BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Ambrosia psilostachya | Western Ragweed 



GENERAL BOTANICAL CHARACTERISTICS : 
Western ragweed is a warm-season, native perennial forb.  The main stem
rises from shallow (2 inches [5 cm]) or deep, branching rhizomes which
extend down 3 to 6 feet (0.9-1.8 m) [6,70].  Stems are slender and
branched, usually 1 to 2 feet (30-60 cm) tall [59,70].  Plants are
monoecious with unisexual flowers; male flowers occur at the top of the
plant and female flowers are axillary [41].  Achenes have a short beak
and small blunt tubercles on top [41].


RAUNKIAER LIFE FORM : 
   Geophyte


REGENERATION PROCESSES : 
Western ragweed colonizes sites by means of spreading rhizomes in the
surface 2 inches (5 cm) of soil, allowing it to propagate when
conditions are unfavorable to seedling establishment [6,48,127].
Western ragweed exhibits nonrandom replacement of ramets, which allows
it to exploit areas favorable to growth [87].

Seeds are reported to migrate into disturbed areas; however, the means
of dissemination was not identified [6].  In a germination trial using 1
square foot (0.09 sq. m) soil samples, western ragweed seedlings did not
appear until week 6 or 7 [84].  After this time, seeds continued to
germinate for 3 weeks [84].

Once seeded into an area, western ragweed may not set fruit until the
second year [37].  Under dry conditions, seed production is somewhat
inversely proportional to plant density.  A dense stand of western
ragweed in a dry summer resulted in stunted growth, and most plants died
without fruiting [72].


SITE CHARACTERISTICS : 
Western ragweed grows in grasslands, savannas, and woodlands across
North America.  In addition to occurring in its native settings (such as
dry prairies, blowouts, washouts, sandy woods, meadows, and hills),
western ragweed is a widespread weed in waste places, roadsides,
railroads, overgrazed rangeland, and other disturbed places
[41,75,99,107,126].

Climate ranges from continental to coastal with short, warm to hot
summers and long, cold winters [1,3,16,129].  Often, there are
moderately strong surface winds [1].  Humidity is semiarid to moist
subhumid [42,44,86].  Annual precipitation ranges from 5 to 34 inches
(114-880 mm) with 60 to 80 percent occurring during the growing season
[19,23,39,56,75,114,125].  Temperatures vary from an average 72 degrees
Fahrenheit (22 deg C) in July to a January average of 11 degrees
Fahrenheit (-11.5 deg C) [124].

Western ragweed grows at elevations ranging from 850 to 7,400 feet
(259-2,256 m) and in many types of soils [18,30,60,66,69].  Soil
textures are predominantly loams, varying from silty clay loams to fine
sandy loams [23,61,122].  Soil pH ranges from 5.7 to 7.9 [22,105].
Soils often have little organic matter and are low in fertility [34].

Western ragweed occurs in too many grassland ecosystems for associated
species to be reviewed here.  Listed below are some typical examples of
major grasslands and the plant components found with western ragweed.
In addition to this brief listing, the reader is referred to specific
examples of more distinct and diverse grasslands in which western
ragweed occurs [11,15,18,19,20,22,25,26,28,29,32,33,39,42,49,129].

Southern Great Plains 
Shortgrass prairie is dominated by buffalograss (Buchloe dactyloides)
and blue grama (Bouteloua gracilis) with sand dropseed (Sporobolus
cryptandrus [132].  When trees occur, sand shinnery oak (Quercus
havardii) is dominant [45,118].

Mixed-grass prairie is dominated by sideoats grama (Bouteloua
curtipendula), buffalograss, little bluestem, and tobosagrass (Hilaria
mutica) [132].  When an overstory is present, dominant trees are honey
mesquite (Prosopis glandulifera), Ashe juniper (Juniperus ashei), post
oak (Quercus stellata), blackjack oak (Q. marilandica), and live oak (Q.
virginiana) [59,67,86,112].  Shrubs include cholla (Opuntia imbricata),
common broomweed (Xanthocephalum dracunculoides), and whitebrush
(Aloysia lycoiodes) [47,66,123].  An associated forb is Riddel daisy
(Aphanostephus riddellii) [47,65].

Tallgrass prairie is dominated by little bluestem, silver bluestem
(Andropogon saccharoides), and fewflowered panic (Dicanthelium
oligosanthes) [36].

Central Great Plains Shortgrass dominated by blue grama with
buffalograss, sand reedgrass (Calamovilfa longifolia), and prairie
dropseed (Sporobolus heterolepis) [82,122,132].  An associated forb is
horseweed (Conyza canadensis) [43].

Northern Great Plains Tallgrass prairie is dominated by big bluestem
with little bluestem, Indiangrass (Sorghastrum nutans), and switchgrass
(Panicum virgatum) [132].  A shrub co-occurring with western ragweed is
Louisiana sandwort (Artemisia ludoviciana) [120].  Codominant forbs are
heath aster (Aster ericoides), purple prairie-clover (Petalostemum
purpureum), and goldenrods (Solidago spp.)  [1,35,106,120].

Western ragweed occurs on floodplain woodlands with sand reedgrass and
Canadian wildrye (Elymus canadensis) [4,117].  The overstory is
dominated by floodplain cottonwood (Populus deltoides) with green ash
(Fraxinus pennsylvanica) [117].


SUCCESSIONAL STATUS : 
Facultative Seral Species

Differing sets of seral stages have been suggested for secondary
succession in prairie ecosystems, and western ragweed has been reported
to occur in all of them and in climax communities [97,102].  Western
ragweed establishes in closed communities that are opened up by heavy
grazing or other disturbance [103].  In old field succession, western
ragweed was present as a principal forb in stands aged 0 to 5 years and
was present with 4 to 15 percent cover after 23 to 29 years [33,48].  On
abandoned black-tailed prairie dog towns, western ragweed was codominant
with an annual grass, prairie threeawn (Aristida oligantha), in an
intermediate seral stage [10,91].  In tallgrass sand prairie, western
ragweed was present in pioneer stages, occurred with greatest frequency
in an intermediate phase, but had greatest cover in the climax phase
[25].  Although reported as a pioneer species, western ragweed occurs on
secondary sand dunes but does not occur on less stable sites such as
primary dunes or tidal flats [27].  Additionally, western ragweed occurs
outside of buffalo wallows, which are considered safe sites for ruderal
species [36].

Western ragweed may have alleopathic or other inhibitory effects on
other pioneer species.  Leachate from western ragweed leaves and roots
significantly (P<0.05) reduced growth of soil bluegreen bacteria (Lyngby
spp.)  cultures [102].  While soil collected in July near western
ragweed was stimulatory to pioneer weedy species (for example, Japanese
brome [Bromus japonicus]), soil collected in January had an inhibitory
or no effect on seedlings of the same species [102].  Leaf leachate from
leaves that overwintered on western ragweed plants inhibited
germination, seedling topgrowth, and mature plant root formation of the
pioneer species [102].


SEASONAL DEVELOPMENT : 
Western ragweed is widespread, so specific dates for phenological stages
vary; however, stages of growth occur seasonally.  The months in
parentheses represent the center of its distribution.  Western ragweed,
a warm season plant, overwinters as a rosette [103].  In mid-spring
(April), seedlings germinate, and rosettes begin active growth of main
stems.  In late summer (August), western ragweed flowers, and seedlings
may germinate with adequate rainfall [5].  It is at this time of year
that western ragweed usually has its greatest biomass [96].  Flowering
continues through autumn [17,41,51,81,89,99].  Fruits form and seeds
disseminate through the late fall and winter (October to December) [5].
Aerial stems are killed by frost.

Related categories for Species: Ambrosia psilostachya | Western Ragweed