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Yavapai County Native & Naturalized Plants

Potamogeton illinoensis - Illinois pondweed

Synonyms: Potamogeton angustifolius, P. fragimillus, P. heterophyllus, P. lucens var. connecticutcutensis, P. lucens var. floridanus, P. methyensis
Other Common Names: shining pondweed, Methy Lake pondweed
Plant Form: Forb or Subshrub

Family: Potamogetonaceae


   
 
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Forb Plant Description

Origin: Native   Life Cycle: Perennial   Similar Species: Potamogeton gramineus, P. nodosus, P.amplifolius
General Desc: Aquatic plant forming colonies that spread by underwater horizontal stems (rhizomes). Stems are round, often branched and jointed. Leaves are often underwater but may float. Whorls of tiny greenish flowers grow on spikes under or above the water.
Identification notes: Leaf margins smooth; floating leaves may be absent, if present wedge-shaped and taper towards stems; submersed leaves flat, narrow to round, not stemmed, 7 to 19 veins; stems round; fruits compressed, greenish-brown, grayish-green to olive-green.
Height: From 1 to 6 feet


Habitat

Habitat Description: Found in the quiet waters of streams, lakes and ponds as well as in slow to moderately fast moving rivers in water up to 15 feet deep. Often found in calcareous waters (hard or alkaline waters higher in dissolved minerals especially calcium).
Plant Communities: Riparian
Elevation: 1300 - 7700 feet


Flower Characteristics

Color: Greenish     Shape: Regular in elongated clusters     Tubular: N     Flowering Period: Jul - Sep
Description: Flower stalk is a thick, fleshy, cylindrical spike 1 to 3 inches long growing underwater or above the water surface. Tiny greenish flowers are crowded in whorls of 8 to 15 along the length of the spike. Each flower has 4 leaf-like sepals and no petals.


Leaf and Stem Characteristics

Leaf Color: Reddish-brown, green     Leaf Type: Simple     Leaf Shape: Other     Leaf Margin: Smooth     Leaf Attachment: Alternate and opposite     Leaves Clasp: N
Hairs: No     Spines: N
Leaf Description: Floating and submersed (underwater) leaves are produced but floating leaves may be absent. Submersed leaves are reddish-brown, flat, narrow to round, may have wavy or smooth edges and a prominent midvein. Floating leaves are green, firm and wedge-shaped.


Fruit and Seed Characteristics

Fruit Color: Grayish-green to olive green   
Fruit Type: Berry-like
Fruit Notes: Fleshy fruit (drupe) 1/4 inch long has an egg-like, slightly flattened shape. It has 1 narrow ridge (keel) on its back edge and a pair of keels on its sides. A pointed projection (beak) about 1/16 inch at its tip is either upright or curves backwards.
Seed Notes: 1 seed is produced in a hard shell. The fruit does not split open at maturity to release the seed.



  Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr #C
Prescott, AZ 86305
(928) 445-6590
Version 8.0  
http://cales.arizona.edu/yavapaiplants/SpeciesDetailForb.php  
Last Updated: Dec 13, 2022
Content Questions/Comments: Email Matt Halldorson  
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