Americanized Russian Knapweed Gardens: Tough Ground Cover on Rough Sites

Summary: Canada, Mexico and the United States view Americanized Russian knapweed gardens as allelopathic, tough, toxic aster family ground covers for rough sites.

closeup of Russian knapweed (Rhaponticum repens) flowers and foliage: Steve Dewey, Utah State University, Bugwood.org, CC BY 3.0, via Forestry Images

Americanized Russian knapweed gardens aid soil aeration, drainage and fertility in croplands, fencerows, meadows, pastures, rangelands, roadsides and wastelands even though critics accuse them of plant growth-inhibiting allelopathy and of livestock-fatal toxins.

Various federal, provincial and state governments in North America ban Russian knapweed and 54 other species in the Asteraceae family of asterlike herbs, shrubs and vines. Canada fleabane, Jerusalem artichokes and nonnative beggar-ticks, bluet, cornflowers, eclipta, emilia, fall hawkbit, hardheads, North African knapweed, velvet dock and wig knapweed cull no weed sanctions. Alaska, Arizona, California, Colorado, Hawaii, Idaho, Iowa, Kansas, Montana, Nevada, New Mexico, North Dakota, Oregon, South Carolina and Utah state governments deem Russian knapweed a weed.

Russian knapweed experiences sanctions from the Alberta, British Columbia, Manitoba, Ontario and Saskatchewan provincial, Canadian and Mexican federal and South Dakota, Washington and Wyoming state governments.

Gray-green, lance-shaped, 0.09- to 0.43-inch- (2.5- to 11-millimeter-) long, 0.08- to 0.29-inch- (2- to 7.5-millimeter-) wide embryonic leaves, called cotyledons, atop curved stems fill seedling stages.

Russian knapweed, commonly named Russian centaurea, Turkestan knapweed and Turkestan thistle, gets opposite-arranged, smooth-margined first leaf stages and 6.56- to 8.21-foot- (2- to 2.5-meter-) deep roots. Mature alternate-arranged, 1.97- to 3.94-inch- (5- to 10-centimeter-) long, 0.39- to 0.98-inch- (1- to 2.5-centimeter-) wide rosettes have none of the earlier stages' white, woolly hairs. Deep-lobed, deep-toothed, 1.58- to 5.91-inch- (4- to 15-centimeter-) long, 1.58-inch- (4-centimeter-) wide lower, and smooth-margined, 0.39- to 1.18-inch- (1- to 3-centimeter-) long upper, foliage is hairless.

Mature, multi-branched, 15.75- to 35.43-inch- (40- to 90-centimeter-) tall stems likewise no longer juggle the cobwebby hairs of their earlier stages in Americanized Russian knapweed gardens.

Silver green-budding inflorescences called heads, each 0.39 to 0.59 (10 to 15 millimeters) across, perennially keep Russian knapweed foliage company during August to October bloom times.

Fifteen to 36 perfect, purple to pink, regular, tube-shaped, 0.43- to 0.55-inch- (11- to 14-millimeter-) long disc florets line up into bowl-shaped, branch-tip, flower-clustering, long-stalked corymbs. Fine hair-tipped inner, and green, undivided, white-edge, 0.39-inch- (1-centimeter-) long outer, modified leaves called bracts merge into six to eight overlapping rows beneath each flower head. Nutrients from black, scaly rhizomes and lateral roots and photosynthates from shoots nurture Russian knapweed, named Rhaponticum repens (creeping rhubarb), into fruiting dry, nonexplosive, one-seeded achenes.

Americanized Russian knapweed gardens offer rooting 2.76- to 7.87-inch- (7- to 20-centimeter-) deep root fragments and 8.2- to 19.68-foot- (2.5- to 6-meter-) deep rhizomes and seeding.

Russian knapweed, described by Swedish botanist Carl Linnaeus (May 23, 1707-Jan. 10, 1778) and by French geneticist Oriane Hidalgo, produces over 1,200 seeds every growing season.

Temperatures between 32.9 and 95 degrees Fahrenheit (0.5 and 35 degrees Celsius) quicken germination of club-shaped, gray-brown to white seeds with two- to three-year in-soil viabilities. The ridged, 0.08- to 0.09-inch- (2- to 2.4-millimeter-) long, 0.024- to 0.028-inch- (0.6- to 0.7-millimeter-) wide seeds reveal 0.24- to 0.43-inch- (6- to 11-millimeter-) long bristles. Alternating temperatures spring seeds from dormancy while soil temperatures around 57.2 degrees Fahrenheit (14 degrees Celsius) at 1.97-inch (5-centimeter) depths start rhizome and root shoots underground.

Americanized Russian knapweed gardens trample competitors allelopathically and treat horses and sheep toxically but transform wastelands vegetatively and transmit vague artichoke, calendula, lettuce and sunflower looks.

Russian knapweed (Rhaponticum repens) in non-native U.S. landscape: Eric Coombs, Oregon Department of Agriculture, Bugwood.org, CC BY 3.0, via Forestry Images

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

closeup of Russian knapweed (Rhaponticum repens) flowers and foliage: Steve Dewey, Utah State University, Bugwood.org, CC BY 3.0, via Forestry Images @ http://www.forestryimages.org/browse/detail.cfm?imgnum=1459265

Russian knapweed (Rhaponticum repens) in non-native U.S. landscape: Eric Coombs, Oregon Department of Agriculture, Bugwood.org, CC BY 3.0, via Forestry Images @ http://www.forestryimages.org/browse/detail.cfm?imgnum=5454502

For further information:

"Centaurea repens L." Tropicos® > Name Search.
Available @ http://www.tropicos.org/Name/2717301

Dickinson, Richard; and Royer, France. 2014. Weeds of North America. Chicago IL; London, England: The University of Chicago Press.

Hidalgo, Oriane; García-Jacas, Núria; Garnatje, Teresa; and Susanna, Alfonso. 2006. "Phylogeny of Rhaponticum (Asteraceae, Cardueae - Centaureinae) and Related Genera Inferred from Nuclear and Chloroplast DNA Sequence Data: Taxonomic and Biogeographic Implications." Annals of Botany 97(5): 705-714.
Available @ https://academic.oup.com/aob/article-lookup/doi/10.1093/aob/mcl029

Linnaeus, Carl. 1753. "28. Centaurea repens." Species Plantarum, vol. II: 1293. Editio Secunda [Second Edition]. Holmiae [Stockholm, Sweden]: Laurentii Salvii [Laurentius Salvius].
Available via Botanicus Digital Library @ http://www.botanicus.org/page/1165706

"Rhaponticum repens (L.) Hidalgo." Tropicos® > Name Search.
Available @ http://www.tropicos.org/Name/100376167

Weakley, Alan S.; Ludwig, J. Christopher; and Townsend, John F. 2012. Flora of Virginia. Edited by Bland Crowder. Fort Worth TX: BRIT Press, Botanical Research Institute of Texas.