EVALUATION OF HERBICIDES FOR CONTROLLING ALLIGATOR WEED (Alternanthera philoxeroides) AND RESTORING NATIVE WETLAND PLANTS AT EUFAULA NATIONAL WILDLIFE REFUGE. Shannon L. Allen ¹, Gary R. Hepp1, and James H. Miller². 1 School of Forestry and Wildlife Sciences, Auburn University 36849 and 2 USDA Forest Service R&D, Auburn, AL 36849. (smith68@auburn.edu)

ABSTRACT

Alligator weed (Alternanthera philoxeroides) is an exotic aquatic plant that has invaded the southeastern U.S. and has the ability to grow in aquatic, semi-aquatic and terrestrial habitats (1, 2). The perennial herb is native to South America and was first noted in the United States in 1894 (3, 4). It causes economic damages include the interference with many uses of water (5). Alligator weed mats reduce drainage from waterways and irrigation canals resulting in flooding (6). Disintegrating mats interfere with bridges and dams and invade habitats previously unaffected (5). Fishing, swimming, and navigation are prevented along shores where mats are present (7). Alligator weed also is a problem in at least 30 other countries including Australia, Burma, China, India, Indonesia, New Zealand and Thailand.

Alligator weed is a stoloniferous plant that does not produce viable seed in the United States (8). When protected from frost and ice, stems and roots containing nodes survive to generate next season's growth (9). Nodes hinder translocation of herbicides to the underwater portion of the mats, contributing to the difficulty of controlling alligator weed (10, 11). Alligator weed competes with native plants (12), displaces native plants in shallow water and along banks (5), reduced light penetration to native aquatic plants and animals (13), and reduces gaseous exchange (14).

Plants in many communities compete for resources such as nutrients, light, space, and water. Phenology of plants can affect their ability to utilize these resources. For example, perennial evergreen plants may have a competitive advantage over annual and deciduous plants because of early growing season presence (15), high growth rate (16), canopy formation (17, 18), and high biomass (19, 17). Many invasive plants, including alligator weed, exhibit some or all of these characteristics, which may help explain their dominance in invaded communities. Increase in native plant populations after alligator weed control (11) possibly could be due to release from competition. Our results lend some support to this idea.

Alligator weed has become a dominant plant in wetlands managed for migratory waterfowl at Eufaula National Wildlife Refuge (ENWR). It provides little or no nutritional value and displaces native plants that provide food for waterfowl, thereby degrading habitat quality. Providing habitat for waterfowl and other waterbirds is one of the primary management objectives at ENWR; therefore, controlling alligator weed is a major management concern.

In this study, we experimentally tested the abilities of two herbicides (triclopyr amine and imazapyr) applied at three application rates (high, medium and low) and on two dates (April and July) to control alligator weed and restore native plants to managed wetlands at ENWR. Triclopyr amine application rates consisted of 12.4L^-ha (high), 9.6L^-ha (medium), and 4.8L^-ha (low). Imazapyr application rates consisted of 3.6L^-ha (high), 2.4L-ha (medium), and 1.2L^-ha (low). Herbicides were mixed with nonionic surfactant and applied using a 2L CO2-pressurized backpack spray unit with a five-nozzle boom. We used a randomized block design and treatments consisting of herbicides, application rate, and application date were assigned randomly to experimental plots (5m²) within each block (n = 4). Control plots receiving no herbicide also were established. Two quadrats (0.5 m²) per experimental plot were used to measure alligator weed height, stem number, and percent cover of all plants before treatment and 1, 2, and 3 weeks and 1, 2, and 3 months after treatment. In October 2004, two quadrats (0.25m²) were randomly placed in each experimental plot to sample biomass of alligator weed and native plants.

Our results showed that July application of either triclopyr amine or imazapyr resulted in greater control of alligator weed than April application. Imazapyr resulted in greater control of alligator weed than triclopyr amine when applied in April. High application rate resulted in greater control than low application rate. In contrast, our results reveal a different strategy if the goal is to restore native plants important to migrating and wintering waterfowl. An April application at medium or high rates resulted in greater biomass of native plants than an April application of low rate or July applications at any rate. Control of alligator weed early in the growing season may have reduced its competition with native plants resulting in greater native plant biomass later in the year. Use of triclopyr amine resulted in greater native plant biomass than imazapyr.

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