Invasive Plants of California's Wildland

Pampas grass (Cortaderia selloana) is a perennial grass six to thirteen feet tall with long leaves folded at the midrib and arising from a tufted base or tussock. The inflorescence or flower cluster is a plumed panicle at the end of a stiff stem. Stems are equal to or slightly longer than the tussock. Plumes nearly always consist of light violet to silver-white hairy female flowers that rarely produce seed. Pampas grass is easily confused with jubata grass (C. jubata). The two species are distinguished by several features, including stem height, leaf, plume, and spikelet color, florets, leaf tip shape, and presence of viable seed. The tussocks of pampas grass are more erect and fountain-like, not spreading, when compared to tussocks of jubata grass.

Poaceae. Perennial grass. Leaves: blades to 6 ft (1.8 m) long, 1-3 in (3-8 cm) wide, V-shaped in cross-section, bluish green (glaucous), upper surface glabrous at base, lower surface glabrous or hairy towards collar, tips bristly and curled, margins scabrous and sharp. Sheath: variable in hairiness, often glabrous, auricle-like outgrowth present at collar. Inflorescence: dense panicle, 1-4 ft (3-13 dm) long, stiff, light violet to silvery white when immature, white at maturity. Spikelets: numerous, 0.6 in (15-17 mm) long, typically with 6 florets in female plants and 3 in hermaphroditic plants.

Florets: 0.16-0.3 in (4-8 mm) long, glumes white or membranous, lemma long-hairy, awns 0.1-0.2 in (2.5-5 mm) long, stigmas exerted (Hickman 1993, Robinson 1984). Caryopsis: viable seed rarely found and, when present, not easily separated from rachilla.

Pampas grass is common as an ornamental throughout California, including interior regions. It has escaped cultivation and spread along sandy, moist ditch banks throughout coastal regions of southern California (Costas-Lippman 1977) below 1,000 feet (330 m). Its distribution is not as extensive as Cortaderia jubata, but it appears to be expanding (DiTomaso et al. 1998).

Pampas grass is native to Argentina, Brazil, and Uruguay, where it grows in damp soils along river margins (Connor and Charlesworth 1989). It was first introduced to Europe by a Scottish horticulturist between 1775 and 1862. Samples were introduced to California about 1848 by Joseph Sexton, a nurseryman from Santa Barbara. Commercial production began in 1874 in both California and Europe. Until 1895 nurserymen in the Goleta Valley near Santa Barbara were the primary producers of Cortaderia selloana for ornamental use (Madison 1992). In 1946 it was planted by the Soil Conservation Service throughout Ventura and Los Angeles counties to provide supplementary dryland forage and prevent erosion (Costas-Lippman, 1977). Pampas grass has escaped cultivation in many coastal areas in California, presumably by fragmentation of the parent plant or, to a limited extent, by seed.

Although the more aggressive Cortaderia jubata is often called pampas grass, true pampas grass (C. selloana) can also be weedy in California. In other areas of the world, particularly New Zealand and Australia, C. selloana is an important weed problem in forestry operations and conservation areas (Gadgil et al. 1984, Harradine 1991). In forests it competes with seedling trees and can slow their establishment and growth. Pampas grass creates a fire hazard with excessive build-up of dry leaves, leaf bases, and flowering stalks. In addition, heavy infestations can block access to plantations and pose a significant fire hazard. In conservation areas pampas grass competes with native vegetation, reduces the aesthetic and recreational value of these areas, and also increases the fire potential.

Pampas grass is typically propagated for ornamental purposes through division of mature plants (Robacker and Corley, 1992). In nature it produces flowers two to three years after germination. Flowering usually occurs from late August though September (Madison 1992), but occasionally in winter. The species is considered gynodioecious, that is, flowers of some plants consist of both male and female parts on the same flower, but only the male parts are functional (Connor 1973). Other plants bear only female flowers. Thus, this species is functionally dioecious. Over the years, selection for ornamental plants in California has been for the showier plumes of the female plants. Consequently, few opportunities exist for seed production. This may account for the lack of spread of this species in California in past years.

In recent years some nurseries have propagated pampas grass from seed. Since it is impossible to distinguish male from female plants before they flower, the result is an increase in the proportion of male plants in the population. Consequently, there has been an increase in the amount of viable seed produced, and this species has escaped to become an invasive weed along the California coast.

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Populations that escaped from cultivation probably occurred in areas where seeds were produced. This can occur when both male and female plants are present in a population or when an occasional perfect flower (with both male and female parts) is produced on a typically male plant. In New Zealand, selection for female plants has not been as rigorous. As a result, more seed is produced, and pampas grass has become a significant weed problem (McKinnon 1984). Little is known of the germination of Cortaderia selloana from seed. Vegetative reproduction can occur when fragmented tillers receive adequate moisture and develop adventitious roots at the base of the shoot.

Establishment of seedlings generally occurs in spring and requires sandy soils, ample moisture, and light. Seedling survival is low in shaded areas or in competition with grasses (Gadgil et al. 1990) or sedges. Since few seeds are produced in California, little is known of the growth requirements. Unlike Cortaderia jubata, C. selloana can tolerate winter frost (Costas-Lippman 1977); it also tolerates warmer summer temperatures, more intense sunlight, and moderate drought. This accounts for its success as an ornamental in the Central Valley of California and its establishment as a weed along the American River near Sacramento. Once established, roots of a single plant can occupy a soil volume of about 1,100 square feet (103 m2). Lateral roots can spread to thirteen feet (4 m) in diameter and eleven and one-half feet (3.5 m) in depth (Harradine 1991). Plants are capable of surviving about fifteen years (Moore 1994).

Control of pampas grass is similar to that of jubata grass. Few strategies are available for the control of Cortaderia selloana. Burning does not provide long-term control, as plants resprout shortly thereafter. Infestations sometimes can be averted by overseeding open disturbed sites with desirable vegetation to prevent establishment of seedlings.

Manual methods: Pulling or hand grubbing Cortaderia selloana seedlings is highly effective. For larger plants however, a pulaski, mattock, or shovel are the safest and most effective tools for removing established clumps. To prevent resprouting, it is important to remove the entire crown and top section of the roots. Detached plants left lying on the soil surface may take root and reestablish under moist soil conditions (Harradine 1991). A large chainsaw or weedeater can expose the base of the plant, allow better access for removal of the crown, and make disposal of the detached plant more manageable (Moore 1994). Cutting and removing or burning the inflorescence prior to seed maturation in late summer may be important if seed production occurs in escaped populations of pampas grass. To reduce labor, the top of the foliage can be removed and the remaining crown treated with diesel oil (Cowan 1976).

Insects and fungi: No insect or fungal control efforts have been investigated for any species of Cortaderia.
Grazing: The success of grazing has not been reported in the United States, but cattle have been shown to provide effective control for pampas grass in commercial forests of New Zealand (Harradine 1991, Gadgil et al. 1984).

Control of pampas grass can be achieved by spot treatment with a post-emergence application of glyphosate at about 2 percent solution or eight qts/100 gal. The addition of a non-ionic or silicone-based surfactant may enhance foliar penetration of the herbicide. For most effective control, plants should be sprayed to wet, but not to the point of herbicide runoff. In one study, over 90 percent control was obtained during the first season, but continued spot applications were necessary to prevent rapid reestablishment (Madison 1992).

Fall applications result in better control compared to summer applications (Costello 1986) because photosynthetic assimilates are translocating downward at a faster rate late in the season. However, if viable seeds are produced, it may be necessary to apply the herbicide prior to seed maturation. Although studies were conducted on jubata grass, it is likely that low-volume (20 gal/ac) treatment with glyphosate at 4 percent can also provide excellent control of pampas grass. The reduced volume can lower the amount of herbicide used as well as the cost of the treatment (Drewitz et al. unpubl. data). Rope wick applications of glyphosate have also proven effective, but good coverage is essential or tillers will recover (Drewitz et al. unpubl. data).

Other registered post-emergence herbicides useful for control of Cortaderia jubata, may also be effective in the control of pampas grass.

For large clumps, the top foliage can be removed by cutting or burning and the regrowth treated with a systemic post-emergence herbicide. This method reduces the amount of herbicide applied compared to herbicide treatment alone (Harradine 1991).