Weed of the Month

Learn about noxious weeds and steps you can take to manage or eradicate.  Each month we will bring you a new profile and helpful resources.

June Weed Profile 

Eurasian watermilfoil 

Myriophyllum spicatum

This exotic species is an aquatic submerged plant that is one of the most destructive aquatic weeds known.  Eurasian watermilfoil (EWM) has leaves whorled around the stem with 14 or more leaflets on each side.  Fruiting stems with small yellow flowers form spikes above the water.  EWM is often confused with native milfoils.  Native species typically have fewer than 14 (5 to 10) leaflet pairs per leaf.  The exotic species can hybridize with the native milfoils.  Hybrids may require DNA testing to distinguish from non-hybrids.

EWM aggressively forms dense mats that block the ability of sunlight to enter the water and stop the growth of native species, displaces wildlife habitat, and impede all forms of water-based recreation.  After fruiting, EWM makes fragments of the plant, these fragments are responsible for new colonies as they float to other areas, sink, and start new plants. Transport on boating equipment is a significant means of plant establishment in new bodies of water.  Any equipment used on or in the water can collect and spread this aquatic species.

This aggressive invader establishes in moving and standing waters and grows at a rate of approximately one foot per week.  It can infest an entire lake within a couple of years after introduction, EWM has less value as a food source for waterfowl than the native plants that it replaces.

Remember to always clean, drain, and dry your boats, trailer, gear, and any equipment when playing or working in aquatic systems. It is not legal to transport watercraft over land with water drain plugs or aquatic vegetation on board per Colorado Parks and Wildlife (CPW) Aquatic Nuisance Species (ANS) regulations.

According to CPW, this is the highest priority aquatic noxious weed in Colorado.  Millions of dollars are spent nationwide for control efforts.  If you detect EWM or any other ANS, please report it to Invasive.Species@state.co.us. The CPW ANS Program will verify the detection per regulation and will collaborate with CDA, the County, and other pertinent landowners or stakeholders to determine and implement the appropriate management response.


U.S. Geological Survey, Nonindigenous Aquatic Species Database


Items of Interest

Weed of the Month


January Weed Profile – Dalmatian Toadflax

February Weed Profile – Common Mulllein

March Weed Profile – Mediterranean Sage

April Weed Profile – Saltcedar

May Weed Profile – Absinth Wormwood 


January Weed Profile – Cheatgrass

February Weed Profile – Myrtle Spurge

March Weed Profile – Hoary Cress

April Weed Profile – Leafy Spurge

May Weed Profile – Scotch Thistle

June Weed Profile – Dyer’s Woad

July Weed Profile – Orange Hawkweed

August Weed Profile – Purple Loosestrife

September Weed Profile – Teasel

October Weed Profile – Canada Thistle

November Weed Profile – Russian Knapweed

December Weed Profile – Russian Olive