Alligator Weed Alternanthera philoxeroides (Mart.) Griseb.

Abstract

Problems caused Alligatorweed forms dense mats along the shoreline of lakes, ponds, streams, ditches, and wetlands, with the mat extending out into open water. The dense growth suppresses native plant species, reduces the quality of habitat for water-fowl, wildlife, and fish, and will reduce oxygen levels in the water under the mat. The dense mats interfere with navigation and recreational use, and will exacerbate flooding. Regulations Alligatorweed is not listed as a federal noxious weed, although the related sessile joyweed (Alternanthera sessilis) was listed out of concern that it too would become a widespread weed. It is listed as a noxious weed in AR and SC, and a Class C weed in AL. Alligatorweed is widely recognized as an invasive weed by state Exotic Pest Plant Councils (EPPC). Description Vegetative growth Alligatorweed is an emergent herbaceous perennial plant, forming dense stands up to 3' tall. The stems vary in color, are approximately ¼" thick, and often hollow, particular in the floating mat stage. The stem nodes are ½" thick, and hollow. Stems will root from the nodes, and in standing water the stems will float on the surface, forming a dense mat, with upright branches. Leaves are opposite, without a petiole, and the orientation of the leaves will shift 90˚from90˚from one node to the next. Leaves are entire, elliptical in shape, and approximately 4 '' long. The leaf base forms a sheath around the stem at the node. Flowering An inflorescence may be formed, one per node, with at least one node separating flowering nodes. The flower stalk is ½-3" long, and the flower spike is a collection of numerous flowers forming a roundish cluster. The flower itself has no petals , but has five white or colorless sepals, which are ¼" long. One seed is produced per flower. Flowering will occur throughout the growing season. Dispersal mechanisms Stems that break off can root at the nodes and form a new colony. Seeds are fertile, and may be transported by animals or water, or germinate from the seedbank if the plants are removed. Spread by Plants are spread naturally by animals or water. Stems may also be transported on boats, boat trailers, and other equipment .