Gouan

Sahara mustard, Asian mustard

none known

0

8

CalEPPC List A-2,CDFA nl

Andrew Sanders,Richard Minnich

Sahara mustard (Brassica tournefortii) is an annual herb
with stems four to forty inches tall. Plants flower early, but the flowers are
small and dull yellow, making them inconspicuous compared to most other true
mustards. Petals are less than one-quarter inch. Individual flower stalks are
longer than the sepals and spread away from the stem. Fruits have an obvious
beak at the tip. The pedicels of the fruits are one-half to four-183 fifths of
an inch long and diverge stiffly from the stem at a forty-five degree angle.

Sahara mustard is an abundant annual weed at low elevations
throughout southwestern deserts of North America, including southern California,
southern Nevada, Arizona, New Mexico, west Texas, and northwestern Mexico. In
California it occurs in both the Mojave and Sonoran deserts, but is more common
in the latter. It is becoming increasingly frequent in semi-arid south coastal
California. It is found as high as 3,300 feet (1,000 m) elevation, but is
especially abundant below 1,000 feet (305 m).

Apparently uncommon at mid-century, this plant attracted little
attention in southern California until recent decades when it began to be
collected widely, mostly by botanists and students. In the late 1950s it was
known only from Riverside, Imperial, and southwestern San Bernardino counties
(Munz 1959). It was not reported in the Sonoran Desert flora until the mid-1960s
(Shreve and Wiggins 1964). It seemed to have experienced a population explosion
from 1977 to 1983, during successive years of above-normal precipitation,
becoming well established in all counties of southern California. It was
described as being a “rare adventive, usually in desert waste areas” as late as
the mid-1980s in San Diego County (Beauchamp 1986).

Now abundant in the Coachella, Borrego, and Imperial valleys of
southeastern California, it is especially common in areas with wind-blown
sediments. It is also invading exotic annual grassland and coastal sage scrub on
the coastal slope of southern California. Plants were observed in Colton Dunes
in southwestern San Bernardino County in the 1950s. First noticed in the
Riverside area in 1988, it has spread throughout the city, including Box Spring
Mountain and Mt. Rubidoux. It is abundant in Baja California, on both coasts. It
often forms almost pure stands on abandoned sandy fields. Anderson (pers. comm.)
observed a dense stand of the species near San Felipe on the northeast coast in
about 1990. It had reached the extreme southern edge of Sonora by 1993 as an
uncommon weed along roadsides. As far north as Coso, near the southern Owens
Valley of the Mojave Desert, it is an uncommon weed of roadsides.

Sahara mustard is most common in wind-blown sand deposits and in
disturbed sites such as roadsides and abandoned fields. It is scarce on alluvial
fans and rocky hillslopes.

Sahara mustard is native to semi-arid and arid deserts of North
Africa and the Middle East, as well as Mediterranean lands of southern Europe
(Townsend and Guest 1980, Tutin et al. 1964, Zohary 1966) in habitats similar to
those it now occupies in North America. The plant apparently was first collected
in North America at Coachella in Riverside County by J.B. Feudge (#1660, RSA) on
25 February 1927. This collection was incorrectly identified as Brassica
arvensis and was only recently corrected (by Andrew Sanders). Sahara mustard
probably was introduced with date palms brought from the Middle East in the
early part of this century with the development of the date industry in the
Coachella Valley.

During rains, a sticky gel forms over the seed case that permits
seeds to disperse long distances by adhering to animals. The rapid spread of
Brassica tournefortii through the Sonoran Desert, with first occurrences
along roadsides, may be related to its ability to adhere to automobiles during
rare periods of wet weather.

Dense stands in the Coachella and Imperial valleys appear to
suppress native wildflowers. Because of its early phenology, it appears to
monopolize available soil moisture as it builds canopy and matures seed long
before many native species have begun to flower. In coastal southern California,
it locally dominates exotic grasslands in dry, open sites, especially disturbed
areas. It expands over larger areas when drought suppresses other exotic annuals
such as Bromus rubens, Avena fatua, Brassica geniculata, and Erodium
cicutarium.

Sahara mustard increases fuel loads and fire hazard in desert
scrub and coastal sage scrub. It also establishes from a soil seedbank after
fire.

Plants flower or fruit as early as December or January and

than 25 percent of normal) winters of 1995-96 and 1996-97.

The density of Sahara mustard plants can vary with annual climate and fire history. For example, two years of drought during 1989-91 in Riverside County killed off existing red brome (Bromus rubens) cover on a dry southern exposure. Sahara mustard populations in this area subsequently increased by almost thirty-five times. During the wet winters of 1991-92 and 1992-93, while plant densities increased, overall biomass decreased, apparently reduced by intraspecific competition. A hot spell in February 1993 caused Sahara mustard to flower early, aborting a potentially productive growing season in moist soils over the following months. After a fire in November 1993, plants continued to decline in both density and biomass, apparently because of the proliferation of other exotics, especially red brome.

(click on photos to view larger image)

Manual methods: Hand pulling might be
effective in limited areas when seed pools have been suppressed.

Prescribed burning: The occurrence of this annual in harsh
deserts of the Old World has no doubt selected it to survive long periods in
soil seedbanks. Therefore, planned burns may not be a useful option. Although
fires cause high seed loss, stem densities reach pre-burn levels within one or
two growing seasons. Partial seed survival after fire may be related to its hard
seed coat.

Insects and fungi: Sahara mustard is
closely related to a number of important vegetable crops (broccoli, cauliflower,
brussels sprouts, etc.), so it will be difficult to find an agent that will
attack this plant but not damage food crops. Even the possibility of transfer of
a control agent to a valuable food crop may create political pressures that
could prevent importation of the agent.

Grazing: Since Sahara mustard establishes from a seedbank, it is
doubtful that grazing could suppress the spread of this annual. Experiments
could be undertaken to determine whether foraging interferes with recruitment
and growing season biomass by placing livestock in fields of Sahara mustard
during early winter (e.g., January).

Plant competition: Establishment of dense cover of exotic annual
grasses apparently suppresses this species.

The extremely early development of this
species might make early chemical control a possibility, especially when
desirable native species have not yet begun to develop. This should be
investigated experimentally.