1. Rick and Diddi Price

    Rick and Diddi Price Member

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    Location:
    Squamish, B.C.
    We have a large garden in Squamish, and are inundated with a weed that we think might be Angelica - low growing (up to 12") and invasive. It has a broad, entirely green leaf with three shallow lobes and serrated edges. It closely resembles another plant that has variagated green/yellow leaves, but the variagated one is not invasive and is easy to manage.

    We wish to isolate some of our areas from this invasion with a barrier of some sort. Our questions are:
    1. How deep a barrier is needed in our silty soil to prevent the spread of this weed?
    2. Does anyone have any suggestions as to a kind of barrier material that will be easy to install (we'll need a lot of it), effective, and not too expensive?
    3. In some of our beds, we plan to kill it by covering the bed with plastic for a long period (after removing the good plants). How long will we need to keep it covered to be sure the roots are dead?
     
  2. Douglas Justice

    Douglas Justice Well-Known Member UBC Botanical Garden Forums Administrator Forums Moderator VCBF Cherry Scout Maple Society 10 Years

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    Location:
    Vancouver, Canada
    Your problem sounds like Aegopodium podagraria (gout weed), a particularly nasty weed that loves sun and rich, moist soil. See this image to confirm identification.

    Long-term smothering works well with such light-hungry plants. Instead of plastic, try layers of newspapers for a more biodegradable solution. You need to lay the newspaper down at least 5 to 10 pages thick and overlap them adequately, but it's low cost and can be a fun project for kids. Before you cover the area, make sure to cut down the plants before they go to seed (you can leave the trash in situ as long as there are no seeds).

    Spread a layer of mulch over top of the papers and it will stay put (in contrast to on top of plastic, which is slippery). If you have a source of weed-free soil, you can actually spread enough over top of the paper mulch to grow a crop. Try annual flowers adapted to dry conditions (you don't want to introduce another noxious weed). Prevent any remaining gout weed plants from flowering (or at least, producing seed) and be religious about weeding out any seedlings. Make sure you cover the area for at least a full growing season to kill off the rhizomes.

    One of the problems with barriers is that often, they aren't. And all barriers eventually fail. However, once your soil is clean, you can create a barrier to slow the rate of ingress by encroaching weeds. Dig down below the level of the rhizome mat (this is easy to find and recognize). Create a narrow trench between the clean and infested soil at least 20cm deeper than the rhizomes and lay your plastic sheeting along the trench so that it comes up at least 30 cm above both sides of the trench. Any overlaps should be at least 1m to prevent the rhizomes from creeping through. Fill the plastic sided trench with stones or similar inert material (you want a relatively sterile barrier). If you see new plants emerging from the clean side of the barrier, you probably haven't dug the trench deep enough, or the rhizomes are creeping through holes or poor overlaps.

    Good luck!
     

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