The term “Arrowhead Weed” most commonly refers to plants belonging to the genus Sagittaria, a group of perennial aquatic or semi-aquatic herbs. These plants are widely distributed and thrive in freshwater environments, often growing in shallow water or saturated soil. While some species are native and valued for their ecological contributions, the common name “weed” arises when the plant’s aggressive growth habits cause it to dominate a waterway or wetland area.
Identification and Habitat
The primary feature used to identify plants in the Sagittaria genus is the distinctive, emergent leaf shape, which gives the plant its common name. These leaves are typically large and shaped like an arrowhead, or sagitta in Latin, with two pointed lobes extending backward from the base. The leaves grow on long, rigid stalks that rise above the water surface, often reaching heights of up to three feet.
The plant produces small, showy flowers on a separate, leafless stalk, arranged in whorls of three along the stem. Each individual flower has three white petals, sometimes with a purplish spot at the base, and blooms occur from mid-summer into early autumn. Sagittaria is typically rooted in the mud of shallow wetlands, marshes, pond edges, and slow-moving streams, and it spreads aggressively through a network of underground rhizomes and starchy tubers.
Ecological Role and Concern
Many native Sagittaria species play a positive role in wetland ecosystems, providing food and habitat for wildlife. The starchy tubers, sometimes called “duck potatoes,” are a valuable food source for numerous waterfowl species, muskrats, and other animals. These plants also help stabilize shorelines and can be used in constructed wetlands for their ability to take up excess nutrients, such as phosphorus, from the water.
However, when certain native species overgrow, or when non-native species are introduced, the dense growth becomes problematic. Aggressive colonies can form thick stands that outcompete and displace other desirable native aquatic vegetation, severely reducing biodiversity. These dense mats of foliage and roots can obstruct water flow in irrigation channels and drainage ditches, leading to silt accumulation and increased flooding risk. The extensive spread of the plant also interferes with recreational activities, making boating, swimming, and fishing difficult.
Management and Removal Strategies
Controlling Sagittaria is challenging because of its ability to regenerate from its root system, which includes rhizomes and tubers. For small, isolated infestations, manual or mechanical removal is a viable option, but it must be meticulously executed. Hand-pulling requires the complete removal of all underground plant material, including the extensive rhizome network and the tubers, as any fragment left behind can sprout a new plant.
Larger infestations often require a multi-faceted approach, incorporating environmental controls such as water level manipulation. While Sagittaria thrives in standing or shallow water, altering water levels through a sustained drawdown can stress the plant, although this method may also trigger the production of more tubers as a survival mechanism. Conversely, maintaining water levels deeper than 3 feet can suppress the growth of the emergent-leaf form.
Chemical control provides an effective option when used in accordance with local regulations for aquatic environments. Systemic aquatic herbicides, such as those containing glyphosate, imazamox, or 2,4-D, are preferred because they are absorbed by the leaves and translocated to the root system. Application should be timed for mid- to late-season when the plants are actively growing and have maximum foliage to absorb the chemical. Using a surfactant, an additive that helps the herbicide stick to the waxy leaves, improves the effectiveness of the treatment. Any use of herbicides in water must be done with an aquatic-approved product and strict adherence to the label to minimize risk to non-target species and water quality.