Sahara mustard (Brassica tournefortii) is a drought tolerant weed from the deserts of North Africa and the Middle East. The earliest record in California is from Coachella Valley in the 1920s. By the 1970s it was widespread in the low deserts of Arizona, Baja California, and Sonora, Mexico. As of 2010 it is literally taking over large areas of Borrego Valley and is rapidly moving up the canyons and alluvial fans of Anza-Borrego Desert State Park. It is adapted to poor soils in very arid habitats, and is even tolerant of saline soils. According to the Arizona-Sonora Desert Museum, it is also allelopathic and suppresses the growth of other competing annuals and perennials. Dried plants break off at ground level and roll in the wind, thus dispersing seeds like tumbleweeds. A hard freeze will severely damage Sahara mustard, possibly giving native annuals a chance to grow and reproduce in some years.

Locoweeds belong to the large and diverse genus of flowering plants Astragalus (Fabaceae), with approximately 2,000 different species in the northern hemisphere, 380 species in North America, and 94 species in California. Some are rare endemics that only occur in one isolated mountain range or canyon. They are sometimes called milk vetches from the notion that milk secretion in goats was increased when they fed on the common Old World forage species (Astragalus cicer). Like many legumes, the leaves are typically divided into a dozen or more leaflets, and the flowers resemble small pea blossoms. Some species produce inflated seed pods that make a distinct popping sound if you step on them. Locoweeds are also called rattleweed because of the bladder-like, inflated pods, particularly when a gust of wind rattles the seeds inside. Astragalus is derived from a Greek word meaning anklebone, the plural of which means dice. Perhaps the dice connotation refers to the rattling of seeds inside papery pods, like the sound of dice in a thrower. In anatomy, the astragalus or talus is one of seven bones in the ankle joint. Anklebones were apparently used for dice by ancient Greeks, and to this day, veteran crapshooters in Las Vegas refer to dice as "bones." Since adult astragalus bones are a little too large for dice, some of the smaller, cuboidal or cuneiform anklebones were probably used.

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