The Royal Horticultural Society's A-Z Encyclopedia of Garden Plants (ed. Christopher Brickell; DK Publishing, 1996) indicates that Ranunculus ficaria spreads a foot or more, but the following information, from a local gardener who grows several cultivars of this plant, suggests that "more" may be quite a bit more. Excerpt: Due to their being potentially invasive, we placed them where it would not matter, but might even be rather nice, if they spread a great deal.

The species is considered invasive, as these sites indicate:

From the Plant Conservation Alliance's "Least Wanted" list

From the USDA site.

It is possible the cultivar is less invasive than the species, but I would certainly keep an eye on it.

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Date 2007-04-13
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First of all, it is important to know that clinging plants, such as Boston ivy and English ivy have the potential to "damage old, soft mortar and strip off pebbledash". (Gardening with Climbers by Christopher Grey-Wilson and Victoria Matthews) It is also suggested that these vines have a "structurally sound surface and must be prevented from reaching under house eaves and roof tiles and into window casements." (The New Royal Horticultural Society Dictionary Manual of Climbers and Wall Plants edited by JK Burras and Mark Griffiths)

I looked at the list of invasive plant species on the Native Plant Society of New Jersey's website. They recommend Boston ivy over English ivy, as English ivy has been identified as an invasive plant in the state of New Jersey. Invasive or noxious plants have "been determined by the State to be detrimental to agriculture, your health, or the environment...It is [an] introduced plant species that is aggressive."

In addition to taking this information into consideration, it would also be important to identify the amount of sunlight and the extent to which the side of the house will be exposed to harsh winter winds and temperatures. Neither Boston nor English ivy is recommended for full sunlight. Boston ivy will give you more fall color and interest and will withstand cold winters. (Simon & Schuster's Guide to Climbing Plants by Enrico Banfi and Francesca Consolino)

If you wanted to consider an alternative vining plant, the Native Plant Society of New Jersey recommends the native vine Coral Honeysuckle (Lonicera sempervirens), it has red-orange Trumpet-shaped blossoms that bloom June-August and grows 8-10'; it is a hummingbird favorite throughout its long bloom period.

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Date 2007-07-02
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Here is a list of helpful resources:

Washington State Noxious Weed List from the USDA

State noxious weed list and schedule of monetary penalties from the WSL

Class A, Class B, and Class C

Washington Department of Ecology (aquatic plants)

Washington Invasive Species Coalition

King County Noxious Weed Lists

National Invasive Species Lists

Plant Conservation Alliance Alien Plant Invaders list

Invasives in British Columbia

The lists which are national in scope are useful too, as some plants not yet officially listed as invasive here may still be plants to watch out for.

There are a great many books on this subject. A recent one, co-authored by a faculty member here, is Invasive species in the Pacific Northwest edited by P.D. Boersma, S.H. Reichard, and A.N. Van Buren; Rebecca L. Gamboa, photo editor. University of Washington Press, c2006.

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Date 2007-10-10
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Acanthus mollis, or Bear's breeches, is not listed as noxious in King County, Washington State, or on the federal list of noxious plants. This is not the same as saying it isn't potentially aggressive, although I've never heard about it being a serious problem here. It is considered invasive in parts of Australia, though.

Plants for a Future Database (a British source) offers the following information on this plant and its growing habits:
"Plants can become invasive, spreading by suckers, and they are difficult to eradicate due to their deep roots."

According to the Pacific Northwest site, Rainyside Gardeners, it is sometimes difficult to get this plant to bloom. A Washington State University Extension site says that Acanthus mollis is potentially invasive in climates warmer than ours.
"This species is classified as a groundcover in that any pieces of root cut from the original plant can easily contribute to further plant spread."

If you want to grow it but are concerned about it spreading, you could try containing the roots with an 8-inch root barrier (similar to what is used to keep running bamboo in check). On the other hand, if you have this plant and decide that you wish to be rid of it, the book Wildly Successful Plants: Northern California Gardens by Pam Peirce and David Goldberg (Sasquatch Books, 2004) says that removing every bit of root over two or three seasons of growth should get rid of the plant. If you cannot eradicate it by continually digging up each new shoot, you may have some luck using a flame weeder (with due caution and appropriate protection). Northwest Coalition for Alternatives to Pesticides describes how to use this tool.

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Date 2008-09-25
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Some sources (such as the State of Oregon's noxious weed control board) have said that "sterile cultivars" of Cytisus scoparius are exempt from regulations governing noxious weeds. However, the Center for Urban Horticulture's Professor Sarah Reichard, an expert on invasive species, says the following:

"The 'sterile cultivar' issue is huge worldwide. The reality is that sterile cultivars depend on the type of sterility: there are many reasons a plant might be sterile. Only a few of them can be considered to be stable under varying environmental conditions.

For future reference, here are links to Washington State Plant Quarantine and Noxious Weed lists.

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Date 2009-05-16
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