Volutaria: a New Very Invasive Knapweed in Borrego Springs and Newport Bay

Frank Harris, Kate Harper, Tom Chester and Ron Vanderhoff

Fig. 1. Vern Konan beginning removal of yet another new field of Volutaria discovered in Borrego Springs on 29 April 2015. Photograph by Frank Harris.

A species of Volutaria, a knapweed from Africa, was first observed in the Borrego Springs area in 2010 as a single plant, although the population was undoubtedly larger at the time. A larger population was found in 2011, and it was identified as a Volutaria species. This was the only known location of this species in the United States until 2015, when another population was discovered in Upper Newport Bay, Orange County.

Its population in Borrego Springs in 2013-2014 had grown to at least 800 plants. In 2015 its population numbered at least 10,000 plants, despite heroic efforts being made by a number of volunteers to remove every single plant beginning in 2013-2014. The seed bank at the end of spring 2015 may contain as many as one million seeds waiting to sprout in future years. New populations continued to be discovered in Borrego Springs throughout 2015 (Fig. 1).

In March 2015 a separate population of over 3,600 plants occupying 0.3 acres was discovered and removed in Upper Newport Bay, Orange County (Fig. 2). This population had obviously been present for at least one year, and maybe more. Unfortunately, seed had already dispersed, so there may be a seed bank that approaches one million seeds here as well.

A naturalized population of what look like the identical plants has also been discovered in the Atacama Desert Region in Chile. It is not known whether these occurrences are related, but they might be via the well-known California - Chile connection of plant species spread by migratory birds.

Fig. 2. Photographs taken by Ron Vanderhoff of the Orange County population.

This species is clearly potentially highly-invasive similar to Brassica tournefortii, Saharan mustard. Each plant produces roughly 2500 seeds, allowing it to expand its population at a very high rate. It is extremely important to try to eradicate this species from the Borrego Springs and Newport Bay areas before it spreads to a larger area.

In April 2015, the California Department of Food and Agriculture listed the species on the state Noxious Weed List and gave it an A rating. A-rated weeds are subject to state or county enforced action involving eradication, quarantine, containment, rejection or other holding actions.

Clearly, more resources are desperately quickly needed to have any chance of extirpating this invader. Very soon it will be nearly impossible to eradicate it.

It is also very important for people in the field to check for Volutaria in other areas. If you think you have found plants of Volutaria, contact any of the authors through Tom Chester at tom@tchester.org.

For much more information, including many pictures of plants at various stages, from seedlings to fruit, see our extensive webpage on this species at tchester.org/bd/species/asteraceae/volutaria_canariensis.html.

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Copyright © 2014-2015 by Frank Harris, Kate Harper, Tom Chester and Ron Vanderhoff.
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Comments and feedback: Tom Chester
Updated 12 October 2015