twoleaf watermilfoil
Myriophyllum heterophyllum Michx.

Overview

Appearance

Myriophyllum heterophyllum is a perennial, aquatic herb that has leaves of two noticeably different forms.

Foliage

The submerged leaves are finely dissected, whorled, reddish/greenish-brown, and 0.5-2.5 in. (1.3-6.4 cm) long. The emergent leaves are small, oval, bright green, whorled and up to 0.25 in. (0.6 cm) wide. Emergent leaves stand 6-8 in. (15.2-20.3 cm) out of the water and may not be apparent until late summer.

Flowers

Flowering occurs from June to September. Flowers are emergent on 2-12 in. (5.1-30.5 cm), green to reddish stalks. Petals are less than 0.1 in. (3 mm) in length and are subtended by downward curved bracts.

Fruit

Fruits are small, nearly round and have a rough surface.

Ecological Threat

Myriophyllum heterophyllum is native to the southern United States, but is considered invasive in much of the northeast. This species is listed as endangered or threatened in three states (Kentucky, Ohio, and Pennsylvania).

Selected Images

Maps

Invasive Listing Sources

• Alberta Fisheries Act
• Connecticut Invasive Plant List
• Connecticut Invasive Plant Working Group
• Connecticut Noxious Weeds
• Idaho Noxious Weeds
• Invasive Species of Concern in Georgia
• Maryland Aquatic Nuisance Species Plan
• Massachusetts Invasive Plants Advisory Group List
• Massachusetts Noxious Weeds
• Massachusetts Prohibited Plant List
• New Hampshire Prohibited Aquatic Species
• New York Regulated and Prohibited Invasive Species - Prohibited
• Pennsylvania's Field Guide to Aquatic Invasive Species
• Rhode Island Natural History Survey
• To be proposed for MISC
• Vermont Noxious Weeds
• Washington Noxious Weeds

References