Astragalus mollissimus aka Purple Locoweed

Astragalus mollissimus is a challenging plant to cultivate outside its native habitat. It requires full sun, excellent drainage, and tolerates drought conditions. Its highly toxic nature means it should never be grown where livestock, pets, or children may access it. This is primarily a wild rangeland species rather than a garden plant.

Propagation is typically done from seed. Some growers also recommend cold stratification for 2-3 weeks to improve germination rates. Sow seeds about 6mm deep in well-draining sandy soil. Germination can be slow, taking up to 9 weeks. Because the plant develops a deep taproot, it should be started in its permanent location or transplanted while very young.

Woolly Locoweed thrives in dry, sandy, or gravelly soils with excellent drainage. In its native habitat, it grows in desert scrub, grasslands, and pinyon-juniper woodlands at elevations from 1,000-2,700 meters. The soil pH should be neutral to slightly alkaline. Like other legumes, it can fix atmospheric nitrogen through root nodule bacteria, allowing it to grow in relatively poor soils.

Woolly Locoweed is native to the western and central United States and northern Mexico. Its range extends from southwestern South Dakota through Kansas, Colorado, Oklahoma, Texas, New Mexico, Arizona, Utah, Nevada, and Wyoming, continuing into central and northern Mexico. It is particularly abundant on short-grass prairies and is commonly found in the Trans-Pecos and Panhandle regions of Texas, as well as the Colorado Plateau.

Woolly Locoweed produces attractive pea-like flowers from March through July in elongated spikes of 7-45 blooms. Flower color is variable, ranging from purple and lavender to pink, and occasionally yellow or cream. Each flower is 1.5-2.5 cm long with the classic legume structure: an upright banner petal with a white center marked by red lines, two forward-pointing wing petals, and two keel petals. The flowers mature into distinctive thick, inflated, densely hairy, moon-shaped seedpods.

Woolly Locoweed is pollinated by insects, primarily bees, moths, and butterflies. The pea-like flowers are characteristic of the Fabaceae family and are adapted for insect pollination. Bees are particularly important pollinators for Astragalus species, with various native bee species visiting the flowers for nectar and pollen. The flowers provide valuable resources for pollinators in the dry grassland and desert habitats where the plant grows.

Woolly Locoweed is a compact, low-growing perennial that typically reaches less than 30 cm (1 foot) in height. The plant forms dense, rounded clumps with sprawling stems 30-45 cm long that arch upward at the tips. A thick, woody taproot anchors the plant and allows it to survive in dry conditions. The leaves are compound with 15-35 small leaflets, reaching up to 28 cm long. The plant's growth is moderate, with populations cycling based on moisture availability.

Astragalus mollissimus is exclusively an outdoor plant, thriving in USDA hardiness zones 4-9 based on its native range from South Dakota to Texas and Arizona. It requires full sun and well-draining sandy or gravelly soil. The plant is found at elevations from 1,000-2,700 meters in desert scrub, grasslands, and pinyon-juniper woodlands. Due to its high toxicity to livestock and potential harm to pets, it should only be grown in areas where animals cannot access it.

Astragalus mollissimus is a variable species with 10 recognized varieties. These include var. bigelovii, var. coryi (Cory's Woolly Locoweed), var. earlei, var. irolanus, var. marcidus (Withered Woolly Milk-vetch), var. matthewsii, var. mogollonicus, var. mollissimus, var. nitens, and var. thompsoniae (Thompson's Woolly Locoweed). The varieties differ in flower color, pubescence, and size. Two varieties (irolanus and nitens) occur only in Mexico, while the others are found in the United States. Var. marcidus has received federal conservation attention.

Astragalus mollissimus is generally resistant to pests and diseases in its native habitat. Interestingly, the plant hosts an endophytic fungus (Undifilum oxytropis) that produces the toxic alkaloid swainsonine. Insect damage can affect population dynamics—Texas A&M notes that populations may die back from insect damage in addition to drought. The plant's toxic nature likely provides some protection from herbivory by mammals.

Woolly Locoweed is adapted to the continental climate of the southwestern United States, tolerating hot summers and cold winters across its range from South Dakota to Texas. It can withstand temperatures typical of USDA zones 4-9, meaning it survives winter lows down to approximately -30°C (-20°F) in the northern parts of its range. The plant goes dormant in winter and emerges early in spring, often flowering as early as March.

In its native habitat, Woolly Locoweed follows natural seasonal cycles without intervention. The plant emerges in late winter to early spring, flowers from March through July, sets seed, and then goes dormant during hot, dry summers and cold winters. Its population is cyclic, increasing in wet years and dying back during drought. If cultivated, the plant needs no supplemental water once established and should be left to follow its natural dormancy cycle.

Purple Locoweed should be watered regularly, allowing the soil to dry out between waterings.

Purple Locoweed requires abundant, bright and direct light. Placing it less than one foot from a window is the best way to make sure the plant receives enough light to survive and thrive. Depending on your region, however, the current weather can affect the placement of the plant in your home. For example, in some areas the sun may be too direct and intense, so it's best to place it farther away from the window. You may also need to adjust your placement according to the season as the sunlight can change intensity over the course of the year. If you live in a region with significant temperature fluctuations, such as a desert, you may need to place the plant a bit farther away from the window to avoid extreme heat or cold. With some care and attention, your Purple Locoweed will be sure to bring you many years of beauty and enjoyment.

Ploi does not possess verified information regarding the toxicity of this particular plant. In the event that you, a member of your family, or a pet ingests plant material with an unclear toxicity level, it is highly recommended to seek the advice of a medical professional.