We can blame the rain for the locoweed.

It rained in Marfa last summer, which is when we get most of our annual rainfall, and then it kept raining in October and November, along with January, February, March, and April too. Don’t get me wrong: rain is welcome in the desert. But its presence in these typically drier months was startling.

It was also tantalizing. Over the course of the three years that we’ve lived at our place, we’ve kept our horses off the land for ten months of every year to encourage the growth of grass in pastures that were grazed to bare earth long ago. I’d hoped that the unusual months of rainfall would allow the seed bank under our pastureland to magically erupt into waving green fields of good grama. Instead, something altogether more vexing sprang up: locoweed.

I didn’t know that locoweed was a real thing until I moved to Marfa a couple of decades ago. Before then, my knowledge of the plant was limited to whatever I’d gleaned from the black and white westerns and reruns of Fury I watched as a child in suburban, air-conditioned Fort Worth. In those old shows, the presence of loco was the source of grave concern for the cowpokes on-screen. The herd will go crazy! They’re all goners! I thought it was a legend, like chupacabras or the fierce cemetery-roaming snipe we were sent to hunt at night during YMCA camp.

I was wrong about loco, and I’m not the only one who was misinformed. While chatting with visitors to Marfa the other day, I cautioned them about the locoweed they’d plucked and held in their hands. “Locoweed? This isn’t marijuana,” one of them chided me.

Well, it’s not marijuana, but you really shouldn’t touch it, much less imbibe it. Every part of locoweed is toxic, from its blooms to its roots. Locoweed is a squat, robust plant that sends up blue, purple, or white flowers. Its seeds lie nestled inside weird pods that, when stepped upon, sound unnervingly like tiny, fragile bones breaking underfoot. Even when locoweed is dead and reduced to dried-up sticks on the ground, it’s still toxic and can stay toxic for years. The seeds can remain dormant forty years or more before germinating.

Locoweed tends to grow in the fall and winter, when it’s practically the only green thing on the range. In winter, grass is weathered and brown, and forbs are few, so livestock and wildlife will give locoweed a taste if it’s the only palatable fare around. Cattle, sheep, elk, and deer can be affected by loco, though horses are particularly susceptible. While some horses can graze in a field pocked with loco and never eat it, others develop a taste for it and, after a few weeks, seek it out exclusively. The plant’s toxicity involves an alkaloid called swainsonine, which, among other things, messes with protein synthesis in animals that ingest it and causes swelling in the nervous system. Selenium can also accumulate in locoweed at levels far greater than is safe for horses. The effects are brutal.

I saw this firsthand a few years ago. A friend’s buckskin mare had been pastured for some months with a band of broodmares that roamed and foraged upon thousands of acres of ranchland. A ranch worker checking on the band noticed that my friend’s mare looked poor and penned her. “Could we check her out?” my friend asked.

When we got there, the mare stood sullen and blinking. Her coat was dark with sweat, although it was not hot, and she was unsteady on her feet. She’d grown ribby, wasted. We slipped a halter on her and urged her to walk to the horse trailer. Before we coaxed her inside, the mare halted and stared with great intensity at a point on the ground. I remember her quivering flanks and her widened, astonished eyes. She appeared to be hallucinating.

At the vet, twenty minutes later, she had noticeably worsened. Instead of walking normally, she goose-stepped into the vet’s pen, and as he began to examine her, she abruptly roared like a predator, bared her teeth, and lunged at him, chasing him and very nearly catching him as he vaulted to safety up and over the panels of the pen. She tried to climb after him. When she didn’t make it, she tried again. Her sides heaved and she glared at the vet and then, with a terrible rhythm, began to bash her head against a steel pole.

“This animal,” the vet said, panting slightly, “is clinically deranged.”

My friend euthanized the poor mare. Since some of her symptoms suggested both locoweed poisoning and rabies, the vet could not be certain of his diagnosis until the mare’s brain was analyzed. The test results came back within a day or two and confirmed that rabies was not the culprit. It was loco.

Horses who are severely locoed, like my friend’s mare, never recover. Those that are caught earlier in the poisoning can experience periods of seeming normalcy, but they can never really be considered safe. Their sensibilities can unravel without notice. Their sanity unspools. You don’t want to be in a small space, like a trailer or a stall, when that happens. You don’t want to be riding the horse when that happens, or have your child nearby. You don’t want any of it to happen at all.

Dozens of varieties of locoweed flourish worldwide. About twenty types grow across the American West, including several in Texas. The woolly and white locoweeds are common varieties, though some, like Big Bend loco, grow only in specific ecoregions.

Wooton’s loco, also called garboncillo, appeared in our south pasture in late fall. Initially it didn’t seem too daunting, the kind of thing you could control with a bit of effort and perseverance. On Thanksgiving Day, I handed hoes to my visiting relatives and, instead of watching football, we went to work grubbing loco as the turkey roasted in the oven. I thought I was on top of the outbreak, but within a few weeks, there was more, and then still more. I spent afternoons hoeing the stuff only to realize a day or two later that I’d hardly made a dent in the invasion.

The county extension agent eventually came out in the spring to see if she could offer any guidance. “Gosh,” she said. “This is really a carpet, isn’t it?”

It was. To be fair, other opportunistic plants arrived as well, forbs and tumbleweeds mostly, and precious little grass. The pastures looked invitingly green from a distance, though nearly all of it was weeds. “At least you’ve got something holding down the topsoil,” the county agent offered. “That’s something.”

The amount of loco on our small acreage means we’re not likely to turn the horses out to graze this summer. That’s a bummer, for the horses prefer green grass to hay, and from an economic standpoint, grass is free while hay is not.

In a stab at optimism, I think about my late friend Sherman, a game warden and rancher, who always said that rain in the fall made for good wildflowers the following year. I wish he were around to see the flowers now, for they’re more abundant and riotous than any time in recent memory. Delicate primroses, the color of a blush; flame-petaled firewheels; pink and orange globe mallows; chocolate daisies, which, with some imagination, smell like chocolate; hardy prairie verbena; and to the south of us, bluebonnets standing hip-high.

By May the loco was yellowed and dying, though sure to return someday. We’ve thought about ways to clobber it into submission—fire, herbicide, mowing—but a soil expert suggested letting the land continue to rest and recover. Spreading horse manure out there, we’re told, will cover the existing loco, enrich the soil, and entice grass. While this treatment requires patience and time, it feels about right. Prescribed burns are useful, but a fire so close to town is too chancy. Herbicides and mowing would wipe out the flowers; let’s not do that.

Traipsing through our pasture recently, we discovered a new flower, one we’ve never seen before, in clumps amid the loco. Its petals are dark-blue curls adorned with teeny silver dots. The dots glisten like dewdrops in sunlight, and that is what I want to see.