A: Crown-of-Thorns, Euphorbia milii, a native of Madagascar, is a common garden plant in southern states, especially Florida. It grows best in cold hardiness zones 10-11 which means South Florida for us. Typically it will die back during harsh, cold temperatures here in Northeast Florida so protect it from freezing temperatures.
Crown-of-Thorns produce flowers best in full sun and allow the soil medium to dry out between watering. Overwatering this plant can quickly cause root decay. It is a low-growing evergreen shrub with very thorny grooved stems and branches. The small flowers are produced in clusters of 2-8 at the tips of green flower stem about 1 inch long. The colors of the flowers are pink, red, yellow and white. It is salt and drought tolerant. Genus Euphorbia includes other commonly available plants such as poinsettia.
All parts of the Crown-of-Thorns plant are poisonous. Generally this group of plants is not appetizing to most animals but they will eat it if their normal food supply becomes limited. Drying does not destroy the toxicity of the plant, and Euphorbia in hay may be slightly more palatable to livestock. Contact with the white, milky sap may cause severe blistering as well as intense pain to open cuts or eyes. Honey made from the flowers of these plants may be toxic. Generally horses, cattle, sheep, cats, dogs and humans are affected by Euphorbia and may experience severe irritation of the mouth and gastrointestinal tract, sometimes with hemorrhage and diarrhea. Other general signs include blistering, swelling about the eyes and mouth, excessive salivation and emesis, abdominal pain and weakness. The sap may cause dermatitis. Death is rare. Work horses may suffer severe blisters and loss of hair on the ankles.