Crown of thorns, Euphorbia milii, is a semi-succulent shrub capable of year-round blooming. Since flowering is solely dependent upon the amount of light the plant receives, pruning won't interfere with its bloom production. This plant benefits from and responds well to judicious pruning. Young specimens usually don't require any pruning until their second or third years. Euphorbia is a slow grower that typically only needs to be pruned lightly every two or three years thereafter. Wait for cool, dry spring weather to minimize the possibility of disease entering open wounds.
Remove any dead, damaged or diseased stems with pruning shears.
Cut out stems that are--or threaten to become--excessively tangled or intertwined.
Prune stems wherever you think the plant could use branching. Shorten any that have become too long or unattractive. Trim back any growth that doesn't appeal to you.
Cut particularly leggy Euphorbias back by half to promote branching of larger main stems. You can safely remove from 1/3 to 2/3 of this plant at a time. New growth with emerge from just below the cuts, resulting in a fuller plant.
Spritz the plant's cut stems with water to staunch the copious flow of sap secretions.
Wash your hands thoroughly after handling this plant.