thick mulch is probably the best method of reducing weeds. Organic mulches, such as wood products, are most popular, but landscape fabrics are effective as well. As with turf, dense ground cover out-competes most weeds.
WEED IDENTIFICATION GUIDE
The following illustrations and descriptions show common, problematic weeds in turf and landscape sites in most parts of the United States. You will encounter other weeds, of course, and a reference specific to your region may be helpful. Leaf characteristics noted in the following descriptions are illustrated in Figures 1 and 2.
Field sandbur—Cenchrus incertus
This summer annual (sometimes biennial) is noted for its painful, spiny burs. It thrives in sandy soils and turf areas in the South. A few related species also can be problems, and all possess irritating burs.
Bermudagrass is a commonly used turfgrass but becomes a tenacious weed in non-turf areas. Further, it has a tendency to invade other turfgrasses and is difficult to control when it does so. It is perennial and spreads aggressively by way of stolons and rhizomes. Bermudagrass exists in most of the states but thrives best in warm climates.
Smooth crabgrass—Digitaria ischaemum and large crabgrass—D. sanguinalis
These two species may be responsible for more pre-emergence applications than any other weed. While smooth crabgrass is somewhat more upright growing, they both appear similar in mowed turf, an environment where they are highly competitive. Both species can spread with stems that root at the nodes. These summer annuals occur throughout most of the United States, and seeds germinate all summer, making them two of the most significant turf weeds.
This summer annual is common in both turf and non-turf areas and sometimes is confused with crabgrass. It germinates a bit later than crabgrass and grows in distinct tufts with silvery or white stems. Goosegrass grows throughout most of the United States.
This is a perennial species with a spreading habit that makes it a troublesome weed in cool-season turf. Mainly a weed of the Eastern United States, nimblewill prefers damp, cool sites.
Annual bluegrass—Poa annua
Annual bluegrass, which actually can be perennial or annual, enjoys cool temperatures. Thus, it may grow as a winter annual in warmer regions. It is an unattractive weed in dormant warm-season turf but also invades cool-season turf, where its apple-green color contrasts with the generally darker-green desirable turfgrasses. Its seed heads, which annual bluegrass is able to produce even at golf-green height, are conspicuous. Perennial forms invade golf greens and slowly outcompete the bentgrass, until the putting surface is actually more annual bluegrass than bentgrass in many cases.
Green foxtail—Setaria viridis
This annual weed and two similar species (giant and yellow foxtail) range over the entire continental United States. They do not usually present a serious problem in dense turf. However, seedling turf and non-turf sites can be subject to serious infestations.
More a weed of open ground than turf, this perennial grass forms large colonies with its creeping underground rhizomes. It is a large plant, up to 5 or 6 feet tall, and requires aggressive treatment to eradicate it from a site.
Yellow nutsedge—Cyperus esculentusand purple nutsedge—C. rotundus
These perennial species resemble grasses but are members of a different family—the sedges. Grasping their distinctly triangular stems is a good method to confirm you’ve got a sedge. Both species enjoy warmth and ample moisture, so they are active during the summer.
Yellow nutsedge is common throughout most of the country, but purple nutsedge is a weed mainly in the Southeast and far West. Yellow nutsedge, pictured, possesses tubers (“nuts”) at the end of rhizomes and produces yellow seedheads, while purple-nutsedge plants grow in chains or series along rhizomes and produce purple seedheads. It is important to be able to distinguish between the species, because some herbicides affect one but not the other.
Cattails are monocots but not grasses. Clearly preferring wet areas, this weed spreads aggressively with perennial rhizomes. It becomes large, up to 7 or 8 feet tall, and can form thick colonies in ditches, ponds and low wet areas. Several species are weedy, but this one is the most widespread and common.
Tumble pigweed—Amaranthus albus
This summer annual often is a problem on newly established sites, including turf. This species and another— redroot pigweed—grow upright, while prostrate pigweed is a mat-forming plant. All three are widespread in the United States.
Giant ragweed—Ambrosia trifida
This large, upright summer annual is a common weed in open areas and newly established sites. It is well known as a cause of allergies. Two related species—common and western ragweed—also are common weeds.
This winter annual is present in all regions and may inhabit almost any site. It is not generally a serious pest in turf but can be a problem in newly seeded grass. A member of the mustard family, these weeds go by familiar names such as cress, rocket, pepperweed, mustard and radish. This group includes both perennials and annuals, and many (but not all) prefer cool weather. Thus, the mustard family includes many winter and spring annuals and perennials that grow in cooler parts of the year.
Mouseear chickweed—Cerastium vulgatum
This herbaceous perennial is a widespread weed in lawns and other landscape sites. Its low growth habit allows it to thrive in closely cut turf. Though it resembles common chickweed— an annual—its hairy leaves make it easy to distinguish.
This tall, upright grower is a summer annual that mostly inhabits beds and other open areas. It can, however, compete with seedling turf. It occurs throughout the United States.
Chicory is a familiar site along roadsides and other low-maintenance, mowed sites. Even though chicory is an upright perennial, it apparently tolerates mowing easily. Its brilliant blue flowers are distinctive.
Canada thistle—Cirsium arvense
Canada thistle is a common and serious weed problem on roadsides and in other open areas. It is perennial and can form large colonies with underground spread. Several other thistle species are weedy pests throughout the United States, though not all are perennial. The spiny leaves of thistle can be painful, so this weed deserves prompt attention whenever it springs up in landscapes.
Field bindweed—Convolvulus arvensis
This perennial vine is a challenge to control. Thriving equally well in turf, rambling over ornamental shrubs or twining along fence lines, this weed dies back to a deep rootstock each winter. If this rootstock is not completely killed—no easy task—the vine will regrow. A member of the morning-glory family, its white flowers are quite showy. Related species include ornamental vines and several other weedy types.
This summer annual is mostly a weed of open places, though it can persist occasionally in low-maintenance turf. Horseweed can reach 6 or 7 feet in height on a narrow stem and so is a conspicuous weed. In turf, it is mowed short but may persist as a low tuft. Horseweed is common throughout the United States.
Prostrate spurge—Euphorbia supina
Prostrate spurge is a common summer annual in the Eastern half of the United States and along the West Coast. Not only does it invade turf—especially in thinned spots—it also is a persistent problem in beds. It forms mats starting in late spring, and seeds continue to germinate throughout the summer. Its milky sap aids identification.
Ground ivy—Glechoma hederacea
Ground ivy is a creeping herbaceous per-ennial that thrives in cool weather. Inhabiting the Eastern half of the United States, ground ivy grows in many situations. However, it presents a difficult problem when it invades cool-season turf, which it out-competes in moist, shady sites with its aggressive spread and tolerance of low mowing. Its extensive roots and rhizomes make it difficult to control.
Prickly lettuce—Lactuca serriola
This winter annual or biennial occurs throughout the United States, but more commonly in Northern areas. An upright grower, it persists in many urban and landscape settings, though it is not a serious turf pest.
Henbit is more common in the Eastern United States but ranges elsewhere. Preferring cooler weather, it typically grows as a winter or spring annual and can be a conspicuous weed in dormant warm-season turf. It germinates in fall and early spring.
Common mallow—Malva neglecta
Mallow is widespread and common in the United States, preferring cooler regions but growing as a winter or spring annual in hot-climate regions. More of a weed in open spaces, it is a substantial problem on newly established sites.
Black medic—Medicago lupulina
This member of the legume family varies from annual to biennial to perennial. Growing in all regions of the United States, the weed resembles white clover but has distinctive yellow flowers. A closely related species, bur clover, has prickly, irritating fruits. This plant has trifoliate leaves.
Carpetweed is low-growing and germinates relatively late in the spring. However, it establishes quickly in many sites, including turf and ornamental beds. The leaves are small and narrow, and the plant may form a thin mat.
Yellow woodsorrel—Oxalis stricta
Yellow woodsorrel is a common turf and landscape weed found throughout the United States. It is perennial with a low growth habit, allowing it to survive mowing in turf. Creeping woodsorrel is a similar species that spreads somewhat more aggressively. Both species resemble clover superficially because of their trifoliate leaves, but woodsorrel flowers are yellow and not similar to clover flowers.
The turf and landscape weeds known as plantain mainly consist of three species—broadleaf, buckhorn (left) and blackseed (right) plantain—of varying distribution. Together, they range over the entire United States and are common turf weeds. Low-growing perennials, they tolerate mowing well.
Prostrate knotweed—Polygonum aviculare
Found throughout the states, this weed germinates in early spring but thrives in summer. Its mats can grow up to several feet wide. It thrives on compacted sites. Many other members of this genus can become weedy, most of them upright growers going by the name of smartweed.
This summer annual is a mat-forming plant with succulent, fleshy stems and leaves. It commonly grows in ornamental beds and open sites where it forms mats up to 1 or 2 feet across. The high water content of this plant’s tissue sometimes reduces herbicidal effectiveness.
Russian thistle, tumbleweed—Salsola kali Mainly a weed of the Western United States, this plant is easily recognized when its dried up “skeletons” blow across the landscape. It is an annual that enjoys heat and thrives in open ground, presenting a control challenge for fence rows and other non-crop areas. The leaves are thread-like and succulent on young plants but become shorter, stiff and pointed on mature plants.
Annual sowthistle—Sonchus oleraceus
Sowthistle is a weed of open places, landscape beds and neglected urban sites but does not succeed in mowed turf. This species and a closely related type—spiny sowthistle—occur throughout the country. They grow as annuals that germinate in spring.
Common chickweed—Stellaria media
Chickweed is a low, tufted winter annual. Unlike hairy mouseear chickweed, common chickweed leaves are hairless. It germinates in the fall but makes most of its growth during the following spring in cold regions. However, it may grow considerably during winter in warmer regions. Chickweed is troublesome not only in beds but also in dormant warm-season turf.
Perhaps the most notorious turf weed, dandelion is a low-growing perennial with a large taproot. It tolerates close mowing and can extensively invade turf. Seeds, its primary mode of spread, are distributed widely through the air and germinate mainly in the spring but also in the fall. Its flowers can create a carpet of yellow in heavily infested turf.
Poison ivy—Toxicodendron radicans
Poison ivy is a woody vine that can appear shrubby at times. Occurring throughout the United States, it is more frequent east of the Rockies. It is more of a problem in fence rows and other less-used areas than in maintained landscapes, but it can persist even in mowed turf. Due to the painful allergic reaction most people have to this plant, you must diligently eliminate it whenever it starts to grow. It sprouts readily from seed. The leaves are trifoliate, and the stem of the center leaflet is longer than the other two.
This mat-forming summer annual has sharp, spiny fruits. Found throughout most of the United States, it is most troublesome in the Southwest. The fruits are sharp, so you must control this weed whenever you spot an infestation. The leaves are pinnately compound.
White clover—Trifolium repens
This perennial is a major weed problem in turf. Its ability to spread and its tolerance to low mowing heights allow it to thrive in the turf environment. Several other related clover species also are turf pests, but white clover is the most widespread. All clovers have trifoliate leaves.
Wild grape—Vitis spp.
Wild grapes include several species that grow rampantly over fences, up trees and across the ground. Wild grapes are not turf pests, of course, but are difficult to eradicate once established. These woody vines commonly inhabit the periphery of landscape sites—fence lines and similar areas.