Horses can eat toxic plants and dangerous flowers either in their grazing pastures or when flowers end up in their cultivated hay. Any unidentified flower in your horses’ pasture should be suspect, and determining whether the flowers are poisonous to your horses can save their lives.
Oleanders (Nerium oleander) are flowering shrubs that have showy, white, yellowish, pink or red blossoms and evergreen, leathery, narrow leaves. All parts of the oleander shrub contain a poisonous sap that's sticky or gummy in consistency. Oleanders are highly toxic to horses when ingested and can cause death.
Water hemlock (Cicuta spp.) is a highly poisonous plant that grows erect and up to 8 feet tall. It has smooth, hollow stems that are lined with purple streaks and blooms in open, umbrella-like whitish flowers. This perennial flower contains an oil-like liquid in its stems that's extremely toxic to horses when eaten, causing colic, nausea, violent convulsions and death. Horses can also become poisoned by drinking the water in ditches or small pools near which the water hemlock plant has been trampled. Also toxic and appearing similar is poison hemlock (Conium maculatum), which has reddish-purple spots instead of streaks on the stems.
St. John's Wort
Common St. John's wort (Hypericum perforatum) has bright-yellow flowers that are 3/4 to 1 inch wide and have black dots on the petal edges. Growing up to 3 feet tall, St. John's wort is often found growing in pastures, abandoned meadows, waste areas and along roadsides. The flowers and leaves contain the poisonous substance hypericin, which in horses causes skin irritation, hair loss or blisters, as well as loss of appetite, blindness, convulsions, hypersensitivity to cold water, coma and even death.
Commonly found growing in and near shady, moist woods, white snakeroot (Eupatorium rugosum) has egg-shaped, sharply-toothed leaves that grow opposite each other in pairs along the tall, smooth stems. White snakeroot doesn’t begin blooming until after July, bearing small white flowers in loose clusters. This poisonous perennial contains tremetol in its leaves and stems, which causes spasms of the leg muscles, sweating, difficulty breathing, constipation, hyperventilation, dilated pupils, stiff gait, weakness and sudden death. The tremetol toxin also becomes highly concentrated in the milk of lactating horses, which can poison the colts that drink it.
Flowers of the nightshade family (Solanaceae) contain the poisonous compound glycoalkaloid, or solanine, in the shoots, leaves and unripe berries. Depending on the nightshade species, the flowers may be white or purple and usually appear in open clusters or have five petals forming a star-shaped blossom. Nightshades are usually found growing along stream banks, roadsides, row-crop and open fields, thickets, wet ditches, pastures, and other disturbed sites. When horses consume nightshade plants, they’ll experience central nervous system disturbances, gastrointestinal problems, hemorrhaging gastroenteritis, weakness, poor coordination, difficulty breathing, excessive salivation and even death.
Tansy Ragwort or Groundsels
Groundsels or tansy ragwort (Senecio spp.) is an erect biennial that reaches 1 to 3 feet tall and blooms in bright lemon-yellow, clustered, up to 1-inch-wide flowers from April until June. All parts of groundsel plants are toxic to horses, containing pyrrolizidine alkaloids, such as senecionine, which cause anorexia, depression, loss of coordination, diarrhea, hemoglobin in the urine and cirrhosis of the liver, often leading to death.