- Author: Devii R. Rao
This past spring a San Benito County rancher came to my office with a weed from her property. She said it had been on the property for decades; she thought she knew what it was, but wasn't 100% sure.
Although I did not know what it was, its structure reminded me of a plant I had seen on weed pamphlets and in plastic weed bouquets back in 2004 when I was an intern with the Marin/Sonoma Weed Management Area (WMA). At that time, a group of people concerned about invasive weeds gave those pamphlets and bouquets to our state legislators, during an Invasive Weed Day at the Capitol.
Since I didn't know the identity of the rancher's weed, I sent photos to a couple botanists. They both agreed that the plant was Russian knapweed (Acroptilon repens) (Figures 1 and 2). Knapweed! That was the plant I had seen back in 2004. I had thought of knapweed as a plant that was more of a problem in other, dryer western states and wondered why it was included in our bouquets back then. It turns out Russian knapweed does grow in California; I just had to go east, away from the higher rainfall coast to find it (Figure 3). Russian knapweed has even been documented in the eastern parts of Marin and Sonoma counties (CalFlora 2018).
In the area where I now work, Russian knapweed can be found in eastern Santa Cruz and Monterey counties, and western San Benito County. It is a deep-rooted perennial plant, originally from central Asia (DiTomaso, Kyser et al. 2013). It can grow in many different settings, including rangelands, farmed areas, and roadsides. It grows well in areas with some moisture, like drainages, but will not grow in areas that are too wet. Russian knapweed will also grow well in sunny, dry areas, making it drought tolerant.
The roots of Russian knapweed can bring zinc from deep in the soil up to the soil surface, which changes the soil properties, preventing other plants from being able to grow. It is poisonous to horses, but doesn't usually cause problems with other livestock since its bitter taste make it unpalatable.
The plant is effective as a weed because of its biology. It has two reproductive strategies: vegetative and seeds. Small pieces of underground, horizontally spreading roots (rhizomes) can form new plants. A minimal number of seeds, which can remain in the seedbank for 2 to 3 years, are also produced. But, it mainly spreads through its root system.
Aminopyralid (Milestone) is effective in controlling Russian knapweed and should be applied from when the flowers are budding to when the flowers are senescing (dying). Aminopyralid can also be effective when applied in the fall. Seeding with other species to compete with Russian knapweed after herbicide application can prevent reinvasion since Russian knapweed needs light, and seeding with other plants can help shade it out. Non-chemical control options tend not to be effective at controlling Russian knapweed. However,, its growth may be somewhat limited by biological control agents (Russian knapweed gall nematode and Russian knapweed mite).
For more information about Russian knapweed, click here to see the Weed Report from the book Weed Control in natural Areas in the Western United States.
Calflora: Information on California plants for education, research and conservation, with data contributed by public and private institutions and individuals, including the Consortium of California Herbaria. [web application]. 2018. Berkeley, California: The Calflora Database [a non-profit organization]. Available: http://www.calflora.org/ (Accessed: Oct 16, 2018).
DiTomaso, J.M., G.B. Kyser et al. 2013. Weed Control in Natural Areas in the Western United States. Weed Research and Information Center, University of California. 544 pp. Available: https://wric.ucdavis.edu/information/natural%20areas/wr_A/Acroptilon.pdf (Accessed: October 16, 2018).