This slow-growing plant can reach 3 to 6 feet tall outdoors ... but avoid lopping off healthy buds. Is Euphorbia milii a cactus? Crown of thorns is a succulent but not a cactus.

The firestick plant (Euphorbia tirucalli ... Once the plant begins to grow in the spring, apply a cactus or all-purpose houseplant fertilizer, diluted to half strength. For outdoor plants ...