BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Bouteloua barbata var. barbata | Sixweeks Grama 



GENERAL BOTANICAL CHARACTERISTICS : 
Sixweeks grama is a small, native, warm-season, annual bunchgrass which
grows up to about 1 foot (0.3 m) in height [26].  The slender, hollow,
tufted culms are usually prostrate but sometimes somewhat erect [3].
Plants are generally light green when growing, but cure to a straw color
[11].  Leaves are 0.4 to 2.4 inch (1-6 cm) long and 0.04 to 0.06 inch
(1-1.5 mm) wide and occur sparingly along the culms [10,21].  The
inflorescence consists of three to seven persistent, comblike spikes per
culm.  The spikes are smaller than those of perennial gramas.  Each is
0.4 to 0.8 inch (1-2 cm) long [3,24].

Robust, erect sixweeks grama plants are sometimes mistaken for Rothrock
grama, a closely related short-lived perennial.  In general, sixweeks
grama has shorter awns and the stems are more branched [3,15].  Several
floras allow proper identification [3,10,15,21].


RAUNKIAER LIFE FORM : 
   Undisturbed State:  Therophyte
   Burned or Clipped State:  Therophyte


REGENERATION PROCESSES : 
Sixweeks grama is a summer annual.  Seeds stored in the soil germinate
after summer rains; plants grow quickly and set seed in the fall before
they die.  In Joshua Tree National Monument at elevations below 1,000
feet (304 m), sixweeks grama grew and produced seeds within 4 weeks of
germination, but above 1,000 feet (304 m) plants took 6 weeks to
complete their life cycle [28].

Germination:  For germination to occur under natural conditions, two
requirements must be met:  (1) a single large rainfall of 0.6 to 1 inch
(1.5-2.5 cm) [17,28] and (2) warm daytime and nighttime soil
temperatures, which normally occur only during the summer or fall
[13,29].  Generally a heavy rainfall which saturates the soil
surrounding sixweeks grama seeds is needed for germination.  However,
following light rains, seeds sometimes germinate along washes or in
depressions [28].  In laboratory experiments, sixweeks grama exhibited
good germination under a temperature regime of warm days (80 degrees F
[27 C] for 8 hours) and warm nights (78 degrees F [26 C] for 16 hours)
but poor germination under a temperature regime of warm days (80 degrees
F [27 C]) and cool nights (55 degrees F [13 C]) [29].  Another study
showed that large numbers of sixweeks grama seeds germinated when
daytime soil temperatures were 86 degrees F (30 C), small to moderate
numbers germinated when daytime soil temperatures were between 62 and 72
degrees F (17 and 23 C), and no seeds germinated when daytime soil
temperatures were 50 degrees F (10 C) [13].

Seed dormancy:  Information concerning the length of time seeds will
remain viable is lacking.  However, considering that numerous authors
have observed that germination of sixweeks grama occurs only during
infrequent years at particular locations [6,7,13], seeds must remain
viable for a number of years.


SITE CHARACTERISTICS : 
Sixweeks grama occupies dry to moist sites [27].  In desert areas, it is
found on open, rocky or sandy slopes, gravelly flats, bajadas, and
disturbed areas with bare soil [3,11,21].  It also occupies washes and
intermittent water courses.  Sixweeks grama is most often found in
desert shrub, semidesert grassland and pinyon-juniper vegetation types
[6,21,25].  Its density is extremely variable from year to year because
of its dependence on summertime precipitation for germination.  On the
Jornada Expermintal Range in New Mexico, sixweeks grama is found in
grasslands dominated by black grama (Bouteloua eriopoda), tobosa grass
(Hilaria mutica), and burrograss (Scleropogon brevifolius) [6].  In
desert areas it is often associated with other summer annuals which
germinate at the same time.  Common associates in the Mojave Desert
include prickly pears (Opuntia spp.) and numerous summer annuals such as
needle grama (Bouteloua aristidoides), cinchweed fetidmarigold (Pectis
papposa), pigweed (Amaranthus fimbriatus), euphorbia (Euphorbia
micromera), and spiderling (Boerhaavia spicata) [24,29].

The elevational range for sixweeks grama in several western states is
presented below:

        elevational range                   state            authority

from 1,000 to 5,500 feet (305-1,676 m)       AZ                [15]
below 3,200 feet (975 m)                Rincon Mtns., AZ       [2]
below 5,000 feet (1,524 m)                  s CA               [21]
below 7,000 feet (2,134 m)                   NM                [5]
from 2,800 to 6,000 feet (850-1,820 m)       UT                [27]


SUCCESSIONAL STATUS : 
Sixweeks grama is an early seral species.  It often occupies disturbed
areas with much bare ground or areas where other vegetation is scarce
[2,7,11].  


SEASONAL DEVELOPMENT : 
Sixweeks grama grows only during the summer or fall.  Following rains of
sufficient quantity to induce germination, plants grow rapidly, set seed
within about 6 weeks, and then die [17,28].  The phenological
development of sixweeks grama in southern New Mexico is presented below
[17]:

    phenological event       timing of event

    seeds germinate          late-August to early-September
    flowering                September
    seeds mature             late-September to early-October
    seeds dispersed          late October

The time of flowering for sixweeks grama in several areas is presented
below:

    time of flowering        location                   authority
 
    June-October         s CA                             [21]
    July-October         AZ                               [15]
    August-October       Kelso Dunes, s CA                [25]
    July-September       Desert Experimental Range, UT    [7]

Related categories for Species: Bouteloua barbata var. barbata | Sixweeks Grama