Weed Work in Upper Mill Creek

Another essential part of the project was to rebuild the capacity of the Marion County Weed Control District which has been inactive in recent years.  Weed control districts are the repository for invasive plant information and the group responsible for finding solutions to noxious weed problems in the county. Each county in Oregon has a weed control district, either standing alone or as part of the county governance, SWCD, or other organization. This project worked to form a lasting partnership that will help the Marion County Weed Control District thrive. 

Surveyors searched for these 14 priority weeds along irrigation canals and waterways: 

  • False brome
  • Garlic mustard
  • Giant hogweed
  • Italian thistle
  • Knotweed
  • Meadow knapweed
  • Milk thistle
  • Old man’s beard
  • Parrotfeather
  • Purple loosestrife
  • Tansy ragwort
  • Water primrose
  • Yellow flag iris
  • Yellow floating heart

The 5 bold species were found during the surveys. There were also large swaths of non-native blackberries, English ivy, reed canarygrass, pennyroyal, and water milfoil alongside other garden and agricultural weeds.  

Here’s a chart of the results:



Total Sq. Ft. Infested

Number of Infestations

False Brome




Meadow Knapweed




Old Man’s Beard








Yellow Flag Iris








The most common priority weed found was tansy ragwort, an herbaceous, yellow-flowered biennial which is toxic to livestock. Tansy populations tend to explode in cycles which make it difficult to contain. You can find more information about tansy ragwort here:


   Yellow Flag Iris           Tansy Ragwort    

Another common weed found in the waterways was yellow flag iris. This noxious plant grows along ditches, streams, and ponds and may choke irrigation ditches.  The large leaf blades and slender yellow flowers differentiate this iris from ornamental iris’ in many gardens. Treatments of yellow flag iris followed the weed surveys during the summer of 2018 and spring of 2019. It was treated using an aquatic licensed herbicide applicator and an aquatic safe herbicide. You can learn more about yellow flag iris here: https://extension.oregonstate.edu/news/gorgeous-yellow-iris-ecological-threat-pnw-wetlands

As part of the capacity-building activities provided by this grant, 2 training events were offered: The Marion County Vegetation Management and Park Maintenance crews were trained to identify, report, and control noxious weeds in road rights-of-way and parks, and landowners within the survey area were invited to an evening workshop to learn about invasive plants on their properties.

The Weed District also purchased outreach materials including a reprint of Marion County Reference Guide: Most Unwanted Noxious Weeds (https://www.co.marion.or.us/PW/Documents/weedguide2012a.pdf) and display weeds to assist educators with weed identification (https://store.msuextension.org/Departments/Weed-Models.aspx). 

While the main objective of this project was to identify and control the spread of invasive weedsthis partnership has fueled collaboration between local and regional partners.We hope to continue this partnership as we forge new paths towards the protection of our natural areas against the spread of noxious and invasive weeds.

This project would not be possible without support from all of the partner organizations including Marion County Weed Board, Marion Soil & Water Conservation District, the North Santiam Watershed Council, with funding from the Oregon Watershed Enhancement Board and cooperation of the Santiam Water Control District.