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Understanding invasive plants


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A presentation for gardeners created in Oregon to understand invasive plant species, their biology, and their control

A presentation for gardeners created in Oregon to understand invasive plant species, their biology, and their control

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  • 1. Linda R. McMahan
    OSU Extension Service
    Presented at Gardeners Mini-College, Corvallis OR 2010
    Note: Some material on control methods developed by and borrowed from retired OSU Extension Professor Susan Aldrich-Markham
    Understanding Invasive Plants
  • 2.
  • 3. Question 1
    Of the following plants, which is considered to be an invasive species in Oregon?
    1. Salal
    2. Purple loosestrife
    3. Red maple
    4. Ajuga
  • 4. Question 2
    Of the following plants, which are considered to be an invasive species in the United Kingdom?
    1. Rhododendron ponticum
    2. skunk cabbage
    3. Cotoneaster
    4. All of the above
    Interesting Fact: An activity known as “Rhodie-bashing” has become common in England.
  • 5. Question 3
    You can best tell if a garden plant is possibly invasive by ?
    1. How fast it grows
    2. Whether it self-seeds
    3. How aggressive it is in your garden
    4. Observing it growing in nearby natural areas
  • 6. What is an invasive plant species? It is one that:
    Spread aggressively, replacing native vegetation
    Seems particularly adept at surviving and reproducing in wild areas
    Is extremely difficult to control and eradicate
    Leads to economic or environmental harm
    All of the above
    Question 4
  • 7. Question 5
    True or False:
    Invasive species are considered to be the number 1 threat to biological diversity in natural areas in the U.S.
  • 8. Question 6
    Invasive plants are spreading over approximately how many acres per year of US wildlife habitat
    500,000 acres
    1.2 million acres
    1.7 million acres
    3.5 million acres
  • 9. Question 7
    Estimates of economic damage annually from invasive plants in the US is measured in:
    Hundreds of thousands of dollars
    Millions of dollars
    Billions of dollars
  • 10. Rubus armeniaca, Himalayan or Armenian blackberry
    Question 8
    Can you name this invasive plant occurring throughout Oregon and the West?
  • 11. Question 9
    Can you name this invasive plant in Willamette Valley oak woodlands?
    Daphne laureola, spurge laurel
  • 12. Question 10
    What percentage of invasive species in the US have been introduced through horticulture and gardening?
  • 13. I am not talking about dandelions!
  • 14. Garlic mustard
    I am talking about plants whose populations explode in wild areas
  • 15. Like knotweed, here seen on Oregon’s Nehalem River, which was rapidly invaded throughout much of the drainage in the late 1990s and early 2000s.
  • 16. As gardeners, we have all been part of the problem because many invasive plants are garden escapees, but we can also be part of the solution. Some examples follow.
  • 17. KatherinaDehnen-Schmutz, Julia Touza, Charles Perrings, and Mark Williamson. 2006. The Horticultural Trade and Ornamental Plant Invasions in Britain, Conservation Biology 21(1):224-231
    Ornamental horticulture has been recognized as the main pathway for plant invasions worldwide. We examined the link between . . . pressure created by the presence of ornamental plants in the market and their ability to escape from cultivation and establish in the wild. . . Species that had escaped from cultivation were more frequently on sale both in the nineteenth century and today than nonescaping species. . . (Abstract Excerpt)
  • 18. Butterfly bush
  • 19. English Ivy
  • 20. Yellow Flag Iris
  • 21. Purple Loosestrife
  • 22. Herb Robert
  • 23. Scot’s Broom
  • 24.
  • 25. Characteristics of Invasive Plants
    Lack natural enemies
    Fast growing
    Fast reproduction and effective dispersal
    Long-lived seeds
    Few habitat restrictions
    Able to form single-species stands
  • 26. 4,000 or more plant species are introduced into the US, 1 in 10 have become invasive, and half of these are of horticultural origin
    The Magnitude of the Problem in the United States
  • 27. How Did They Get Here?
    Intentional planting for ornamental value, forage, or crops
    Seed mixes that often contain plants with invasive potential or unintended weed seeds
    Weed seeds as contaminants in bird seed and seed for crops and gardens
    As hitchhikers on tires, wheels, boots & shoes, other plants, & ships
  • 28. Photo – knotweed infestation in Lincoln Co.
    Oregon Department of Agriculture
    Knotweed infestation in Lincoln County, OR
  • 29. Photo – knotweed infestation in Clackamas Co.
    The Nature Conservancy
    Japanese Knotweed(Polygonum cuspidatum)
    Knotweed infestation in Clackamas County, OR
  • 30. Some Sample Control Methods to Show How Difficult It Can Be
    ControllingHimalayan Blackberry as Example 1
  • 31. Mechanical Control of Blackberry
    Cut or mow the canes back to ground level repeatedly
    Dig up roots and crowns
    Graze with goats (only if desirable plants are not present)
    Establish other plants on the site for competition
  • 32. Chemical Control of Blackberry
    Foliar Application
    Triclopyr (2-3% solution) apply anytime during active growth period – spring to fall
    Or triclopyr ester (4% solution) is effective in the winter, when many native plants are dormant
    Or glyphosate (2-3%) fall application only
  • 33. Chemical Control of Blackberry
    Cut Stem Treatment
    Apply undiluted triclopyr to freshly-cut stems (be sure to get both ends if rooted)
    Not as effective as a foliar application, but does not injure plants growing nearby
    Good as a follow-up treatment after most of the blackberry infestation is removed and other plants are established
  • 34. Blackberry IWM for Large Areas
    Option 1 – Cut or mow early to mid-season, allow to regrow grow to about 18 inches tall, then apply herbicide
    Option 2 – Broadcast herbicide late summer or fall before cutting or burning
    Plant native vegetation to occupy the site
    Control competing vegetation around the desirable plants with regular maintenance
  • 35. Controlling Knotweed
    These methods make controlling blackberry look positive easy!
  • 36. Photo – knotweed infesting the upper Skykomish River, WA
    King County Noxious Weed Program
    Knotweed infesting the Upper Skykomish River, WA
  • 37. Photo – knotweed leaves, 3 species comparison
  • 38. Winter View
    Seattle Public Utilities
    Dense growth pattern
  • 39. Mechanical Control of Knotweed
    Cut knotweed stems at least twice a month between April and August, then once a month until the first frost
    Keep this up for 2 to 3 years
    Dispose of all cut stems and root fragments because they can resprout
    Or cover patchloosely with geo-textile, break off stems growing underneath
  • 40. Woven
    Seattle Public Utilities
    Geotextile Fabric
  • 41. Chemical Control of Knotweed
    Stem injection Method (for small patches)
    Rodeo and Aquamaster have labels
    Inject up to 5 ml into the hollow stem about 6 inches above the ground, just below a node (3 ml is adequate)
    Or cut off stem and fill hollow with 3-5 ml
    Maximum rate is 8 quarts/acre
  • 42. Chemical Control of Knotweed
    Foliar Application Method(The Nature Conservancy)
    Triclopyr or glyphosate (3 to 5% solution), plus a non-ionic surfactant
    Spraying plants at 3 to 6 ft tall is practical, but flowering stage is most effective
    Best to cut or spray in the spring, then spray regrowth again in the fall
  • 43. Where to Find Out More Information
  • 44. Fact Sheet on Ivy Removal in a Home Landscape
    My story. I moved into a new house in May 2008
    that had (perhaps literally) a ton of ivy. It covered
    the fences to the extent that I couldn’t even open
    the gate without removing some of it. Removing
    enough ivy to open the fence was my first project.
    Next, I mowed it back to keep it from spreading
    further into the yard, until the mower decided it
    didn’t like that anymore. Then, since we needed some ivy removal pictures at the office, two of us tackled the English ivy on the Douglas fir tree in the back, using the tree lifesaver method similar to that used by the parks department in the City of Portland.
    English Ivy (Hedera helix) is a European forest
    that has become invasive in the US, where it is
    officially considered to be invasive by most states.
    Because it is such an effective ground cover, it has
    Ivy on tree trunk before removal
  • 45. A collaboration between the City of Portland, Oregon Public Broad-casting, Oregon Assn of Nurseries, Clackamas Community College, OSU Extension Service, and Oregon Sea Grant —Revised Version Coming to you soon!
  • 46. Revised version summer 2010 will include more alternatives applicable to Oregon’s Eastside
  • 47. Invasive Species: Some Facts for the Gardener
    Robert Emanuel, Joy Jones and Linda McMahan
    Invasive nonnative plants are a serious subject for gardeners, farmers, the general public, and land managers. Understanding what invasive species are and how to control them are increasingly important issues for gardeners.
    What is an Invasive Species?
    Invasive species are non‐native organisms whose introduction causes, or is likely to cause economic or environmental harm or harm to human health. Human actions are the primary means by which invasive species are introduced and spread. An invasive plant can also be referred to as a noxious weed because it is listed by state or federal law as particularly
  • 48.
    What does the future have in store? This site is a good one to know what to look out for as probably invasive plants—put together by people who work with natural areas and are on the lookout for unusual plants in nature
  • 49. NPSO Lists species that are considered to be invasive in wild areas but not yet recognized legally as invasive by the ODA – Sample from List
    Acer platanoides– Norway maple
    Aesculushippocastanum– horse chestnut
    Cotoneaster spp.-- cotoneaster
    Ligustrumvulgare– common privit
    Lunnariaannua– money plant
    Lychniscoronaria– rose campion
    Melissa officianalis– lemon balm
    Prunuslaurocerasus – cherry laurel
    Prunuslusitanica– Portugese laurel
    Pyracantha spp.– firethorn
    Rosa multiflora– multiflora rose
    Saponariaofficianalis– soapwort
    Viburnumopulus– snowball tree
  • 50. NPSO Oregon Watch List
    Acer pseudoplatanus– sycamore maple
    Acanthus mollis– bear’s breeches
    Ajugareptans– ajuga
    Alchemillamollis– lady’s mantle
    Arum italicum– Italian lords and ladies
    Betulapendula– European birch
    Centranthusruber– red valerian
    Galiumodoratum– sweet woodruff
    Kniphophiauvaria– red hot poker
    Myosotisscorpioides– forget-me-not
    Verbena bonariensis– tall verbena
    Viola odorata– sweet violet
  • 51. How to Do Your Own Searches
    Complete a web search putting the scientific name in quotation marks and adding “+ invasive”.
    For example, a search for the Ponderosa pine would look like [“Pinus ponderosa” + invasive]. This approach will provide various “hits”; many times these are government and nonprofit organizations that provide this kind of information.
  • 52. How to Do Your Own Searches
    Or you can go directly to one source from USDA at
    Or you can sometimes locate information by typing [invasive species Oregon] or a similar designation into your search engine.
    Other good sources are listed on the following few slides.
  • 53.
  • 54.
  • 55. The Gardener’s Challenge
    Stop the Spread – don’t share, if in doubt, don’t plant
    Remove – in your own garden and through efforts in your community
    Tell others – and recommend alternatives
    Keep informed – Read GardenSmartOregon and tell others about it.
    So, you know it is invasive, now what?
  • 56. The Gardener’s Challenge
    Support nurseries that offer alternatives to invasive plants
    Check seed packet contents—such as for “wildflower” mixes” before purchase
    Be especially careful with water plants—many are invasive and others are well-known “hitch-hikers”
    Watch the ground under your bird feeder
    So, you know it is invasive, now what?
  • 57. Thank you for being GardenSmart!