Weed Management in Turf

Weed Life Cycles

The simplest definition of a weed is a plant that grows where it is not wanted. Creeping bentgrass, a turfgrass used on golf courses, is often considered a weed because it is unwanted in Kentucky bluegrass lawns. Weeds are undesirable because they disrupt turf uniformity and compete with desirable grass species for moisture, light, and nutrients. Some weeds are harmful to people because they attract bees, cause skin irritation, or cause poisoning if ingested.

An effective weed management program depends on your ability to identify weeds and to understand their life cycles. This information is essential for developing a good cultural weed management program. It is also necessary for selecting herbicides and for determining the proper time of year to apply them.

Weed Life Cycles

Turfgrass weeds can be grouped into one of three life cycles: annual, biennial, or perennial. Annuals are plants that complete their life cycle in one growing season. Seeds of summer annuals germinate in spring, produce vegetative growth in spring and summer, then produce seed and die in the fall. Winter annual seeds germinate in fall; grow during fall, winter, and early spring; then produce seed and die in late spring. Examples of summer annual weeds are crabgrass and prostrate knotweed. Two winter annual weeds are annual bluegrass and common chickweed.

Biennials require two growing seasons to complete their life cycles. They usually produce vegetative growth the first year, then flower and set seed during the second year. Examples of biennial weeds are yellow rocket and wild carrot.

Perennial weeds live for three or more years (although leaves and aboveground stems often die back at the end of the growing season). Perennials produce new vegetative growth from growing points at or below the soil surface. Perennials also produce new plants from seed. Examples of perennial weeds are orchardgrass and dandelion.

Weed Identification

To identify weeds you must first distinguish between broadleaf weeds and grass weeds. Broadleaf weeds usually have wider leaf blades than grass weeds. Each leaf typically has a main vein that divides the leaf in half with a network of smaller veins (originating from the main vein) forming a netlike pattern.

Broadleaf weeds have distinct leaf shapes and surface characteristics that can be used in identification. The arrangement of leaves on stems can also be a useful identification aid. Leaves either are alternately arranged (when a leaf grows from a node on one side of the stem and another is produced on the opposite side further up the stem) or arranged opposite one another.

Some broadleaf weeds produce leaves in a circular pattern (rosette) from a central growing point located at or beneath the soil surface. Others grow and spread by means of creeping aboveground stems called stolons or belowground stems called rhizomes. Broadleaf weeds can produce a fibrous root system or a root system dominated by a large, fleshy taproot.

Broadleaf weeds often bear colorful flowers of different sizes and shapes. At certain times of the year flowers can be very useful identification aids.

Grass weeds have long, narrow leaves with veins running parallel to each other (they do not form a netlike pattern). Grasses do not have showy or colorful flowers, and leaf shapes are similar among species. The ability to identify grasses depends on recognizing growth habits, certain vegetative features, and seedheads.

Growth habits of grasses

Growth habits of grasses can be divided into three different categories: bunch-type, rhizomatous, and stoloniferous. In plants having a bunch-type growth habit, new stems are produced by tillering. A tiller is a stem that arises from a bud in the crown and grows vertically, remaining enclosed by the leaf sheath. Although all grasses produce tillers, only those that spread by tillering alone are referred to as bunch-type grasses.

In plants with the rhizomatous growth habit, lateral growth occurs by horizontal creeping underground stems called rhizomes. Rhizomes are produced from buds in the crown that break through the outer leaf sheath. Rhizomes produce nodes that can give rise to new tillers.

In plants having the stoloniferous growth habit, lateral growth occurs by horizontal creeping aboveground stems called stolons. Stolons are produced from buds in the crown that break through the outer leaf sheath. Stolons produce nodes that can give rise to new tillers. Stolons are usually green, whereas rhizomes are usually white.

Vegetative structures of grasses

Several vegetative features can be used to identify grass weeds. The most important are leaf blade characteristics and the structures associated with the collar.

Vernation is a term used to describe the arrangement of the youngest leaf in the bud shoot. Grasses with folded vernation have leaves that are folded in a V-shape. Leaves with rolled vernation are round with no folds. To determine if the grass has folded or rolled vernation, hold the plant between your thumb and index finger and roll it. If it rolls like a straw, it has rolled vernation, if it lies flat and has edges, it is folded. You can also determine vernation by cutting a cross-section of the stem just below the leaf blade.

Leaf blades of grasses can vary in width and hairiness. Some grasses have leaf blades that are dominated by a single prominent vein in the center of the leaf. Others have equal-sized veins running lengthwise over the entire width of the blade. Leaf blade tips may terminate in a sharp point, a blunt tip, or keeled shape (boat-shaped tip).

The collar region is located between the leaf blade and leaf sheath. It may or may not contain structures called ligules and auricles. A ligule is the membranous or hairy tissue located at the junction of the leaf blade and leaf sheath. Ligules can vary considerably in size and shape and may be membranous, a fringe of hairs, or absent.

Auricles are appendages that are considered an extension of the collar. They can be long or clawlike, small or rudimentary, or absent.

Grass seedheads

Seedheads may be useful in distinguishing among grass weeds. They can appear as open-panicle types, compact spikes, and divided spikes.

Weed Descriptions: Grasses and Grasslike Weeds

Crabgrass (Digitaria spp.)

Two species of crabgrass, hairy and smooth, are commonly found in Pennsylvania (smooth crabgrass predominates in turfgrass stands). Both species are summer annuals and have wide (¼- to ½-inch), sparsely hairy, pale-green leaves that taper to a sharp point. Leaves of hairy crabgrass are hairier than smooth crabgrass. The ligules of both species are long and membranous. Crabgrass does not have auricles. Seedheads are divided spikes that project like fingers from the stem, producing thousands of seeds in late summer. Seeds germinate in spring when soil temperatures reach 55° to 58° F for several consecutive days. Crabgrass plants die after the first frost in early fall.

Goosegrass (Eleusine indica)

Goosegrass, also known as silver crabgrass, is common in southern Pennsylvania. Leaves are darker green and narrower than crabgrass (1/8 to ¼ inch) and sheaths have a silvery-green color (especially near the center of the plant). The ligule of goosegrass is membranous and divided in the center. The collar region is sparsely hairy and has no auricles. Goosegrass has a divided spiketype seedhead bearing seeds in straight rows on the seed stalks. Goosegrass seeds germinate four to six weeks later than crabgrass and germination continues throughout the summer.

Barnyardgrass (Echinochloa crusgalli)

Barnyardgrass is a summer annual grass with wide (greater than ½ inch) leaves and sheaths that lie close to the ground. Barnyardgrass seeds germinate later than crabgrass seeds, and plants do not tolerate low mowing heights. This species has no ligule or auricles. The seedhead is composed of compact spikes arising at several locations on the main stalk. Barnyardgrass can be a problem in newly established turf if seed is introduced with the topsoil.

Foxtail (Setariaspp.)

Foxtail is a light-green, leafy, summer annual grass weed that reaches maturity in midsummer. It is often confused with crabgrass. As a weed in turf, foxtail is much less common than crabgrass, but it can proliferate under low-fertility conditions and high mowing heights as well as in spring seedings. This weed can be distinguished from crabgrass by its hairy ligule and short, compact spike seedhead.

Annual bluegrass (Poa annua)

Annual bluegrass is a light-green, small-statured, bunch-type winter annual grass. Annual bluegrass is a highly diverse species with some subspecies functioning as short-lived perennials. The ligule is long and membranous and no auricles are present. Small but conspicuous open-panicle seedheads are evident during most of the growing season. Most seeds germinate in late summer or early fall. Although this species can persist throughout the entire growing season on irrigated sites, it usually dies during hot, dry conditions if not irrigated.

Bentgrass (Agrostis spp.)

Bentgrasses are desirable turfgrass species when used on golf course fairways, putting greens, and croquet courts. However, they are a common perennial grass weed in many home lawns. Like other stoloniferous weeds, bentgrass creeps over desirable turf and forms large light-green patches that usually turn brown in summer. Bentgrass has rolled vernation; long membranous ligules; no auricles; and narrow, flat leaf blades that have equal-sized veins across the entire width of the blade. The seedhead is an open panicle but is rarely seen in lawns.

Orchardgrass (Dactylis glomerata)

Orchardgrass is a bunch-type perennial grass weed that forms light green clumps in lawns. Leaves have folded vernation and are wide (¼ to ½ inch), light green, and pointed at the tip. The sheaths of orchardgrass are strongly compressed and flattened. Other features of orchardgrass are the long, membranous ligule and the open-panicle seedheads.

Nimblewill (Muhlenbergia schreberi)

Nimblewill is a blue-green perennial grass that is common in Pennsylvania lawns during summer. It spreads over existing turf by stolons and forms dense patches. Leaf blades have a medium texture (about ¼ inch wide) and are short (1 ½ to 2 inches) with leaf tips tapering to an abrupt point. The stems are long, slender, and wiry with prominent nodes. Ligules are short, membranous, and jagged. The leaf blades have long hairs at the margins but do not possess auricles. Seedheads are long, slender, and inconspicuous. Nimblewill grows rapidly during the warm summer months and turns brown or tan in winter.

Yellow nutsedge (Cyperus esculentus)

Yellow nutsedge is not a true grass but a member of the sedge family. Plants in this family are characterized by erect, triangular stems and a preference for moist or wet areas. Leaves and stems are yellow-green and shiny. Although leaves and aboveground stems die in winter, new growth occurs in spring and summer from vigorous, scaly rhizomes and nutlets that grow underground. Chestnut-brown seedheads may be present on plants that are not mowed.

Wild garlic (Allium vineale)

This species is a perennial weed that has a strong garlic or onionlike odor when cut. It is one of the first weeds to emerge in early spring. Wild garlic produces long, slender, mostly hollow leaves that are dark green and covered by a waxy substance. Leaves emerge from underground bulblets that are covered by thin, papery scales. Flowers may be present on uncut stems and can be white, pink, or purple.

A closely related species, wild onion (Allium canadense), looks very much like wild garlic. Wild garlic is more common in Pennsylvania and has hollow leaves. Wild onion has flat (not hollow) leaves. Depending on the growth stage and time of year, it may be difficult to distinguish between these two species.

Weed Descriptions: Broadleaf Weeds

Black medic (Medicago lupulina)

Black medic is a low-growing, dark-green, summer annual broadleaf weed in turf. It can sometimes act as a perennial during years when the winter is mild. Black medic leaves have three leaflets with the center leaflet extended on a short stalk. Stems may spread one or two feet from the original growing point but do not root at nodes. Black medic has small, compact, yellow flowers that form in leaf axils.

Prostrate knotweed (Polygonum aviculare)

Prostrate knotweed is a low-growing summer annual that is well adapted to compacted, highly trafficked areas such as along sidewalks, in athletic fields, and in golf course cart paths. The leaves are alternately arranged on stems, small (½ to 1 inch long by ¼ to ½ inch wide), and blue green, and have margins that are not serrated or lobed. Each leaf is elliptical, tapering to a rounded tip. Stems grow prostrate but do not root at nodes. One distinct feature of knotweed is the papery sheath at the base of each leaf. Flowers are very small and white, and grow in the leaf axils.

Prostrate spurge (Euphorbia supina)

Prostrate spurge (sometimes called spotted spurge) is a summer annual weed that spreads in a prostrate fashion over the soil surface or over desirable grass species. Prostrate spurge can be recognized by its oppositely arranged, small (¼- to ¾-inch-long) leaves that have a reddish-brown mark or enlarged spot on the upper surface. When broken or cut, the stems exude a white, milky substance. Flowers of spotted spurge are very small and pink or white. This species is common in newly established turf and frequently occurs in mature turf that has been thinned by insect attack.

Purslane (Portulaca oleracea)

Purslane is a prostrate-growing, succulent (fleshy) summer annual that grows in newly established turf or in thin lawns. Stems are thick, sprawling, and red. Leaves are thick and fleshy, light green and wedge-shaped. Flowers are small and yellow. Purslane is a prolific seed producer and seeds may lie dormant in soil for many years before germinating.

Common chickweed (Stellaria media)

Common chickweed is most often classified as a winter annual, but it can grow and flower at any time during the growing season. Leaves are small and elliptical (tapering to a point), and occur opposite one another on square stems that have a single row of hairs. Leaf surfaces are smooth (not covered with hairs). Common chickweed spreads in turf via branched, creeping, aboveground stems that root at the nodes. Flowers are small and white, and have five petals. Common chickweed forms dense patches in high-cut turf and prefers moist, shaded areas, but it can grow in sunny areas and under very low mowing heights (less than ¼ inch).

Corn speedwell (Veronica arvensis)

Corn speedwell is a low-growing winter annual weed. Upper leaves are small and taper to a point, whereas lower leaves are rounded and lobed. Stems have an up-right growth habit and do not spread more than an inch or two from the crown. Plants are covered with sparse, fine hairs. The plant's most notable features are the small blue flowers that grow in the leaf axils and its heart-shaped seed capsules. This weed grows in thin turf during cool weather.

Dog fennel (Anthemis cotula)

Dog fennel (also known as mayweed) is a winter annual. It has finely divided leaves that give off an acrid odor when crushed. The plant can grow up to six inches tall, but it has the ability to escape damage by lawn mowers. Dog fennel has a taproot and does not root at nodes. Perhaps the plant's most distinctive feature is the large flower with white petals and a yellow center.

Henbit (Lamium amplexicaule)

Henbit is a winter annual weed that has oppositely arranged leaves with lobed margins. Stems are hairy and square; they typically grow in an upright fashion but can grow prostrate and occasionally root at nodes. Flowers occur in the axils of upper leaves. They are pale purple, long (up to 3/8 inch), and trumpet-shaped. Henbit is found in moist soils and can be especially troublesome in turf during early spring.

Mallow (Malva rotundifolia)

Mallow is a biennial most often found in poorly maintained and underfertilized turf. Leaves are large (greater than 1 inch in diameter) and round with serrated margins. Mallow produces sprawling branches that form nodes that do not root. Flowers are pink to lavender and are produced in the axils of leaves and stems.

Yellow rocket (Barbara vulgaris)

Yellow rocket functions as a biennial, winter annual, or perennial weed. It produces a rosette of leaves in turf. Individual leaves are strongly lobed and terminate with a large rounded lobe. In most cases, flowers do not develop under low mowing heights, but bright yellow flowers clustered at the tips of the uppermost branches can be observed around fence posts or other structures that are unmowed.

Broadleaf plantain (Plantago major)

Broadleaf plantain is a large (3- to 6-inch diameter), low-growing, perennial weed in Pennsylvania lawns. Leaves grow in a rosette fashion and are spoon-shaped with wavy margins. Prominent veins run lengthwise on the leaf surface. Seedheads are long (5 to 10 inches) and are covered with seeds that adhere tightly to the stalk. Broadleaf plantain has a thick tap root that grows deep into the soil.

Buckhorn plantain (Plantago lanceolata)

Buckhorn plantain is closely related to broadleaf plantain. It is a large perennial broadleaf weed that grows in a rosette fashion. Leaves are long and much more slender than those of broadleaf plantain. Individual leaves taper to a point and have prominent longitudinal veins and smooth, wavy margins. Seed stalks are long and terminate in small compact seedheads.

Creeping speedwell (Veronica filiformis)

Creeping speedwell, a perennial, is a small-statured plant that can creep over desirable turfgrasses and form dense, light-green patches several feet in diameter. Leaves are oppositely arranged, very small (¼ to ½ inch in diameter), and rounded with scalloped margins. Stems grow horizontally above ground and root at the nodes. Creeping speedwell produces small blue and white flowers in spring. Seed capsules are heart-shaped.

Cinquefoil (Potentilla spp.)

Cinquefoil is a perennial weed that grows in low-fertility soils and spreads by stolons. Depending on the species, leaves have three to five leaflets with serrated margins. Flowers of cinquefoil are small and bright yellow, and have five petals.

Creeping buttercup (Ranunculus repens)

Creeping buttercup spreads by extensive stolons that can quickly take over large areas of thinned turf. Leaves are dark green and divided into three segments. The flowers are small, bright yellow, and cup-shaped.

Dandelion (Taraxacum officinale)

Dandelion is the best-known and perhaps the most common perennial turfgrass weed in Pennsylvania. It forms a rosette of long, narrow, and strongly lobed leaves. Dandelions produce thick taproots that can penetrate up to several inches into the soil. Bright-yellow flowers (1 inch in diameter) are produced on long stems in spring.

Ground ivy (Glecoma hederacea)

Ground ivy is a low-growing, creeping, perennial broadleaf weed. Leaves are oppositely arranged on stems and are round or kidney-shaped with scalloped margins. The upper leaf surface has distinct veins and is sparsely hairy. Stems are square, creeping, and long. Ground ivy produces nodes that root at leaf and stem axils and that can form new stolons. Flowers are blue or purple and trumpet-shaped. This weed is most common in shaded areas, but it can also grow in full sun.

Heal-all (Prunella vulgaris)

Heal-all, a perennial weed, grows horizontally in turf by means of creeping, aboveground stems that can root at nodes. Leaves are oppositely arranged on stems and broad at the base; they taper to a blunt tip. Veins are prominent on the upper leaf surface, and margins are smooth (not lobed or serrated). As with other members of the mint family, stems are square. Flowers are violet to purple and are produced in dense clusters at the tips of branches.

Wild violet (Viola papilionacea)

Wild violets are persistent perennial weeds that are difficult to control in turf. This species is an upright grower that spreads by means of thick underground stems. Leaves are heart-shaped and margins are serrated. The leaf surface is shiny owing to a thick waxy covering. Flower petals are purple, and the center of the flower is white or yellow.

White clover (Trifolium repens)

Clover is a very common weed in nearly all turfed areas. Although some homeowners do not find clover objectionable, its creeping growth habit can overtake turf and form large dark-green patches. Clover leaves are com- posed of three leaflets, each with a small white mark in the center. Stems grow above ground and root at nodes. Clover produces white, compact flowers that are about ½ inch in diameter.

Yellow woodsorrel (Oxalis stricta)

Yellow woodsorrel (sometimes called oxalis) is a light-green, upright perennial weed. Like clover and black medic, each leaf has three leaflets. Leaflets of yellow woodsorrel can be distinguished from other weeds by their distinct heart shape. Flowers are bright yellow with five petals. As flowers mature, they lose their petals and form banana-shaped seedpods that forcibly eject seeds.

Yellow and orange hawkweed (Hieracium spp.)

Two types of hawkweed grow in Pennsylvania, yellow and orange. Both species are perennials that produce a rosette growth habit. Leaves are long and slender but not lobed or serrated. Perhaps the most distinctive feature of these species is the abundant, bristlelike hairs that cover the leaf surface. Flowers are produced on long stems and are bright yellow (yellow hawkweed) or orange (orange hawkweed).

Cultural Practices for Improved Weed Management

Any cultural practice that increases the density and vigor of desirable turfgrasses will discourage competition from weeds. Weeds can only exist if there is space for them. Thus, cultural practices for weed control in turf are aimed at shading and crowding the young weed seedlings by producing a dense sod. Effective cultural control measures include the proper selection and establishment of turfgrasses, adequate liming and fertilization, proper mowing practices, judicious watering, and insect and disease control.

Turfgrasses that are not adapted to the environmental conditions and intended use of the turf may become weak, resulting in a thin stand. When there are voids in the turf, weeds have an opportunity to grow and compete with the desirable species. The use of proper establishment procedures helps to ensure that a dense turf will compete with germinating weed seedlings.

Too high or too low a soil pH and inadequate fertilization lessens the competitiveness of turfgrasses, resulting in reduced density and subsequent weed invasion. Soil testing is the key to proper pH management and fertilization. Soil test recommendations provide guidelines for fertilization and liming to establish and maintain turfgrasses. Adequate nitrogen should be supplied to favor the desirable species in the stand. Phosphorus fertilization increases seedling vigor and is one factor in reducing weed infestations in newly established turf. Lime should be applied when the soil is too acid, and acidifying materials can be used when the pH is too high.

Improper mowing is one of the most common causes of weed invasion. Mowing heights that are too short result in weakened turfgrasses and weed encroachment. Most lawns should be cut at least 2 inches or higher.

Improper watering also contributes to weed invasion. Frequent light watering encourages shallow rooting and promotes weak turf, which becomes susceptible to insect and disease attacks as well as damage from traffic. Frequent light watering also encourages germination and development of weeds at the expense of turfgrasses. Watering deeply (4 to 6 inches) before turfgrasses show signs of wilting is a practical approach to a sound watering program. A soil probe can be used to monitor soil moisture levels.

Most weeds are opportunistic, filling in voids in turf caused by diseases and insects. Diseases can be controlled by cultural practices and with fungicides. Insect damage can be reduced by maintaining a healthy turf, by using insecticides, and by using biorational means of control, such as endophyte-containing ryegrasses and fescues, which discourage leaf and stem-feeding insects.

Chemical Weed Control

Herbicides are chemicals that kill or alter the normal growth of weeds. They can be divided into two main groups: selective and nonselective. Selective herbicides are those that control the target weed(s) without damaging desirable turfgrass species. Nonselective herbicides kill all vegetation (including turfgrasses) and are used in lawn renovation or on weeds not controlled by selective herbicides.

Herbicides can be further divided into preemergence and postemergence categories. Preemergence herbicides are applied prior to germination and emergence of weeds. These are typically used for controlling annual weeds. Postemergence herbicides are used for controlling weeds that have already emerged from the soil. They are either contact or systemic in nature. Postemergence-contact herbicides affect only those plant parts that they contact and are not translocated to other portions of the plant. Postemergence-systemic herbicides are translocated throughout the plant; hence they are effective in controlling perennial weeds that can generate new foliage from underground vegetative structures.

Herbicides can be applied to foliage or soil. Postemergence herbicides are usually foliar applied, whereas preemergence herbicides are soil applied. A foliar-applied herbicide must contact and be absorbed by foliage, and is less effective if washed off the leaf surface by rainfall or irrigation. Soil-applied herbicides can be applied as either liquids or granulars; they should be watered into the soil during or following application.

Chemical control of grass and grasslike weeds

Summer annual grasses

Summer annual grass weeds are usually controlled with preemergence herbicides. These herbicides act by forming a chemical barrier in the soil prior to seed germination or emergence. The barrier prevents grass seedlings from emerging and developing normally.

You can use several preemergence herbicides to control summer annual grass weeds in Pennsylvania. Table 1 lists the chemical (generic) and trade names of some commonly used preemergence herbicides.

Several factors should be considered in choosing a preemergence herbicide. The first is the safety of the chemical for turfgrass species and cultivars. For example, oxadiazon may injure fine fescues, but it is generally safe on Kentucky bluegrass, ryegrass, and tall fescue. Most preemergence herbicides have long residual activity in the soil and may affect newly seeded turfgrasses. Thus, seeding of turfgrasses should be postponed for the amount of time specified on the manufacturer's label. Siduron is the only material that can be safely used during or immediately following seeding.

For maximum effectiveness, preemergence herbicides should be applied uniformly at the label recommended rates. These herbicides are more efficient when watered-in within two to three days of application.

The timing of preemergence herbicide applications is the most critical component of an effective chemical control program. As a general rule, the best time to apply preemergence materials is approximately 10 to 14 days prior to the expected germination period in spring. Crabgrass begins to germinate when soils are moist and the temperature in the upper inch of soil reaches 55° to 58° F at daybreak for four to five days. Forsythia flower petal fall is not a consistently reliable means of determining timing of crabgrass herbicide applications. Normally, preemergence crabgrass treatment in Pennsylvania should take place as follows:

Depending on the product, time of application, and location, reapplication within 60 days may be required for season-long control. Consult product labels to determine if two applications are allowed or needed. Poor weed control may occur with late applications. In these cases, postemergence herbicides may be required.

Goosegrass germinates later than crabgrass. Preemergence herbicide applications to control goosegrass should take place three to four weeks after the normal dates for applying crabgrass control materials.

Postemergence control of crabgrass and some other summer annual weeds involves the use of chemicals that kill growing plants after they have appeared in the turf. Postemergence herbicides can be used to treat only those areas where summer annual grass weeds have emerged. Preemergence herbicides, on the other hand, are usually applied over the entire turf area since the applicator does not know where seeds are or if they are present.

For postemergence herbicides to be effective, crabgrass must be uniformly covered. Thus, these compounds should be applied only when crabgrass is visible in the stand. Table 2 lists the chemical and trade names of some commonly used postemergence herbicides for annual grass control.

The methanearsonates (MSMA and DSMA) act as contact herbicides. The most commonly used methanearsonate, MSMA, may injure desirable species at high temperatures (greater than 80° F), and repeat applications at specified intervals are necessary for complete control (see label). It is important not to water turf for 24 hours after application. MSMA is effective in controlling crabgrass under both adequate and low soil moisture levels.

Fenoxaprop-p-ethyl is a postemergence herbicide that is slowly translocated within the plant. It can effectively control tillered crabgrass with a single application. It is relatively safe on cool-season turfgrasses but may injure some Kentucky bluegrass cultivars, especially at high temperatures early in the season. It should not be applied if cool-season turfs show signs of drought stress. Fenoxaprop-p-ethyl is less effective when tank-mixed with phenoxy-type herbicides such as 2,4-D and MCPP.

Dithiopyr acts as a preemergence and postemergence herbicide. It provides postemergence control of crabgrass only up to the one-tiller stage of development, but it can be combined with fenoxaprop-p-ethyl when two or more tillers are present.

Quinclorac is a postemergence herbicide effective in controlling crabgrass and some broadleaf weeds. Optimum control of crabgrass is achieved when quinclorac is applied before development of the second tiller or when crabgrass plants have five or more tillers. In some cases, quinclorac does not provide complete control of crabgrass at the two-, three-, and four-tiller stages of development. In these situations, a second application may be required for complete control. Quinclorac can be mixed with other herbicides, including pendimethalin and phenoxy herbicides, to improve weed control. For best results, apply quinclorac in combination with a methylated seed oil according to directions on the label.

Postemergence herbicides can be combined with preemergence herbicides to ensure that late-germinating summer annual grasses will be controlled along with weeds that have already emerged. Studies at Penn State have demonstrated improved control of crabgrass with postemergence-preemergence applications over postemergence applications alone. Be sure to follow label directions when considering combinations of herbicides.

Winter annual grasses

Annual bluegrass is a tenacious and difficult-to-control winter annual grass weed in turf. The following should be used only as a guide for determining which products and methods are available for controlling this weed.

Before attempting to manage this species, realize that complete eradication is nearly impossible and that acceptable control may take several years to achieve. A chemical control program will usually be ineffective without a well-designed cultural control program that favors desirable turf species over annual bluegrass. Chemical control of annual bluegrass can be attempted with preemergence herbicides, herbicides that have both preemergence and postemergence activity, and chemical growth regulators.

Several commercial preemergence herbicides can be used for annual bluegrass control. Application should take place in September just prior to the peak period of annual bluegrass germination. Since seeds of this species germinate at different times of the year, complete control with a single application of a preemergence herbicide is unlikely. Although repeat applications will improve control, newly developing roots and rhizomes of desirable turfgrass species may be inhibited. Only the annual subspecies of annual bluegrass can be controlled with preemergence herbicides. Be sure to follow label precautions if overseeding with turfgrasses.

Ethofumesate (Prograss™) is a herbicide that has both pre- and postemergence activity against annual bluegrass. This herbicide can be applied to perennial ryegrass and Kentucky bluegrass, and is labeled for use on creeping bentgrass fairways. At least two applications of ethofumesate are required for successful control. Applications should be about 30 days apart beginning in September and the last application should take place no later than December 1 or after soil temperatures fall below 40° F. Be sure to follow label directions as rates vary depending on the turfgrass species present.

Reduction of annual bluegrass populations in bentgrass fairways can be achieved with applications of the growth regulators paclobutrazol (Trimmit^®) or flurprimidol (Cutless™). The best results occur with both spring and fall applications. Both products reduce growth and cause yellowing of annual bluegrass for up to five weeks after application. The use of growth regulators for annual bluegrass control requires a great deal of care and a fundamental knowledge of golf course management.

Perennial grass weeds

Most perennial grass weeds cannot be controlled with selective herbicides in turf. Spot treatment with a nonselective herbicide such as glyphosate (Roundup™) is the most reliable means of removing these weeds from turf. Glyphosate is an especially effective herbicide for perennial grass weeds since it is translocated through stolons and rhizomes and leaves no harmful soil residual. This herbicide is most effective when applied to actively growing plants.

A common perennial grasslike weed, yellow nutsedge, can be removed selectively from turf with the herbicides halosulfuron (Sedgehammer™) or bentazon (Basagran™). Be sure to follow label directions and note restrictions for nontolerant species.

Chemical control of broadleaf weeds

Broadleaf weeds are usually controlled with selective postemergence herbicides. The most common broadleaf herbicides used in turf include 2,4-D, 2,4-DP (dichlorprop), MCPP (mecoprop), dicamba, clopyralid, and triclopyr. There are many different commercial formulations and mixtures of these compounds (see Table 3).

It is extremely important to identify the weed(s) to be controlled before selecting one of these herbicides. All have the potential to damage trees, shrubs, flowers, and vegetables if they contact the foliage. Trees and shrubs are particularly sensitive to dicamba since this herbicide is mobile in the soil and can be taken up by tree roots.

Liquid postemergence broadleaf herbicides are available as salts and esters. The most popular salt formulation is the amine salt. The amine salt is soluble in water and suitable for spray applications. Amine formulations are nonvolatile, but they can react with calcium and magnesium ions (present in hard water and fertilizer solutions) to form insoluble salts.

Esters are insoluble in water but soluble in organic solvents and oils; hence, they are formulated as emulsifiable concentrates. When added to water, ester formulations form emulsions and do not react with calcium and magnesium ions. Esters provide better penetration of the thick waxy coating on leaves of some weeds when compared with salts.

Esters can be classified as short- or long-chain molecules. Short-chain esters are highly volatile and can damage sensitive nontarget plants. Volatility increases as temperatures increase and therefore esters are usually used during cool weather. Volatility of long-chain ester formulations, such as butoxyethyls, is minor and generally presents no serious problems when used according to environmental restraints listed on the label. Only long-chain esters are used for turf weed control.

The most effective control of broadleaf weeds is obtained when postemergence herbicides are applied as sprays to foliage (and not washed off). Granular formulations of these products are sometimes used to control broadleaf weeds; however, granulars should be applied to moist (dew-covered) foliage for optimum control.

Postemergence broadleaf herbicides are most effective when weeds are actively growing (spring and fall) and when air temperatures are greater than 70° F. During these periods, absorption and translocation of the herbicides by weeds are greatest, and desirable turf species have a chance to grow into the voids left after the weeds are killed. Consult herbicide product labels for optimum environmental conditions and timing of application.

Some manufacturers have attempted to satisfy the demand for faster kill of broadleaf weeds by adding the fast-acting herbicide, carfentrazone, to formulations containing slower-acting, systemic herbicides. Carfentrazone belongs to a class of herbicides called protox inhibitors and acts on broadleaf weeds by disrupting chlorophyll synthesis, resulting in rapid yellowing and desiccation. This herbicide does not translocate throughout the plant; thus, it does not provide good broadleaf weed control by itself. However, when it is combined with the systemic broadleaf herbicides 2,4-D, MCPA, MCPP, and dicamba, the result is fast burndown and, eventually, complete kill.

Clopyralid is a broad-spectrum broadleaf herbicide present in numerous herbicide formulations and used widely in cool-season turf. In 2003, labels of products containing clopyralid were amended to eliminate use on residential turf in the United States. This was done in response to concerns about the potential for damage to sensitive plants from clopyralid-treated turf residues in compost. The label change was based on several reports of plant damage from compost containing trace amounts of clopyralid and not on human health concerns. Clopyralid-containing products can still be used in non-residential turf markets, such as institutional grounds, athletic fields, and golf courses.

A few annual broadleaf weeds are controlled with preemergence herbicides. Spotted spurge, for example, is controlled with prodiamine, a common annual grass herbicide. One preemergence broadleaf herbicide, isoxaben (Gallery™), controls a wide spectrum of annual broadleaf weeds; however, this material has no activity on broadleaf weeds when generated from underground vegetative structures.

Chemical control, by weed species

Herbicides or combinations of herbicides used to control weeds are listed, by weed species, in Tables 4 and 5. For convenience, additional information is provided on tolerant species, dates of application, and methods of application. These suggestions are not a substitute for pesticide labels. The label provides detailed information on safety and proper use of the herbicide. Read the entire label before applying any pesticide.

Table 4. Suggestions for selective control of turfgrass weeds in Pennsylvania, by weed species.

Table 5. Suggestions for nonselective control of turfgrass weeds in Pennsylvania, by weed species.

Acknowledgements

Grateful acknowledgment is made to the Scotts Company for permission to reproduce drawings of broadleaf weeds from Scotts® Guide to the Identification of Dicot Turf Weeds.