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.

October Weed Profile

Yellow Flag Iris

Iris pseudacorus

Yellow flag iris is a Watch List species being considered for addition to the state’s weed list by the Colorado Department of Agriculture.

It was imported to North America as an ornamental plant in the late-1700s but has escaped and now infests ditches, streams, and ponds. In some areas it has formed large monocultures and changed the adjacent ecosystems. Thick growths of yellow flag can clog irrigation systems and streams and, by trapping sediment in the roots, can narrow waterways. All parts of the plant are toxic to livestock and other animals.

Plants grow to about 3 feet tall with long sword-like leaves that grow from thick rhizomes in a fan-like arrangement. Leaves are about ½ to 1¼ inch wide, flat with a pronounced midrib. Plants resemble cattails when not in bloom.

The rhizomes can live for over 10 years in the soil and can remain viable for 3 months or more when dry.

Plants generally form flowers after three years. The 2-3-inch-wide flowers are yellow to whitish with three upward facing petals and three downward facing sepals. The sepals usually have dark purplish-brown streaking. Flowering is summer through fall.

Seeds are formed in three-sided pods. Each plant can form several hundred seeds that can survive and float for more than a year, enabling new infestations to establish long distances from existing occurrences.


New York Invasive Species (Is) Information


Washington State Noxious Weed Control Board


University of Florida / IFAS / Center for Aquatic & Invasive Plants


Vermont Invasives



Flower – Shaun Winterton, Aquarium and Pond Plants of the World, Edition 3, USDA APHIS PPQ, Bugwood.org

All Others – Jefferson County Invasive Species Management

Items of Interest

Weed of the Month


January Weed Profile – Knotweed

February Weed Profile – Rush Skeletonweed

March Weed Profile – Black Henbane

April Weed Profile – Chinese clematis

May Weed Profile – Houndstongue

June Weed Profile – Milk Thistle

July Weed Profile – Oxeye Daisy

August Weed Profile – Spotted Knapweed

September Weed Profile – Perennial Pepperweed

October Weed Profile – Yellow Flag Iris


January Weed Profile – Dalmatian Toadflax

February Weed Profile – Common Mulllein

March Weed Profile – Mediterranean Sage

April Weed Profile – Saltcedar

May Weed Profile – Absinth Wormwood 

June Weed Profile – Eurasian watermilfoil 

July Weed Profile – Yellow starthistle 

August Weed Profile – Hairy willowherb

September Weed Profile – Yellow toadflax

October Weed Profile – Chamomile

November Weed Profile – Diffuse knapweed

December Weed Profile – Poison hemlock


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