Locoweed is a low-growing, herbaceous plant in the pea family, recognizable by its clustered compound leaves covered in fine silvery hairs and its showy spikes of tubular flowers in white, purple, pink, or blue. The name “locoweed” is not a single species but a blanket term covering dozens of plants in two closely related genera, Astragalus and Oxytropis, that share a family resemblance but differ in telling details. Learning those details matters because locoweed is one of the most economically damaging poisonous plants across the western United States and parts of Canada, and telling it apart from harmless prairie wildflowers takes a closer look than most people expect.
The Pea-Family Blueprint
Every locoweed species belongs to the legume family (Fabaceae), the same family as garden peas, clovers, and lupines. That family tree gives locoweed its basic architecture. The leaves are pinnately compound, meaning a single leaf stalk carries many small, paired leaflets arranged like a feather. Depending on the species, a single leaf may hold anywhere from seven to over thirty leaflets, each one narrow and oval, usually less than an inch long. Most locoweed species are covered in fine, silvery or grayish hairs that give the foliage a fuzzy, almost frosted look. On a bright day in open grassland, that silvery sheen is often the first thing that catches your eye.
The flowers are another signature. Locoweed blooms in dense, upright clusters called racemes that rise above the leaf rosette on leafless stalks. Each individual flower has the classic pea-flower shape: a broad upper petal (the banner), two narrow side petals (the wings), and a lower boat-shaped structure (the keel) formed by two fused petals. Flower color varies by species, from pure white to deep purple, with some species blooming in shades of pink, lavender, or yellowish cream. Bloom time is typically late spring through midsummer in most western habitats, though timing shifts with elevation and latitude.
After flowering, locoweed produces inflated, papery seed pods that look like small, puffy bladders. These pods are distinctive in their own right and persist on the plant well into fall, making them a useful identification feature even after the flowers have dropped. In many species the pods are partially enclosed by the dried flower calyx, and some species produce pods that are noticeably hairy or mottled.
The Key Distinction Between Astragalus and Oxytropis
The single most useful identification trait for separating the two main locoweed genera is the tip of the keel petal. In Oxytropis species, the keel comes to a distinct, sharp point, almost like a tiny beak. In Astragalus species, the keel tip is blunt or rounded. You need to look closely at a fresh flower to see this, but once you know what to look for, it is a reliable field character that works across species.
Growth habit also helps. Most Oxytropis locoweeds are acaulescent, meaning they lack a true above-ground stem. Their leaves and flower stalks all radiate directly from a basal rosette at ground level, giving the plant a tufted, cushion-like shape. Many Astragalus species, by contrast, have leafy stems that rise above the ground, giving them a more upright or sprawling look. There are exceptions in both genera, but the “stemless rosette with a pointed keel” combination reliably points you toward Oxytropis.
White Locoweed
White locoweed (Oxytropis sericea) is probably the most widespread and commonly encountered species in North America. Its range stretches from the Yukon Territory east to Manitoba and south through the Rocky Mountains and Great Plains to Nevada, New Mexico, and Texas.1PubMed Central. Oxytropis sericea, white locoweed The name comes from its creamy white flowers, though some populations show a faint lavender or bluish tinge, especially at higher elevations.
The plant forms a dense basal rosette typically six to twelve inches tall. Leaves carry nine to nineteen narrow leaflets densely coated in silky, appressed hairs that give them a silvery green color. The flower stalks are also hairy and stand slightly taller than the leaves, each topped with a tight raceme of ten to thirty white, pea-shaped flowers. After pollination, the pods are somewhat leathery and partially divided inside by a longitudinal partition, a feature shared with several Oxytropis species.
White locoweed favors open, well-drained slopes on sandy, gravelly, or rocky soils. It grows best on sandy loams, tolerates moderately saline and nutrient-poor conditions, and avoids waterlogged clay soils.1PubMed Central. Oxytropis sericea, white locoweed You will find it on prairie uplands, foothill grasslands, stream banks, and grassy subalpine openings, sometimes growing alongside sagebrush, grama grasses, or scattered conifers. Elevation range is broad: from around 3,000 feet in Montana up to 11,000 feet in Colorado.1PubMed Central. Oxytropis sericea, white locoweed
Purple Locoweed and Lambert’s Locoweed
Lambert’s locoweed (Oxytropis lambertii) is the other major species most people encounter in the western U.S. Its flowers are usually a vivid purple or magenta, though pink and bicolor forms exist. The overall plant structure is similar to white locoweed: a stemless rosette of compound, silky-haired leaves with flower stalks rising above the foliage. Lambert’s locoweed tends to be slightly more slender and delicate-looking than white locoweed, and its leaflets are often narrower.
Lambert’s locoweed includes several recognized varieties. Var. lambertii is the most widespread, found across the central and northern Great Plains. Var. bigelovii occurs in the southern part of the range, particularly in southern Utah, Arizona, and southwestern New Mexico. Var. articulata has a more restricted distribution.2Springer. Distribution of locoweed toxin swainsonine in populations of Oxytropis lambertii These varieties look quite similar in the field, and telling them apart usually requires examining pod characteristics or knowing the geographic location.
Other purple-flowered locoweeds exist as well. Woolly locoweed (Astragalus mollissimus) is an Astragalus species with lavender to purple flowers and conspicuously woolly foliage. Because it belongs to Astragalus rather than Oxytropis, it typically has a short leafy stem rather than a purely basal rosette, and its keel petal is blunt rather than pointed. The dense woolly hairs are longer and shaggier than the silky hairs of Oxytropis species, giving the plant a distinctly furry texture.
Common Look-Alikes and How to Tell Them Apart
Locoweed shares its grassland habitat with several harmless or even beneficial legumes that can confuse identification. Here are the most common sources of mix-ups:
- Lupines: Lupine leaves are palmately compound, meaning the leaflets radiate from a single point like fingers on a hand, rather than being arranged in pairs along a central stalk. Once you know this distinction, lupines and locoweeds are easy to separate even without flowers.
- Clovers and vetches: Clovers have trifoliate leaves (three leaflets per leaf) and round flower heads, not elongated racemes. Vetches are climbing or trailing plants with tendrils at the tips of their leaves, something no locoweed produces.
- Non-toxic Astragalus species: This is the trickiest category. The genus Astragalus contains hundreds of species in North America, and many are not toxic. Without checking flower structure, pod shape, and hair characteristics against a regional field guide, separating a toxic Astragalus locoweed from a benign milkvetch is difficult even for experienced botanists. When in doubt on rangeland, treat any low-growing, hairy-leaved legume with inflated pods as potentially toxic until confirmed otherwise.
Habitat context helps narrow things down. If you are on dry, well-drained rangeland in the western states between 3,000 and 10,000 feet of elevation and you see a stemless rosette of silvery, compound leaves with white or purple pea flowers, the probability that you are looking at locoweed is high.
Where Locoweed Grows
Locoweed poisoning has historically been concentrated in the states west of the Mississippi River, where Astragalus and Oxytropis species are both abundant and diverse.1PubMed Central. Oxytropis sericea, white locoweed The core zone for livestock losses runs from Montana and the Dakotas south through Wyoming, Colorado, Utah, and New Mexico, with significant presence in Arizona, Nevada, Idaho, and parts of Texas. White locoweed reaches north into Canada’s prairie provinces and the Yukon, meaning the hazard extends well beyond the U.S. border.
Locoweed is a classic disturbance-adapted plant. It thrives on overgrazed pastures, drought-stressed rangelands, and areas where competing vegetation has been thinned out by fire or poor soil conditions. In years of drought, when palatable grasses are scarce, locoweed may be the greenest thing available to cattle and horses, which is when poisoning incidents spike. Understanding where locoweed is likely to flourish is almost as useful as knowing what it looks like, because the two pieces of information together turn a fuzzy visual resemblance into a confident identification.
Why Identification Matters: the Swainsonine Connection
The reason people care about identifying locoweed is not just botanical curiosity. These plants can produce swainsonine, an alkaloid toxin that causes a chronic neurological syndrome in livestock known as locoism. Animals that graze on locoweed over a period of weeks develop staggering gait, weight loss, depression, impaired vision, and reproductive failure. The word “loco” is Spanish for crazy, a reference to the erratic behavior of affected animals.
Interestingly, locoweed does not manufacture swainsonine on its own. The toxin is produced by an endophytic fungus living inside the plant’s tissues. In Oxytropis species, this endophyte belongs to Alternaria sect. Undifilum.3PubMed Central. Rethinking of the Roles of Endophyte Symbiosis and Mycotoxin in Oxytropis Plants The fungus lives quietly within the plant without causing visible disease, and the amount of swainsonine a given plant produces is closely tied to the endophyte strain it harbors.4PubMed. Production of swainsonine by fungal endophytes of locoweed High-quality genome assemblies of both the locoweed host and its fungal partner have confirmed this symbiotic relationship at the molecular level.5PubMed. Assembly of high-quality genomes of the locoweed Oxytropis ochrocephala and its endophyte Alternaria oxytropis provides new evidence for their symbiotic relationship and swainsonine biosynthesis
This matters for identification because it means you cannot tell whether a locoweed plant is toxic just by looking at it. Two plants of the same species growing a few miles apart may differ dramatically in toxicity depending on which fungal strain they carry, or whether they carry the endophyte at all.
Not Every Locoweed Plant Is Equally Dangerous
A field survey of Lambert’s locoweed populations across the western U.S. found striking geographic variation in toxicity. Among the three recognized varieties, only var. bigelovii contained detectable levels of swainsonine, and only in populations from the southwestern portion of its range in southern Utah, Arizona, and southwestern New Mexico. Populations of var. bigelovii from eastern Utah, Colorado, and northeastern New Mexico had no detectable swainsonine. The other two varieties, lambertii and articulata, also tested negative.2Springer. Distribution of locoweed toxin swainsonine in populations of Oxytropis lambertii
This finding underscores something counterintuitive: a plant can look exactly like a textbook locoweed, belong to a species with a long history of poisoning livestock, and still be non-toxic. The toxin depends on the fungal endophyte, and the endophyte’s presence varies by population, region, and even individual plant. For ranchers, this means that local knowledge and regional testing data are more useful than species identification alone when assessing pasture risk. For hikers or curious naturalists, it means that finding a locoweed plant does not necessarily mean the area is a minefield for animals, though caution is still warranted.
What Locoism Actually Does to Animals
Animals that eat locoweed consistently over several weeks develop a progressive poisoning syndrome. The swainsonine toxin inhibits enzymes involved in sugar processing inside cells, leading to abnormal accumulation of complex sugars in cellular compartments. This creates widespread cellular swelling and damage, particularly in nerve cells. Affected animals show neurological symptoms first: stumbling, head-bobbing, exaggerated reactions to stimuli, and apparent confusion.
The damage is not limited to the brain. Studies of cattle, sheep, and horses with locoweed poisoning have documented distinctive cellular changes in the eyes, including marked swelling and vacuolation of retinal ganglion cells, the cells of the ciliary body that help focus the lens, and the secretory tissue of the tear glands.6American Journal of Veterinary Research. Ophthalmic Lesions in Locoweed Poisoning of Cattle, Sheep, and Horses These eye changes help explain the impaired vision that is a hallmark of locoism. Reproductive problems are another major consequence: pregnant animals that eat locoweed are at elevated risk of abortion, and surviving offspring may have congenital abnormalities.
If animals are removed from locoweed pastures early enough, some recovery is possible, though neurological damage may be permanent in severe cases. The practical challenge is that livestock often develop a preference for locoweed after initial exposure, actively seeking it out even when other forage is available. This behavioral component makes management difficult and makes early identification and removal of locoweed from pastures so important.
Seasonal Changes in Appearance
Locoweed looks different depending on the time of year, which complicates identification for people who only know the flowering form. In early spring, the plant emerges as a tight rosette of fresh, silvery-green compound leaves pressed close to the ground. At this stage, it could easily be mistaken for a young milkvetch, a prairie clover seedling, or any number of small-leaved legumes. The silky hair covering and the paired-leaflet arrangement are the best clues at this point.
By late spring and into summer, the flower stalks elongate and the racemes bloom, making identification much easier. The combination of a basal rosette, hairy foliage, and a dense spike of pea-shaped flowers is distinctive even at a distance. This is the easiest time of year to spot and confirm locoweed.
By late summer and fall, the flowers have faded and the inflated seed pods become the most prominent feature. The pods dry to a papery, straw-colored brown and may rattle when shaken, earning some species the folk name “rattleweed.” In winter, the above-ground parts die back in most areas, but the dried stalks and pods may persist, and the perennial root crown remains alive underground, ready to send up new growth the following spring. On heavily infested rangeland, the dried remnants of locoweed are visible even under a light snow cover as small clusters of brown, brittle stems poking through.
Practical Tips for Field Identification
If you are trying to confirm a locoweed identification in the field, here is a practical approach that works without a microscope or a botany degree:
- Check the leaves: Compound, with paired leaflets along a central stalk. Covered in fine, silky or woolly hairs. If the leaves are smooth and hairless, it is probably not locoweed.
- Check for a stem: If the plant is stemless with all leaves and flower stalks arising from a ground-level rosette, you are likely looking at an Oxytropis species. If it has a leafy stem rising above the ground, it could be an Astragalus species or something else entirely.
- Examine a flower: Pea-shaped, in a dense upright cluster. If the keel petal comes to a distinct point, that confirms Oxytropis.
- Look at the pods: Inflated, papery, sometimes hairy. Many locoweed pods have a partial internal partition running lengthwise.
- Consider the habitat: Dry, open rangeland with well-drained soil at moderate to high elevations in western North America is prime locoweed territory.
No single feature is definitive by itself. The combination of silvery compound leaves, a basal growth habit, pea-family flowers, inflated pods, and a western rangeland setting is what makes the identification convincing. When you need certainty for livestock management decisions, send a sample to your state’s agricultural extension service. They can confirm the species and, in some cases, test for swainsonine content, which tells you whether the population in your pasture actually poses a toxicity risk.
Locoweed in Chinese Rangelands
Locoweed is not exclusively a North American problem. Several Oxytropis species are major livestock hazards on rangelands in northern and western China, where the same endophytic fungus relationship drives toxicity.3PubMed Central. Rethinking of the Roles of Endophyte Symbiosis and Mycotoxin in Oxytropis Plants Chinese species like Oxytropis ochrocephala (yellow-flowered locoweed) and Oxytropis kansuensis share the same general growth form and toxic mechanism as their North American relatives, though the flower colors and specific leaflet counts differ. Research into these Chinese populations has contributed significantly to understanding how the fungal endophyte produces swainsonine, with high-quality genome assemblies of both plant and fungus now available.5PubMed. Assembly of high-quality genomes of the locoweed Oxytropis ochrocephala and its endophyte Alternaria oxytropis provides new evidence for their symbiotic relationship and swainsonine biosynthesis
For anyone trying to identify locoweed outside the familiar North American context, the same general approach works: look for the combination of compound silky leaves, pea-family flowers on leafless stalks, inflated pods, and a preference for dry, well-drained grassland. The genus Oxytropis is large and widespread across the Northern Hemisphere’s temperate and alpine zones, and while not every species is toxic, the visual toolkit for recognizing them is remarkably consistent across continents.