Crown of thorns is the common name for the species Euphorbia milii. A member of the Euphorbiaceae family, it is one species among the over 2,000 species in the family. The genus Euphorbia, according to the GRIN website (APG III Taxonomic System) lists 528 species, subspecies, and varieties in the genus.
The genus name Euphorbia was first published by Carolus Linnaeus in the first edition of Species Plantarum in 1753. However, it can be traced back as far as 79 AD to the Roman officer, Pliny the Elder, as it is mentioned in his book, Natural History of Pliny. The generic name, Euphorbia, honors an African physician named Euphorbus. The species name, milii, commemorates Baron Milius, once governor of Reunion, who in 1821 introduced the species to France.
Aficionados refer to this group of plants as “euphorbs” and most of the leafy species are referred to as “spurges.” The vast majority originate from tropical and subtropical regions of Madagascar and eastern Africa, but some can also be found in tropical Asia and the Americas.
Recently I taught a National Garden Club’s Flower Show School Symposium in Palm Beach Gardens, Florida. One topic of study was Euphorbia milii, or crown of thorns. I was not surprised to see crown of thorns planted in highway and street medians, in residential landscapes and in the extensive landscaping of the hotel where the symposium was held.
The horticulture assistant responsible for assembling plants to aid in the study of crown of thorns was an avid collector who for many years collected all the different types she could find. I was amazed when I walked into the auditorium and saw her collection of 10- to 15-year-old specimens lined up across the front of the stage. She must have had 30 or more specimens ranging in size from five to six feet tall to miniature specimens only three to five inches tall. Blooms in colors of red, all shades of pink, peach, yellow, and white were exhibited.
Some time ago the use of crown of thorns as a landscape plant waned, but a recent resurgence in their popularity has occurred as new, compact types in a wide range of colors have been developed. Drought tolerance has made it a favorite of landscapers and homeowners wanting to conserve water. Salt tolerance has made it a useful shrub for landscapes near bodies of salt water, as many Florida landscapes are.
A succulent shrub or subshrub, crown of thorns has thick, fleshy, grayish brown, five- to seven-sided stems that store water. Noticeable to most people are the fierce, prominent spines that line the stems on all sides. Leaves are mostly obovate (oval shaped like an egg with a narrow end at the base where it attaches to the stem) and smooth in colors that range from grayish green to bright green. Leaves are arranged spirally on the stem, and foliage is produced on new growth.
The colorful part of the crown of thorns “flower” is not petals in the usual sense, but leaflike bracts called cyathophylls. The flower itself is different from other flowers in that it is composed of a structure called a cyathium. Geoff Stein gives a thorough discussion of this complex structure in his article entitled “Euphorbia ‘Flowers,’ an Introduction to the Amazing Cyathia.”
Crown of thorns flowers year round. In my Zone 8B garden, it stays outside during the summer, but in winter it comes inside to my sunny Florida room where it continues to bloom. Gradually, as the plant matures, more and more leaves drop and the spiny stem is exposed.
Most gardeners working with crown of thorns do not need to be reminded to wear gloves. Not only are the spines a menace to the ungloved hand, but the plant produces a poisonous milky sap that can irritate the skin. If sap gets on your hands, rinse it off immediately. If allowed to congeal, it becomes a more difficult task, for it is insoluble in water. An emulsifier like milk or soap will assist its removal. Avoid getting this sap in cuts or in the eyes.
Crown of thorns is easily started from tip cuttings. Like many other succulents, the cutting should be allowed to dry for three to four days until the cut end has callused over. A dip in rooting compound containing a fungicide is recommended before placing the cutting in a slightly moist, very well draining potting mix.
Maintenance is usually low for established plantings. Infrequent applications of water are needed. While plants are very drought tolerant, the leaves will drop in extreme drought. In the landscape, fertilize very lightly with complete fertilizer in May and October. Prune during cool, dry weather by removing dead and overly tangled stems. A few insects to watch for are scale, mealybugs, spider mites, and occasionally thrips. Disease problems may occur, expecially if plants are grown in places where the soil or the foliage stays wet for extended times. Protect plants if the temperature drops below 30°F.
Crown of thorns on the market today are mostly hybrids of Euphorbia milii and E. lophogona and are usually designated as Euphorbia ×lomi. Three main groups are recognized.
California hybrids developed starting around 1960 by Humel are often called giant crown of thorns. Prized for their stout stems and large cyathophylls, the name E. ×lomi California Group has been proposed for this group of plants.
Another group, referred to by some experts as E. ×lomi Heidelberg Group are similar in appearance but have thicker leaves and thinner stems. Collected in the wild and propagated commercially in Germany, this group is harder to find in United States markets but may be available through specialist growers.
Much work has been done in Thailand in recent years. The Thai hybrids include plants with larger cyathophyls and come in a wide range of color combinations. They have an upright form that does not become a tangled mass like some of the other types. Leaves tend to be larger and brighter green. The botanical name E. ×lomi Poysean Group has been proposed for this group of cultivars.
Choose a crown of thorn that fits your space. Expect color for most of the year from this accommodating group of plants. At my garden club’s recent Christmas flower show, flowering specimens added pizzazz and held their own among the Christmas cacti, poinsettias, kalanchoes, and other seasonal bloomers.
PLEASE MOUSE OVER PICTURES FOR IDENTIFICATION OF THE PLANT AS WELL AS CREDIT TO THE PHOTOGRAPHER.
About Marie Harrison
Serving as a board member for Valparaiso Garden Club, the Florida Federation of Garden Clubs and the Deep South Region, and National Garden Clubs takes a chunk of my time and attention. Being a Master Flower Show Judge, a Floral Design Instructor, instructor of horticulture for National Garden Clubs, and a University of Florida Master Gardener crowds a bit more into my busy days. In addition to these activities, I contribute regularly to Florida Gardening magazine and other publications. I am author of four gardening books, all published by Pineapple Press, Sarasota, Florida. Read about them and visit me at www.mariesgardenanddesign.com.