Plant 384

Astragalus species (Fabaceae)

Milk vetch

Astragalus, the largest genus of herbaceous plants in the pea and bean family, includes approximately 3,000 species distributed across Europe, Africa, Asia and North and South America. These species are annual or perennial, herbs or subshrubs with odd- or even-pinnate leaves. Their flowerheads of pea-like flowers are white through yellow and red to purple and blue. The fruit pods contains kidney-shaped seeds.

Rarely grown in gardens, Astragalus species have many uses. Belief that Astragalus improves the quality of the milk of humans and grazing livestock gives rise to one of the potential origins of the generic name, ‘star-milk’ (from the Greek astron and gala). Another potential origin is from the Greek meaning ‘ankle-bone’, possibly a reference to the shape of the seeds or the roots. Astragalus species are used to treat respiratory infections and kidney diseases, whilst Astragalus mongholicus, an important herb in Chinese medicine, is believed to regulate the immune system. These attributed claims of the therapeutic properties of Astragalus are unsupported by rigorous evidence. Some Astragalus species are however toxic.

Astragalus is unquestionably useful in the form of gum tragacanth. This odourless, tasteless, viscous substance, which is rich in polysaccharides, is extracted from the sap and roots of several species of Middle Eastern Astragalus, especially Astragalus tragacantha. There are records of the use of gum tragacanth as early as the third century BCE, a longevity matched by its versatility. Gum tragacanth is used in leatherworking, textiles, the production of artists’ pastels, paper marbling, cigar-making, and as a food additive (under the alias E413). In cake decorating, it is used as a fondant additive to alter texture, making the icing suitable for moulding delicate structures such as flower petals.

However, there are some species of Astragalus you would not want on your cake. These are the so-called ‘locoweeds’, which lead to neurological problems in animals that graze upon them. Many of these Astralagus species accumulate high levels of toxic selenium from the soil. Other locoweeds produce the toxic alkaloid swainsonine, although in some cases this alkaloid is produced by fungi living inside the plants, rather than the plants themselves. Selenium hyperaccumulation makes the genus useful in plant-based prospecting, a technique which uses plant populations to locate interesting mineral deposits. This can be achieved simply by looking for the presence of ‘indicator plants’, including certain Astragalus species, or by taking samples of plant material and conducting chemical analyses for selenium.

Further reading

Beach DC 1954. History, production, and uses of tragacanth. In: Natural plant hydrocolloids. American Chemical Society pp. 38-44.

White PJ 2016. Selenium accumulation by plants. Annals of Botany 117: 217-235.

Ruth Calder