ISSN 1188-603X

No. 373 February 26, 2007 Victoria, B.C.
Dr. A. Ceska, P.O.Box 8546, Victoria, B.C. Canada V8W 3S2


From: Dave Polster []

Carpet burweed, (Soliva sessilis Ruiz & Pavón), was first found in Canada at Ruckle Provincial Park on Saltspring Island, BC in 1996 (Polster, in press).

It is a winter annual that originally came from South America. It was first noted in California in 1836 and probably arrived with shipments of hides (Ray 1987). It is well adapted to the mild moist winters and dry summers of coastal British Columbia as it flowers and sets seed in the late spring and early summer. It passes the dry summer months as a seed, germinating any time from mid-October to mid-May.

A variety of efforts have been made using a range of techniques since it was first found to try and eradicate it. In 2005 burweed was found at several other provincial parks as well as at a major City of Victoria park. Early in 2006 the Invasive Plant Council of BC (IPC) decided that carpet burweed would be a good candidate to address at what was believed to be an early stage of establishment. Four levels of government provided funding for a program designed to eventually eradicate this plant in Canada. In addition, two forums were held by the IPC (Invasive Plant Council) with all interested parties to garner support for this work. Dr. Adolf and Oluna Ceska were hired to search for the plant in coastal bluff ecosystems and other likely locations on Southern Vancouver Island and the associated Gulf Islands. The Ceskas found that private campgrounds and recreational vehicle parks were significantly infested by burweed and were likely the centres of distribution of this plant rather than Ruckle Provincial Park on Saltspring Island as had been previously thought. An additional search crew (2 people) was hired to visit all of the private campgrounds and recreational vehicle parks on Southern Vancouver Island. Over 175 sites have been searched by the two search teams. Burweed was found at a number of sites in Canada in 2006 (Table 1).

In addition to treatment crews from various municipal and provincial parks agencies, a crew (2 people) was hired to treat the private campground sites where burweed was found. Treatments consisted of either hand pulling where population levels were low or burning with propane fired torches ("Tiger" torches). A total of 53 person days have been spent killing burweed by this crew. Additional efforts to control burweed were undertaken by municipal and provincial workers. A Carpet Burweed Response Team was formed to provide direction in dealing with this plant. Plans have been developed by the team for a forum on burweed to be held on February 9th 2007. Four options for dealing with this plant have been developed: 1) do nothing; 2) provide education and awareness; 3) search for and treat burweed in all high priority sites; and 4) search for and treat burweed in all locations. Details of future programs will be developed by the group of interested parties at this forum. It is expected that continued work, at least at the high priority sites will be undertaken in 2007. Work on eradication has already been initiated at some sites for fall and winter germinating plants (Fred Hook, City of Victoria pers. com.). Although ideally this invasive species would be eradicated from Canada, success is not assured. However failure to act now to control this plant will ensure that it becomes well established in suitable locations in Canada.

Literature Cited

Polster, D. [in press]:
Eradicating (?) a new weed for Canada; Soliva sessilis Ruiz & Pavón. Submitted to Topics in Canadian Weed Science Watch for it at:
Ray, M.F. 1987.
Soliva (Asteraceae: Anthemideae) in California. Madrono 34: 228-239.

Table 1. Sites where Burweed Was Present in 2006




Area of infestation (m2) (approx.)



Beachcomber RV

N48 33 29.3

W123 21 47.9

Private campground


Beacon Hill Park (dog run)

N48 24 31.3

W123 22 01.6

City of Victoria park


Beacon Hill Park (Playground)

N48 24 51.2

W123 21 45.4

City of Victoria park


Bella Pacifica Campground Tofino

N49 07 41.0

W125 53 57.7

Private campground


Big Tent RV and Campground

N49 19 07.4

W124 17 09.0

Private campground


Cattle Point (Uplands Park)

N48 26 17.4

W123 17 35.9

Muni. Of Oak Bay Park


Cowichan River Provincial Park

N48 46 21.7

W123 53 53.6

Provincial Park


D’Arcy Island (GI National Park)

N48 34 06.6

W123 16 21.6

National Park


French Beach Provincial Park

N48 23 34.9

W123 56 42.4

Provincial Park


Island View RV Campground

N48 34 37.2

W123 22 06.1

Private campground


Jordan River Campground

N48 25 14.8

W124 03 17.1

Forest Company site


KOA Victoria West Campground

N48 32 49.4

W123 33 58.1

Private campground


Living Forests RV Site

N49 08 02.6

W123 54 53.7

Private campground


Park Canada site Tsawwassen

N49 02 07.8

W123 05 29.8

Private campground


Rathtrevor Group Site 1

N49 19 00.7

W124 16 13.9

Provincial Park


Rathtrevor Group Site 2

N49 19 01.4

W124 16 13.1

Provincial Park


Rathtrevor Group Site 3

N49 19 01.1

W124 16 10.4

Provincial Park


Rathtrevor Walk-in Site 1

N49 19 21.1

W124 16 00.0

Provincial Park


Rathtrevor Walk-in Site 2

N49 19 21.2

W124 16 01.3

Provincial Park


Rathtrevor Walk-in Site 3

N49 19 25.9

W124 15 58.1

Provincial Park


Rathtrevor Parking Lot 5

N49 18 56.8

W124 15 53.7

Provincial Park


Ruckle Provincial Park

N48 46 15.7

W123 22 06.1

Provincial Park


Ruckle Provincial Park (exclosure)

N48 46 17.3

W123 22 03.1

Provincial Park


Smelt Bay Provincial Park (Cortez)

N50 01 53.9

W124 59 48.0

Provincial Park


Sooke River RV Site

N48 23 35.3

W123 42 34.6

Private campground


Sunny Shores Resort & Marina

N48 46 21.7

W123 53 53.6

Private campground


Thetis Lake Regional Park

N48 27 49.3

W123 28 07.7

Regional Park (CRD)


Thetis Lake RV & Campground

N48 27 44.9

W123 28 07.4

Private campground


Westwood Lake RV & Campground

N49 09 49.4

W123 59 41.5

Private campground


Ucluelet Campground

N48 56 49.5

W125 33 29.1

Private campground




81,170 m2

1 Site treated by stakeholder or finder

2 Site treated by ICP Crew

3 Work on burweed reductions at Ruckle Provincial Park has reduced numbers but not the extent of the infestation so although the infested area is large, the plant density is very low.



From: Adolf Ceska & Oluna Ceska []

Carpet burweed (Soliva sessilis Ruiz & Pav.) is a South American plant that has spread in Australia, New Zealand and In North America. In North America, carpet burweed occurs from California to British Columbia, and in other parts of the U.S. in southeastern states from Virginia to Texas. In Douglas et al. (1998) it is listed from Arizona, but there are no specimens listed in the databases of the main Arizona herbaria.

In British Columbia, Carpet burweed was first collected in Ruckle Provincial Park, Saltspring Island, on May 18, 1996 by Frank Lomer, who later reported this find in BEN (Botanical Electronic News) # 163 (Lomer 1997).

Carpet burweed stands out from other invasive plants by having a sharp spine on its achenes that penetrates skin on bare feet or on paws of dogs. The presence of carpet burweed in parks, playgrounds and golf courses significantly diminishes the recreational value of those facilities.

British Columbia Parks, who administer Ruckle Park, responded to carpet burweed soon after the infestation was detected. Carpet burweed was weeded out by hand in sensitive areas with native plants, and the areas with massive infestation were covered with black plastic tarps. Mulching did not prove effective, merely giving the managers a false impression that carpet burweed problem in Ruckle Park was under control. In addition, the BC Parks budget was reduced and did not allow funds to be spent on carpet burweed control. Within a few years, the seed bank in the mulched areas germinated, and the burweed problem in Ruckle Park erupted again. The history of the carpet burweed infestation in Ruckle Park as well as the history of the various treatments until the year 2004 has been summarized by Stevens et al. (2004 or 2005).

In March 2005, Oluna Ceska found carpet burweed at the edge of the Thetis Lake Park parking lot, and almost simultaneously carpet burweed was found in Rathtrevor Beach Provincial Park near Parksville, in Skutz Falls Provincial Park, in Beacon Hill (Victoria City Park), at French Beach, and on D'Arcy Island. Consequent discovery of carpet burweed on Cattle Point, Oak Bay, Victoria by Adolf Ceska showed that carpet burweed is more widespread than previously thought.

We were hired by the Invasive Plant Council of British Columbia to do field work to assess the actual distribution of carpet burweed on southern Vancouver Island. It was suggested that we should concentrate on parks around Victoria and on the eastern part of Vancouver Island.

We were looking for the source of the carpet burweed infestation in Thetis Lake Park, and we realized that there was a far more massive infestation in the private Thetis Lake RV Park adjacent to the public park. We then visited Island View Beach Regional Park. We did not find any carpet burweed in this park, but the adjacent private RV Park was infested with carpet burweed to such a degree that large parts of the original lawn have been overgrown and replaced with a solid "carpet" of carpet burweed.

When we realized that RV parks were vulnerable to carpet burweed infestations, we switched our focus from parks and public spaces, and in the time available in our contract, we focused our search in RV parks. We found that about one third of RV parks we visited had carpet burweed in them. Our findings were confirmed by the crew that was hired to continue in the carpet burweed search, who found carpet burweed in many other sites. Results of our search efforts have been incorporated in Dave Polster's article in this issue of BEN.

Carpet burweed is an annual plant that behaves in our area as a typical winter-annual. It starts to germinate in fall after the first rains, and its vegetation season lasts until the dry late spring and summer. During this period, carpet burweed germinates, produces greenish flowers, produces "seeds" (those are in fact fruits, achenes or - to be more precise cypselae [sing.: cypsela] in Asteraceae). Casual observations in our area indicate that the plants do not exhibit photoperiodicity; plants can flower soon after their germination and they can produce flower heads throughout the whole vegetation period. This may set it apart from native winter-annuals that display strict photoperiodicity. The ability to flower throughout the whole vegetation season gives carpet burweed a competitive advantage over native winter annuals. In addition, carpet burweed has broad ecological amplitude (especially in term of available moisture) that overlaps with large number of native plants. Carpet burweed thus poses a serious threat to a large number of plant species.

Annual species, also called therophytes, form the most significant life-form group of all the life-forms that exist in the Garry-oak ecosystems (see Roemer 1972, page 50). Most of them are, like carpet burweed, the so- called winter-annuals, i.e., they germinate in fall and winter and finish their life- cycle by the end of spring. Quite a few bryophytes have a similar life-cycle, although they cannot be called therophytes- the term is restricted for the vascular plants only.

Therophytes (i.e., annual vascular plants) are not only the most significant group among native vascular plants, but they also represent the largest group of vascular plants on southern Vancouver Island listed as rare by the BC Conservation Data Centre. According to the British Columbia Ministry of Environment (2007), there are 21 RED-listed therophytes in Garry oak ecosystems, and about 13 BLUE-listed ones, for the total of 34 CDC-listed species. If one adds several annual bryophytes (e.g., Entosthodon fascicularis), there are well over 35 annual rare plant species that have similar life cycles and similar ecological requirements as those of the invasive carpet burweed. Land managers and even environmental consultants who are unaware of the importance of therophytes in the floristic composition of the site can come to a wrong conclusion that there are no plant species that would call for the protection of the areas infested with carpet burweed. Nevertheless, we believe that this is an important factor that should be considered when choosing the eradication methods on each particular site.

During our survey, we found carpet burweed growing with British Columbia rare plants at the following sites:

Carpet burweed threatens native rare plants in the following three ways:

  1. Direct competition;
  2. Destruction: rare plants may be destroyed when improper techniques are used in places where rare plants occur with carpet burweed (e.g., use of herbicides, torch burning, etc.);
  3. Open places dominated by carpet burweed may be developed or seeded, since the areas appear to be barren if seen outside the vegetation season.

During our search for carpet burweed we found other invasive plants that grew in many sites we have investigated. We found numerous populations of burrowing clover (Trifolium subterraneum), a species that was first reported in British Columbia by Ceska (1975), and many populations of moss pygmyweed (Crassula tillaea), first reported from British Columbia only quite recently by Zika (2002).

Yellow wall bedstraw, Galium murale (L.) All., an introduced European species previously known in North America only from California, was found as new to British Columbia and Canada growing in the lawn of the Thetis Lake RV Park.


Our fieldwork was supported by the Invasive Plant Council of British Columbia.

Literature Cited

British Columbia Ministry of Environment. 2007.
BC Species and Ecosystems Explorer. Web page:
Ceska, A. 1975.
Additions to the adventive flora of Vancouver Island, British Columbia. Canadian Field Naturalist 89:451-453.
Douglas, G.W., G.B. Straley, D.V. Meidinger, & J. Pojar. 1998.
Illustrated Flora of British Columbia. Vol. 1: Gymnosperms and Dicotyledons (Aceraceae through Asteraceae). B.C. Ministry of Environment, Lands and Parks and B.C. Ministry of Forests. Victoria. 436 p.
Lomer, S.F. 1997.
Six new introduced species in British Columbia. BEN Electronic News) # 163 (April 19, 1997) May 18, 1996
Roemer, H.L. 1972.
Forest vegetation and environments on the Saanich Peninsula, Vancouver Island. Ph.D. Thesis, University of Victoria, Victoria, BC. xvi+405 p.
Stevens, V., E. Lofroth, B. Costanzo, & S. Walsh. [2004 or 2005].
Invasive burweed in Ruckle Provincial Park, British Columbia. [Poster] Available from:
Zika, P.F. 2002.
Crassula tillaea, a small addition to the flora of British Columbia. BEN # 289, May 26, 2002

Send submissions to
BEN is archived at