Buckwheat is a very fast growing plant that has hairless stems. The leaves vary in size and shape, ranging from an arrow-like shape to a heart shape.
The flowers are small and white, and the seeds are small 3-sided angular shapes, which turn brown when they are ripe.
This plant can be found throughout North America. In some areas it is considered an escaped weed, while in some areas it is purposefully planted to provide ground cover or for harvest for buckwheat flour.
It usually can be found in disturbed soils, but very rarely does it persist for long. It is usually found along roadsides and ditches.
All parts of the plant are toxic in both fresh and dried states. The only exception is the ripe seed, which does not contain any of the toxic substance.
This plant is toxic because it contains a pigment called "fagopyrin" (thus its scientific name!). Fagopyrin causes photosensitive dermatitis in horses. The photosensitive dermatitis is caused by the toxin in the blood vessels reacting with ultraviolent rays. The resulting radiant energy causes damage (sometimes severe enough to slough off skin!) to the blood vessels and the skin.
The most prominent symptom of this photosensitive dermatitis is a weeping itchy skin inflammation on any area that has been exposed to sun. Horses will also become very agitated in the sunlight, often trying to get under or behind anything that will block the sun exposure.
Horses with white skin are most often affected. If a horse with dark skin suffers from buckwheat poisoning, his eyes may be affected if he does not have pigmented eyes, but otherwise he will be relatively unaffected.
The horse should be immediately moved out of the sun, and kept away from all UV light. Any St. John's Wort should be immediately removed from the diet.
Antihistamines as well as anti-inflammatory drugs may be useful in the initial stages after poisoning.
Another image, showing more arrow-like leaves:
USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. Vol. 1: 672.