Crown of thorns (Euphorbia milii) is a pretty succulent plant that can bloom almost year-round, even indoors. The thick, bright green leaves grow along the new stem growth, but watch out for thorns. The true flowers are small and green, surrounded by showy bracts in red, orange, pink, yellow, or white. This slow-growing plant will grow into a shrub reaching 3 to 6 feet tall outdoors. As a houseplant, expect it to reach only about 2 feet in height. Euphorbia is poisonous to humans and pets.