Invasive weed management in riparian areas – a few take-homes

Weeds pose a major challenge to successful restoration of riparian vegetation.  Armenian blackberry, knotweed, poison hemlock, Reed canary grass and other non-native invasive species crowd out native species and compete aggressively with planted trees and shrubs.   To address this issue, OSU Extension and The Freshwater Trust sponsored a workshop on “Weed Management in Riparian Areas” at the OSU Extension auditorium, June 10, 2014.  There were 49 attendees including representatives from watershed councils, local agencies and municipalities, as well as landowners and contractors.  We spend the morning in the classroom and afternoon in the field at sites along Jackson Creek and the Applegate River at Red Lily Winery.  There was a lot of great information presented and for details, visit the links to the presentations below. 

Among the many interesting points for me were some of the basics about management of annual and perennial weeds presented by Carri Pirosko of the Oregon Department of Agriculure.  These are concepts we already know, but they bear repeating.


  1. Invasive species are adapted to disturbance, such as clearing an area of blackberry.  This often exposes mineral soil, which is an ideal seedbed for new weeds that were either already present in the soil seedbank or were blown in from other areas.  Bare mineral soil provides plenty of sunlight, moisture, and a respite from competition – ideal conditions for rapid weed growth.  See the photo sequence above, which shows (from left to right) the conversion of a blackberry patch to a patch of poison hemlock and prickly lettuce over about 18 months.  So it’s easy to replace one weed problem with another.  Preventing or minimizing disturbance can reduce invasions of new weeds.  And retention of litter, mulch or other organic matter over disturbed soil can help minimize the germination of new weeds.
  2. Perennial weeds are like icebergs – the above ground part of the plant is often only a small part of the total mass, which is dominated by an extensive root system.  In spring, sugars move from the roots to the shoots.  And in late summer and fall, they move from the shoots to the roots.  One implication of this is that stored carbohydrates are at a maximum during dormancy, and at a minimum in summer.  So, timing control during the summer stage when reserves as minimized makes more sense than a dormant season treatment.  A second implication is that herbicide treatments during late summer or early fall when the plant is sending sugars downward to the roots often results in better translocation of the herbicide.  That’s not always the case but it explains why, for example, early summer treatments of blackberry with glyphosate (active ingredient in Roundup and many other products) are pretty useless while early fall treatments are much more effective. 
  3. Another implication of the iceberg concept (below ground root mass) is lack of effectiveness of repeated cutting or mowing for controlling perennial weeds.  That is, mowing or otherwise cutting a weed like blackberry once or twice a season just encourages it.  To get effective control for tough perennial needs, you would need to mow every 3-4 weeks, all season long.  And repeat that over several seasons.
  4. To effectively control weeds prior to planting desirable native trees and shrubs, a longer timeframe needs to be considered.  The first year could be taken up with treating blackberry for example, which is the Rogue Basin’s most common and abundant invasive weed along with Reed canary grass.  Then year 2 is spent dealing with the annual and biennial weeds that inevitably spring up.  Site preparation is the best opportunity to control weeds, so that’s the time to hit them hard and get them under control, prior to planting.

There’s more in the individual presentations.  Check them out below.  If you are interested in participating in future activities of the Riparian Practitioners Network, contact me at or (541) 776-7371 x221.

Riparian publications & booklets

How reference sites help determine weed treatment approach.  Olivia Duren, Ecologist, Freshwater Trust

What can the biology of invasive plants teach us about controlling them?  Carri Pirosko, Integrated Weed Management Coordinator, Oregon Department of Agriculture

Weed management in riparian areas with a focus on non-chemical strategiesMax Bennett, Forestry/Natural Resources Agent, OSU Extension Service 

Riparian weed control utilizing herbicides.  Kyle Strauss, Strauss Ecological Services 

Undertanding pesticide fate for the protection of water resources.  Jeff Jenkins, Department of Environmental and Molecular Toxicology, Oregon State University.  

Laws and regulationsLinda White, Pesticide Certification and Licensing, Oregon Department of Agriculture. 





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