Foliage
Its leaves are opposite, entire, and somewhat diverse in form, depending on whether they are submerged or floating. The floating leaves are spatulate-obovate in shape, have 5 to 7 veins, are 0.12-0.31 in. (3-8 mm) wide and up to 0.79 in. (2 cm) long. The submerged leaves are typically linear, have a single vein, and grow 0.16-0.39 in. (4-10 mm) in length; however, they can also be broader and more closely resemble the floating leaves.

Flowers
Callitriche stagnalis is monoecious. The staminate and pistillate flowers are tiny, simple in structure, and typically located right next to one another in the leaf axils. This close proximity facilitates aerial self-pollination. The flowers have two small bracts at their bases, which may help the flowers to float on the water, also facilitating pollination.

Fruit
The fruits are nearly round (suborbicular) and 0.06-0.08 in. (1.5-2 mm) thick. Each mericarp of the fruit has a thin (0.004 in. / 0.1 mm) winged margin running from its base to its apex.

Ecological Threat
Callitriche stagnalis is capable of creating locally dense mats of vegetation that may crowd out native aquatic vegetation.

Citations
Rawlins, K.A., R.L. Winston, C.T. Bargeron, D.J. Moorhead, and R. Carroll. 2018. New Invaders of the Southeast. USDA Forest Service, Forest Health Assessment and Applied Sciences Team, Morgantown, West Virginia. FHTET-2017-05. Retrieved from http://bugwoodcloud.org/resource/pdf/FHTET-2017-05_New%20Invaders_SE.pdf

Rawlins, K.A., R.L. Winston, C.T. Bargeron, D.J. Moorhead, and R. Carroll. 2018. New Invaders of the Northeast and Northcentral United States. USDA Forest Service, Forest Health Assessment and Applied Sciences Team, Morgantown, West Virginia. FHTET-2017-04. Retrieved from https://bugwoodcloud.org/resource/pdf/FHTET-2017-04_New%20Invaders_NE.pdf

Selected Images

Maps

Invasive Listing Sources

• Connecticut Invasive Plant List
• Connecticut Invasive Plant Working Group
• Connecticut Noxious Weeds
• Maryland Aquatic Nuisance Species Plan
• Maryland Department of Natural Resources Policy: Restriction on Planting Exotic Invasive Plants
• Mid-Atlantic Field Guide to Aquatic Invasive Species
• National Park Service, Mid-Atlantic Exotic Plant Management Team Invasive Plant List
• Native Plant Society of Oregon, 2008
• New Invaders of the Northeast and Northcentral
• New Invaders of the Southeast
• New Jersey Invasive Species Strike Team 2017 Invasive Species List
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• To be proposed for MISC
• WeedUS - Database of Plants Invading Natural Areas in the United States

References