BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Sonchus arvensis | Perennial Sowthistle 



GENERAL BOTANICAL CHARACTERISTICS : 
Perennial sowthistle is an introduced, perennial, deep-rooted forb.  The
erect, hollow stems grow 1 to 6 feet (0.3-1.8 m) tall and contain a
milky fluid [20].  Inflorescences have several to many flowerheads
[19].  The achene has a mostly persistent pappus which is four times
longer than the achene.  The pappus hairs have hooked cells which aid in
animal dispersal [20].

Perennial sowthistle either has rhizomes or creeping roots with buds.
The literature has not discriminated between these two types of
vegetative reproductive organs.  This report uses the term rhizomes but
recognizes the possibility that the underground vegetative buds may
originate from root tissue and not stem tissue.

The extensive creeping rhizomes, 0.1 to 0.2 inch (0.25-0.5 cm) in
diameter, originate from short, spindle-shaped, somewhat branched
primary roots.  These rhizomes are generally 2 to 5 inches (5-12 cm)
deep, but perennial sowthistle can produce vegetative buds 20 inches (50
cm) below the ground surface.  Vertical roots penetrate 6 feet (2 m)
deep [20].


RAUNKIAER LIFE FORM : 
   Hemicryptophyte


REGENERATION PROCESSES : 
Perennial sowthistle reproduces vegetatively and by seed.

Perennial sowthistle typically produces an average of 30 seeds per
flowerhead.  The main stalk of a perennial sowthistle in North Dakota
produced 62 flowerheads and 9,750 well-developed seeds.  However, large
clones often do not produce many seeds because of self-incompatability.
Viability of perennial sowthistle seeds is generally high (70-90%) [20].

Seed dispersal is primarily by wind, but hooked cells on pappus hairs
also cling to clothes and animal fur [20].  Sheldon and Burrows [36]
computed maximum dispersal distances for perennial sowthistle based on
the rate of fall of achenes in still air.  Assuming a mean plant height
of 35 inches (90 cm) and a vertical convection speed of 6.8 miles per
hour (10 km/hr), perennial sowthistle achenes are dispersed the
following distances at each wind speed [36]:

  		Wind Speed                     Dispersal Distance
	   miles per hour (km/hr)                   feet (m)

		 3.4  (5.5)                         11  (3.3)
		 6.8 (10.9)                         22  (6.7)
		10.3 (16.4)                         33 (10.0)

Buried perennial sowthistle seeds persist an undetermined length of time
in the seedbank [28,45].  In the Delta Marsh, Manitoba, there was an
average of 3.4 perennial sowthistle seeds per square foot (38/sq m) in
the hybrid cattail (Typha X glauca) zone and 2.3 per square foot (25/sq
m) in the common reed (Phragmites australis) zone [28].

Seed germination occurs when soils are warm.  Air temperatures ranging
from 77 to 86 degrees Fahrenheit (25-30 deg C) are optimal for
germination [20].  Hogenbirk and Wein [17] studied the emergence of
seedlings from soil seedbanks at different temperatures.  Seeds were
collected from a 13.2 square inch (85 sq cm), 4 inch (10 cm) deep sample
in a willow (Salix spp.) savanna on the Peace-Athabasca Delta in
Alberta.  Eight and seven perennial sowthistle seedlings emerged at
day/night air temperatures of 68/50 degrees Fahrenheit (20/10 deg C) and
86/59 degrees Fahrenheit (30/15 deg C), respectively [17].

Light stimulates germination.  The rate of emergence decreases rapidly
at soil depths greater than 0.2 inch (0.5 cm) [20].

Perennial sowthistle seeds do not germinate under submerged conditions
but germinate readily when the soil surface is exposed [28,45].
Perennial sowthistle seeds germinated under simulated drawdown
conditions in 17 percent of substrate samples collected at Delta Marsh,
Manitoba [28].  Perennial sowthistle established on artificially exposed
mudflats on Delta Marsh at low densities (less than 1 seedling per
square foot [<10/sq m]) [23].  Perennial sowthistle was present on a
site in northwestern Minnesota 2 years after artificial drawdown in a
common cattail (Typha latifolia) and narrowleaf cattail (T.
angustifolia) community [14].

Seedlings survive best where soil is moist.  Perennial sowthistle forms
a rosette early in its development.  Flowering stems develop when plants
have 12 to 15 leaves, usually during the second growing season [20].

Perennial sowthistle colonizes a site rapidly by vegetative
reproduction.  The rate of vegetative spread from the border of a
perennial sowthistle clone in North Dakota varied from 1.6 to 9.2 feet
per year (0.5-2.8 m/yr).  Perennial sowthistle persists in cultivated
fields because new plants can develop from rhizome sections less than 1
inch (2.5 cm) long if buds are present [20].

Vegetative and reproductive growth is enhanced by full light, moderate
temperatures, and water-saturated soil.  Growth is more rapid at
day/night temperatures of 68/59 degrees Fahrenheit (20/15 deg C) than at
either 86/77 degrees Fahrenheit (30/25 deg C) or 50/41 degrees
Fahrenheit (10/5 deg C) [49].


SITE CHARACTERISTICS : 
Perennial sowthistle occurs along roads, marsh edges, lakeshores, and
riverbanks, and in cultivated fields, meadows, and pastures.  Although
considered an upland species, perennial sowthistle grows well on moist
sites.  In a North Dakota grassland, perennial sowthistle occurs in the
intermediate moisture zone.  It does not occur in locations which are
permanently flooded nor where drainage is excessive [7].  Perennial
sowthistle is present at low frequencies in the emergent zone of oxbow
lakes in the Athabasca River drainage in Alberta [21].

Perennial sowthistle is most competitive on sites with poorly drained
soils, high water-holding capacities, and moderate alkalinity [49].  It
does well on light, well-drained soil if there is a good supply of
moisture.  In southern Quebec, perennial sowthistle colonized low angle,
sandy riverbanks with 0.4 percent organic matter and less than 25
percent vegetative cover [34].  Perennial sowthistle commonly occurs on
saline soils [34,41].  It was encountered in 29 percent of inventoried
saline sites in the Canadian Prairie Provinces [3].

Extremely cold climates may limit perennial sowthistle expansion.  Seeds
and rhizomes had not developed on perennial sowthistle observed in
August in northern Manitoba.  The conditions required to reduce the
survival of perennial sowthistle roots and perennating buds by 50
percent is 1 degree Fahrenheit (-17 deg C) for 2 hours [35].


SUCCESSIONAL STATUS : 
Perennial sowthistle is an early successional species [20].  It is not
reported to occur under forest canopies and thus may be intolerant of
shade.  

Perennial sowthistle rapidly colonizes disturbed sites [20] as well as
recently exposed mud flats [14,23,45].  Drought was simulated in a
wetland community on the Peace-Athabasca Delta, Alberta, by
transplanting soil blocks upslope.  Average perennial sowthistle cover
increased from 2 to 10 percent in 1 year, possibly in response to
decreased height growth of bluejoint reedgrass (Calamagrostis
canadensis) [16].  Flooding eventually eliminates perennial sowthistle
from mud flats [45].


SEASONAL DEVELOPMENT : 
Perennial sowthistle begins to flower around July 1 in the northern
United States and continues until frost, although flowering is mostly
complete by late summer.  Fruits mature about 10 days after flowering.
Seedlings do not emerge until mid- to late May in the Great Plains of
the United States and in Saskatchewan.  Shoots and new roots and
rhizomes begin to develop when the soil warms, usually in late April.
New shoots develop until late summer [20].

Related categories for Species: Sonchus arvensis | Perennial Sowthistle