Montana Field Guide

PLANTS: Rhizomatous perennials that grow in terrestrial habitats (FNA 2006; Lesica et al. 2012). Stems are erect, branched, and grow about 23-100 cm tall. Plants are somewhat cobwebby-tomentose and resin-gland-dotted.

LEAVES: Leaves of the rosette (basal) and lower stem are often absent by maturity (FNA 2006; Lesica et al. 2012). Stem leaves are oblong, 3-10(15) cm long, becoming sessile and smaller upwards. Leaf margins are entire to pinnately lobed.

INFLORESCENCE: Paniculiform or corymbiform. Flower heads are solitary on the ends of leafy branches (Jacobs 2007b). Involucres are broadly ovoid, 9–13 mm high, and somewhat cobwebby (FNA 2006; Lesica et al. 2012). Involucral bracts (phyllaries) are broadly ovate, green or tan below, scarious above, and sharp-pointed. The receptacle is flat with fine scales.

Flowering May to September (FNA 2006).

Until more recently the American literature has placed Russian knapweed within the genus Centaurea. Molecular phylogenetic studies of the relationships of Cynareae genera support the separation of Acroptilon from Centaurea (Susanna et al. 1995; FNA 2006). Acroptilon is also morphologically distinct in that attachment scars on the cypselae occur near the base as opposed to the sides and flower heads lack sterile outer florets (FNA 2006). In addition, Centaurea species have a dense bristly receptacle while Acroptilon species have a receptacle that is naked, with fine scales, or pubescent (Lesica et al. 2012).

Grasslands, roadsides, meadows, fields, and most often on stream terraces; plains, valleys (Lesica et al. 2012). It invades open, disturbed land, particularly where dry (Sheley and Petroff 1999).

Plants are allelopathic. Leaves produce three types of sesquiterpene lactones, repin, acroptilin, and hyrcanin (Jacobs and Denny 2007). Roots exude cnicin.

Russian knapweed plants have proteins levels similar to alfalfa hay but are too bitter for livestock to eat (Sheley and Petroff 1999).

FLOWERS
Flower heads consist of disk flowers. Each head is composed of 15-36 disk flowers that are white or light purple. All florets are fertile and the petals are 11-14 mm long, slender with an expanded limb. The styles are united almost to their tips. Fruits (cypselae) are ivory to grayish or brown, about 2-4 mm long. The pappus is deciduous and consists of white flattened bristles with plumose tips, 6-11 mm long.

ROOT SYSTEM
Reproduction is by seed and adventitious buds on creeping rhizomes. However, growth by adventitious buds is stronger than by seed production. Rhizomes are scaly (bark covered) and dark brown to black (Jacobs and Denny 2007). In a plant’s first year roots grow to 6.5 to 8.3 feet deep, and in the second year grow to 16 to 23 feet deep (Sheley and Petroff 1999). The adventitious buds grow into leafy shoots. It is the root system that allows populations to persist (even up to 75 years), to complete aggressively for water and nutrients, and to exude phytotoxic, allelopathic chemicals (Jacobs and Denny 2007).

LIFE CYCLE and DISPERSAL [adapted from Jacobs and Denny 2007]
When soil temperatures remain above freezing shoots emerge from the adventitious buds in early spring. The first leaves have entire margins, but soon develop as rosettes with pinnately divided leaves. Stems bolt from rosettes from late May to mid-June. Flowering occurs from early July through September. Seeds are produced from August through September. However, Russian knapweed has poor seed dispersing mechanisms. Many ovules abort and a study from British Columbia found 5-8 seeds per head and 100-300 seeds per plant develop. Another study found 1,200 seeds per plant (Ivanova 1966). Seeds apparently are viable for 2-3 years. Germination requires that seeds are covered by wet soil or litter for an extended period. The involucral bracts remain closed and the pappus is minute and deciduous, both of which impede dispersal by wind. However, an intact seed head might have the potential to float. To date, most dispersal is by way of infested hay!

Successful management of Russian knapweed targets its extensive root system because it poorly reproduces by seed (Jacobs and Denny 2007). Developing land-use objectives, a desired plant community, and an integrated weed management strategy will make control of Russian knapweed possible (Sheley and Petroff 1999).

BIOCONTOL [Adapted from Jacobs and Denny 2007]
Biological control has been limited and is not effective since Russian knapweed poorly develops and disperses by seed. The Russian Knapweed Stem-gall Nematode (Subanguina picridis) has been introduced and established with limited success.

CHEMICAL CONTROL [Adapted from Jacobs and Denny 2007]
Herbicides are effective when properly managed with other tactics. Herbicides that translocate the chemical directly to the roots will be most cost-effective. Surfactants improve the uptake of each chemical listed below. The herbicide type and concentration, timing of chemical control, soil properties, and other factors will determine its effectiveness and impact to non-target species. Strict adherence to application requirements defined on the herbicide label will reduce risks to human and environmental health. Many herbicides must be applied by applicators with an Aquatic Pest Control license. Consult your County Extension Agent and/or Weed District for more information on herbicidal control.

Picloram (0.50 pound per acre) applied to the bloom stage has been shown to provide long-term suppression in Montana.

Aminopyralid, clopyralid, and imazapic have also suppressed Russian knapweed when applied during the stages from flower bud to mid-flowering or during fall re-growth; and could be used where Picloram cannot. In Fergus County, clopyralid plus 2,4,-D applied to the bloom stage reduced the density of plants. Further, it accompanied an increase in perennial grasses.

Chlorsulfuron applied in the bloom to post-bloom stage may provide short-term control.

CULTURAL and GRAZING CONTROLS [Adapted from Jacobs and Denny 2007]
Hand-pulling is not a practical control method because of its deep, rhizomatous growth.

Mowing is not very effective at reducing plant density. It may suppress growth initially, but later stimulates regeneration. It might be useful in combination with a herbicide treatment.

Burning can reduce plant biomass, but is not effective at preventing growth, germination, or flowering.

Grazing. There is little information available. Horses that have grazed Russian knapweed for an extended period developed equine chewing disease (Nigropallidal encephalomalacia) which is a fatal neurodegenerative disorder (Jacobs and Denny 2007). Russian Knapweed plants have proteins levels similar to alfalfa hay but are too bitter for livestock to eat (Sheley and Petroff 1999).

Revegetation. Grasses used for long-term control must have the following characteristics: a) adapted to the soil and climate, b) easy to establish, c) competitive with weeds, d) palatable and nutritive, particularly for late-season use, e) dry matter productivity, and f) stand longevity (Sheley and Petroff 1999). Although Russian knapweed is allelopathic, control through plant competition should be exploited (Sheley and Petroff 1999). Studies have found that monocultures of Winter Rye (Secale cereal) or Wheat (Triticum aestivum) reduced Russian Knapweed (Sheley and Petroff 1999). Other research found that Russian Knapweed and Smooth Brome (Bromus inermis) competed for limited resources while Russian Knapweed and Western Wheatgrass (Elymus smithii) did not compete (Sheley and Petroff 1999).

Useful Links:
Montana Invasive Species website
Montana Biological Weed Control Coordination Project
Montana Department of Agriculture - Noxious Weeds
Montana Weed Control Association
Montana Weed Control Association Contacts Webpage.
Montana Fish, Wildlife, and Parks - Noxious Weeds
Montana State University Integrated Pest Management Extension
Weed Publications at Montana State University Extension - MontGuides