Originating in Asia and Europe, Rhaponticum repens has become established in nearly all areas of the western states. Plants are very similar to those in the centaurea genus, but lack sterile outer florets.
I. Appearance and Characteristics
Rhaponticum repens is commonly called russian knapweed and is native to Eurasia. It has been introduced to North American and is a deep-rooted, long-lived plant. Some of these plants have been in existence for over 75 years.
The species was first described by Carl Linnaeus in 1763 as Centaurea repens. Augustin Pyramus de Candolle separated it from the genus Centaurea in 1838, placing it in the genus Acroptilon. The genus name derives from acro- (high, here meaning tip) and ptilo- (feather). A 1995 molecular phylogenetic study, the structure of the flower, and the chromosome number support separating it from the genus Centaurea. Some sources then continue to place it as the sole member of the monotypic genus Acroptilon. A phylogenetic study published in 2006 concluded that Acroptilon belongs in the genus Rhaponticum.
Rhaponticum repens, synonyms including Acroptilon repens and Leuzea repens, with the common name Russian knapweed, is a species of bushy rhizomatous perennial, up to 80 cm tall. Stems and leaves are finely arachnoid-tomentose becoming glabrous and green with age. The rosette leaves are oblongolate, pinnately lobed to entire, 2–3 cm wide by 3–8 cm long. The lower cauline leaves are smaller, pinnately lobed; the upper leaves become much reduced, sessile, serrate to entire. The heads are numerous terminating the branches.
Flowers are pink to purplish, the marginal ones not enlarged. The outer and middle involucral bracts are broad, striate, smooth with broadly rounded tips; the inner bracts are narrower with hairy tips. Pappus present with bristles 6–11 mm long. Fruit is a whitish, slightly ridged achene.
Russian knapweed is a deep-rooted long-lived perennial. Some stands have been in existence for 75 years. It forms dense colonies in cultivated fields, orchards, pastures, and roadsides.
A native to Eurasia, Russian knapweed was introduced into North America in the late 19th century. Absent only from southeastern U.S., it has become widespread in other regions, especially in the western United States.
Nigropallidal encephalomalacia, also called chewing disease, a movement disorder similar to Parkinson’s disease, is caused in horses ingesting Russian knapweed for prolonged periods. A sesquiterpene lactone, repin, in the plant is likely responsible for this toxicity.
II. How to Grow and Care
Russian knapweed is a hardy, perennial weed known for its resilience and adaptability to various environmental conditions. It requires minimal maintenance, making it an easy plant to care for. Special care points include ensuring proper control measures are in place, as russian knapweed can be invasive and may require consistent management to prevent over-spreading. Adequate soil moisture is also essential for its growth, although it’s tolerant to drought once established.
Sunlight
Russian knapweed thrives best under full sun conditions, where it receives direct sunlight for at least 6 to 8 hours a day. It exhibits an admirable tolerance to partial sun, but deviations from its ideal exposure could lead to reduced vigor and flowering potential. The plant is adept at coping with various light scenarios, often adjusting its foliage to maximize light absorption. Outdoors, it is advisable to plant russian knapweed in positions that ensure ample sunlight, away from any significant shade-causing obstructions, to maintain its growth and health capabilities. Its hardiness under sunlit conditions makes russian knapweed a resilient addition to landscapes where direct sun is predominant.
Watering
Originating from arid regions, russian knapweed has adapted to survive with less frequent watering, exhibiting a high drought tolerance. Its water habits are balanced between resilience and the need for consistent moisture, particularly during its active growth phases. For optimal health, Russian knapweed should be watered every week. Typically grown outdoors due to its robust nature, russian knapweed thrives in well-draining soil that mimics its native habitat’s conditions, where occasional rainfall provides the necessary hydration without the risk of waterlogging.
Fertilizing
Fertilization enhances Russian knapweed’s vigor and flowering. Apply a balanced nutrition fertilizer monthly during the growing season. Use according to label rates, avoiding over-fertilization which may harm russian knapweed. In dormant periods, reduce frequency. A slow-release formula can provide steady nutrients. Always water russian knapweed after applying fertilizer to help absorption and prevent root burn. Seasonal adjustments ensure optimal plant health and blooming.
Propagation
Belonging to the Asteraceae family, russian knapweed is a perennial weed broadly known for its resilience and invasiveness, necessitating particular caution during propagation. Successful multiplication typically occurs through vegetative means, leveraging its rhizomatous root system. While sowing is a potential method, it is rarely recommended due to russian knapweed’s aggressive nature. Gardeners should isolate new plants to prevent unintentional spreading, using barriers to contain its rhizomes. Ensuring healthy parent plants and appropriate soil conditions will elevate the chances of prosperous propagation.
Transplanting
For Russian knapweed, the optimum time to replant is when the days are long and warm — typically from late spring until the cusp of summer. Choose a sun-kissed spot with well-draining soil. When transplanting, ensure to give the roots ample space to re-establish.
III. Management
Rhaponticum repens is more difficult to eliminate than the other knapweeds because of its rhizomatous growth pattern. Studies for biological control in Montana and Wyoming have shown a gall-forming nematode and gall fly to have some impact on this plant. The nematode, Subanguina picridis, was introduced in the United States in 1984 and only Russian knapweed is susceptible. The nematode induces gall formation and it overwinters as a larva in root galls in the upper layers of the soil. In early spring, the larvae move up into stems, leaves, and new shoots.
Unfortunately, even though it is very effective in the laboratory; its effect on Russian knapweed populations are largely unnoticeable. In Wyoming the gall midge, Jaapiella ivannikovi has been released as a potential biological control for Russian Knapweed. It was initially released in 2010 after years of research this insect seem promising; but the real-world successes of this insect are still not known.
Fire is not a solution for the management of knapweeds. A single, low-intensity fire does not control spotted, diffuse or Russian knapweeds and instead may increase cover and density. Also, cultivation alone will only increase the rate of spread and establishment of Acroptilon repens. Cultivation in combination with re-seeding of native competitive grasses, can reduce the success of a knapweed re-invasion.
Herbicide treatment does not inhibit the growth of Russian knapweed if used by itself. For this knapweed, a single herbicide treatment followed by reseeding with rhizomatous grasses (such as streambank wheatgrass) can provide long term control and avoid annual reapplication of herbicide. Overall, for the management of Russian knapweed a combined strategy using chemical, cultural and mechanical methods is necessary.
