BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Yucca glauca | Soapweed Yucca 



GENERAL BOTANICAL CHARACTERISTICS : 
Soapweed yucca is a small, acaulescent, perennial shrub which grows from
1 to 6 feet (.3-1.8 m) in height [1,45].  Plants are simple, or grow in
dense clumps 2.6 to 8.2 feet (0.8-2.5 m) in diameter, with short often
prostrate stems [23,45].  Soapweed yucca is typically deep-rooted [3],
and long-lived.  Researchers estimated that a plant 5 feet (1.5 m) in
height was more than 50 years old [25].

The glaucous green leaves 1 to 3 feet (.3-1 m) in length are broad,
stiff, and sharply pointed with fibrous margins [21,23,48].  The
inflorescence is either racemose or paniculate, and up to .3 feet (1 m)
in length on a 1 to 4 feet (.3-1.2 m) tall flowerstalk [23,47].  The
flowers themselves are large, globose or campanulate and greenish-white
with a purple tinge [21,45].  The oblong fruit develops into a woody
capsule which contains numerous flat black, winged seeds [23,45,47].  


RAUNKIAER LIFE FORM : 
   Phanerophyte
   Geophyte


REGENERATION PROCESSES : 
Soapweed yucca is capable of reproducing sexually or through vegetative
means.  Typically, rosettes grow for several years, bloom once and die,
but are replaced by new rosettes derived vegetatively [28].  Winged seed
is shiny, black, and averages .006 ounce (16 mg) in weight [2,23,27].
Seed is contained in a dehiscent, woody capsular fruit 2 to 3 inches
(5-7 cm) long [23,26].  Plants generally first flower at 5 or 6 years of
age [45].

Germination:  Soapweed yucca seed may lack a mechanism for dormancy
[28], and generally germinates well under favorable temperature and
moisture conditions [27,41].  Germination rates range from 45 to 98% but
generally average 80 to 90% under laboratory conditions [1].  However,
Arnott [1] observed that only 20% of the seed actually produced viable
seedlings in greenhouse experiments.  Various treatments can influence
germination.  Without pretreatment, germination generally begins within
1 or 2 weeks [1].  Seed soaked in water for 24 hours often germinates
within only 4 days [45].  Tolstead [41] reported that germination of
untreated seeds averaged 67.1% while seed vernalized for 2 months
averaged 86.6%.  Germination may be adversely affected by exposure to
heat as shown below [27]:

                        % germination
exposure time    control     80 C     90 C   100 C    110 C     120 C
2 hours            79        57       42      --       --        --  
5 minutes          --        --       41      59       28         3

Pollination:  One of the most interesting and well-studied aspects of
yucca ecology centers around the symbiotic relationship between yuccas
and their yucca moth pollinators.  Soapweed yucca relies solely on the
yucca moth (Tegeticula yuccasella) for pollination [36].  Seed
production is thus totally dependent on the availability of this
pollinator which, in the larval stage, feeds on a small percentage of
seeds (generally around 7%) [26].  In some years, less than 1% of the
racemes set fruit due to lack of pollinators, or poor weather conditions
[28].

Vegetative regeneration:  Evidence suggests that on newly disturbed
sites, which lack competing grasses, soapweed yucca regenerates
primarily from seed.  Vegetative regeneration allow soapweed yucca to
survive and expand on sites with significant competition such as prairie
grasslands.  On these sites a low ratio of seedlings to ramets is common
[28].  Vegetative regeneration occurs through the sprouting of
underground horizontal or oblique rhizomes which form a tangled network
approximately 4-24 inches (10-60 cm) below the soil surface.  Rhizomes
are covered with a thick, rough, protective bark, branch and spread
horizontally with vertical branches growing to the soil surface where
new leafheads are formed.  The connection to the mother plant gradually
becomes less important with time and eventually dies [45].  Ramets are
produced in late summer from lateral buds at the base of the
inflorescence or from rhizomes near senescent rosettes.  The basal
diameter of individual rosettes is a good predictor of potential for
vegetative regeneration, with larger rosettes exhibiting a much greater
tendency to form new plants [28].  Soapweed yucca can resprout
relatively quickly even after successive defoliations.  There is little
evidence to link season of injury or climatic factors with recovery
[45].


SITE CHARACTERISTICS : 
Soapweed yucca grows on dry rocky outcrops, ridges, slopes, lower
mountains, prairies, and plains [23,38,44,47].  It is often found thinly
scattered in rolling grasslands or in open coniferous woodlands but also
occurs in dense stands in some areas [21,45].

Soils:  This yucca grows well on a variety of soils including coarse
gravel, sand or porous loam [10,44].  Growth is ofen best on compact
sands and soapweed yucca is frequently described as an indicator of sand
[10].

Elevation:  Generalized elevational ranges by state are as follows [14]:

        from: 4,000 to 9,500 ft (1,219-2,896 m) in CO
              2,500 to 5,000 ft (762-1,524 m) in MT
              3,800 to 6,400 ft (1,158-1,951 m) in WY


SUCCESSIONAL STATUS : 
Soapweed yucca can occur as a colonizer on disturbed sites [28], or as a
common climax dominant in a variety of plains grasslands, drier forest
communities and prairies [34,35].  Seedlings often thrive on disturbed
sites in which soil is exposed such as road cuts, overgrazed sites, or
slide zones.  However, small seedlings often compete poorly with grasses
or older conspecifics.  Existing plants or clones of this versatile
plant can still increase in area through vegetative regeneration in
grasslands or prairies.  Interestingly, this yucca is described as both
an increaser in response to grazing, and a climax species.  Geographic
variation in its reproductive modes and successional status is possible
[28].


SEASONAL DEVELOPMENT : 
Soapweed yucca, unlike many of its plant associates, remains green
throughout the winter months [16].  Lateral buds located at the base of
the inflorescence produce new ramets during the late summer [28].

Flowering in this yucca typically occurs after a rosette has grown
vegetatively for a number of years.  The individual rosette dies after
flowering but is replaced by new rosettes [28].  Generalized flowering
dates are as follows [14]:

location        beginning of flowering          end of flowering
  CO                    May                           July
  MT                    June                          July
  ND                    June                          July
  WY                    June                          July

Fruit of soapweed yucca ripens during July and August, and seed is
dispersed in September [1].

Related categories for Species: Yucca glauca | Soapweed Yucca