Dead not Alive:
Wilson H. Faircloth,
Graduate Research Assistant,
Michael G. Patterson, Extension Weed Specialist,
James H. Miller,
David H. Teem,
(Imperata cylindrica) has
become a major problem for landowners, land managers, foresters, and
governmental agencies since its introduction into
Cogongrass spreads by both wind-blown seeds and underground creeping rhizomes. The rhizomes can form a dense mat in the upper 6-8 inches of soil and may comprise as much as 80% of the total plant mass (1,2). It is the rhizome system that makes this plant particularly hard to control. Elimination of aboveground portions of the plant can be easily accomplished, but if the rhizomes are not killed or removed, rapid re-sprouting and regrowth will occur.
Conservative estimates put the
infested acreage between 500,000 and one million in
Cogongrass has some distinctive vegetative features that aid identification. Cogongrass rarely is found as a single plant but quickly forms patches or infestations, often circular in outline. Plants vary in height, even in the same patch from 1 to 4 ft. tall (1,5). Taller leaves will lean over in late summer. Leaves measure ½-1 inch in width and are commonly 12-30 inches long. They rarely have a lush green color; instead, they appear mostly yellowish green. A reddening of the leaves sometimes has been observed in the fall, and is correlated to extreme changes in temperature. The whitish upper midrib of a mature leaf is often not centered on the blade as with most grasses thus making identification somewhat easier. Also leaf margins are rough to the touch due to tiny saw-like serrations, which is a common trait of other grasses as well. It is this rough margin, which may cut the tongue of a grazing animal, along with a high silica content that make cogongrass a useless forage crop. The leaves appear to arise directly from the soil, giving the impression that the plant is stemless, but short stems are present. A few short hairs may arise at the node, or the place where the leaf arises from the stem, but otherwise the plant is hairless.
Another key identifying feature is the production of fluffy, white, plume-like seedheads in early spring. This spring flowering is contrary to most summer grasses, which flower later in the season. Cogongrass also has been documented to initiate flowering at other times of the year in response to disturbance such as herbicide application, fire, mowing, or the first hard frost (4). Seedheads range from 2 to 8 inches in length and may contain as many as 3000 seed. Each seed has silky, white hairs that aid in wind dispersal. Seed viability is variable and seed must land on bare ground for germination (3). Rhizomes of cogongrass are white, segmented and branched and have been found extending 48 inches below the soil surface, but more commonly completely occupy the upper 6-8 inches (2,5). Rhizomes are sharp-pointed and often pierce the roots of other plants and unprotected human feet and hands. Each rhizome segment can give rise to a new plant, which can occur with cultivation or partial herbicide control (4).
Cogongrass is an opportunistic
plant and invades a wide range of non-cultivated habitats including
rights-of-way, forests, pastures, orchards, and waste areas. Cogongrass thrives in full sunlight, but may
extend well into a mature forest stand, especially if there is no intermediate
tree or shrub layer. Cogongrass
will not grow in saturated soils, but tolerates periodic flooding reasonably
well. Although cogongrass will not tolerate continued
soil disturbance, it is a rapid invader of recently disturbed soil such as that
found in road construction areas, industrial lands, mechanically site prepared
forest land, and even the container-grown ornamental industry. Cogongrass has never been a pest of row-crop
agriculture in the
Cogongrass control can be variable due to the age and rhizome mat density and depth. Young infestations are usually easier to control than older entrenched fields. For newer patches, tillage can eliminate cogongrass from an area if continued during the course of a growing season. The initial tillage should begin in the spring (March-May) with an implement that inverts the soil to a depth of at least six inches. Additional tillage should be performed every six to eight weeks with a disk harrow or other appropriate implement. Dry periods during the summer will aid in the control of cogongrass. The area can be planted to a fall cover crop and then followed the next season with perennial or annual grass or broadleaf crops. Mowing may help reduce cogongrass stands, but areas must be mowed frequently and at a low height to reduce the stand.
Tillage may not be an option on many sites such as steep slopes, established tree plantings, or around dwellings. Out of dozens of herbicides tested for significant activity on cogongrass only two, the active ingredients glyphosate (Roundup[a], Glypro, Accord[b], etc…) and imazapyr (Arsenal, Arsenal AC, and Chopper[c]), have much effect on this grass (12). Even at high rates and combinations, cogongrass often regenerates within a year following a single application of either product. A minimum of two applications per year is needed, realizing that older infestations may require 2-3 years of treatment to eliminate rhizomes. Glyphosate has no soil residual activity and permits planting replacement species after application. Imazapyr has both soil and foliar activity and can severely injure susceptible plant species that are planted too soon after the last treatment. Most vegetables, row crops, and ornamentals WILL BE INJURED if planted with 24 months following an imazapyr application. As with all pesticides, proper handling and usage is of utmost importance and ALWAYS READ AND FOLLOW LABEL DIRECTIONS.
Small Area Infestations and Home Landscapes
Cogongrass in small (less than 20 ft diameter) patches can be treated with a glyphosate solution in early fall (August-October). A 2% solution of 41% active ingredient material (3 fl. oz. per gallon of water) or a 4% solution of a 21% active ingredient glyphosate (6 fl. oz. per gallon of water) sprayed on the green leaves and allowed to dry for 2 to 3 hours will kill the top growth of cogongrass. Regrowth must be treated the following spring and possibly the next fall to ensure rhizome kill. CAUTION: glyphosate herbicide spray mixtures should be considered non-selective when sprayed on green tissue. Spray and spray drift should be kept off any desirable plants. Larger infestations can be treated with glyphosate using a tractor-mounted boom sprayer calibrated to deliver 10 to 15 gallons of spray solution (water + herbicide) per acre. Glyphosate at the rate of 3-4 pounds active ingredient per acre should be used. This translates into 3-4 quarts of herbicide per acre for the 41% active material. Replacement species should be planted in the area following the last treatment (either spring or 2nd fall) to suppress reinfestation. Crimson clover or ryegrass may be planted after the fall application of glyphosate to stabilize sloping areas through winter. This cover crop can be replaced with a perennial grass such as bahiagrass or with shrubs planted in a high-density pattern to provide shade over the area.
Rights-of-Way, Industrial Sites, and other Non-Crop Areas
Where selective treatment is not needed, tank mixes of glyphosate (3-4 lb. active ingredient per acre) plus imazapyr (Arsenal 1-4 pt. per acre) are effective. Sulfometuron (Oust XPÒ[d]) at 2 oz. per acre has been shown to increase cogongrass control when applied with imazapyr, but should not be applied as a stand-alone treatment. For selective treatment of cogongrass in unimproved bahiagrass and bermudagrass turf, Arsenal may be applied at a rate of 8 fl. oz. per acre. DO NOT apply imazapyr to bahiagrass after full green-up and DO NOT apply to fields that will be grazed or cut for hay. Burning or mowing prior to herbicide application may increase chemical effectiveness by eliminating thatch and causing the production of new growth, which better absorbs pesticide. A cogongrass fire is HOT and FAST; PROCEED WITH EXTREME CAUTION AND CAREFUL PLANNING AND PREPARATION. In addition, always consult with local authorities on the rules concerning burning in your area.
Cogongrass management in southern pines is more difficult. When cogongrass is present at harvest, if feasible, prescribe burn during winter prior to treatment to eliminate logging debris and cogongrass thatch for more effective applications. Chemically site prepare with Arsenal AC (1 pint per acre) or Chopper (2 pints per acre) plus glyphosate at 4 lbs active ingredient per acre in the fall prior to planting in late winter (at least 3 months after application). This combination may be applied aerially and should include a suitable surfactant not to exceed 0.5%. After planting, release treatments may be applied to seedling longleaf or loblolly pine that are NOT actively growing or initiating new buds, usually in late summer after rainfall resumes. Arsenal AC at 4 fl. oz. per acre is effective, especially if the cogongrass was previously treated with site preparation sprays. DO NOT use surfactant with this treatment, as severe tree injury and/or mortality will result. Oust XP (1-2 oz. per acre) may be tank-mixed with Arsenal AC (4-6 fl. oz. per acre) for improved control in LOBLOLLY PINE ONLY. On well-established pines (DBH > 5 in.), apply the site prep mixture described above from August-October, taking care not to contact pine foliage. Fire is NOT recommended in standing timber unless cogongrass is in a suppressed state from prior treatment.
Rehabilitation is the most important final phase of control and reclamation program for cogongrass and other nonnative invasive plants. The rehabilitation phase requires establishment and/or release of fast growing native plants that can outcompete and outlast any surviving cogongrass plants while stabilizing and protecting the soil. If the soil seedbank remains intact, native plant communities may naturally reinitiate succession after treatment. Light seeded native species are usually present in the seedbank while heavier seeded plants will gradually be deposited on a site by birds and other animals. In recent years, native plant seed and seedlings have become increasingly available for rehabilitation sowing and planting, but a limited number of species and absence of well-developed establishment procedures often hinder use. Tree nurseries operated by State forestry agencies are a good source of many species of native trees and shrubs. Often it is necessary to establish fast growing tree species during the later control phase to hinder reestablishment of cogongrass. Reestablishing native grasses and forbs is equally important, and these species are available from commercial nurseries specializing in native plants, utilizing local sources when possible. Seedling native plants can be also collected and transplanted from suitable field sites. Their establishment will be more challenging than the commonly available nonnative plants so often used for soil stabilization and wildlife food plots. Constant surveillance, treatment of new unwanted arrivals, and finally rehabilitation following control are critical to preventing reinfestation on a specific site.
b. Rhizomes are also segmented with paper-like sheaths.
Roundup is a registered trademark of Monsanto Co.,
[b] Glypro and Accord are registered trademarks of Dow Agrosciences,
Arsenal, Arsenal AC, and Chopper are registered trademarks of BASF,
Oust XP is a registered trademark of Dupont,
Photo 1 Photo 2 Photo 3a Photo 3b
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