Plant Highlight: Euphorbia
Euphorbia is a large genus, with about 2,000 species and a bewildering array of forms. Some are pernicious weeds with tiny leaves, while others are shrubby or even grow to be impressive trees. A significant number of them are succulent, though these constitute a minority of the species in the genus. The succulent types are particularly concentrated in Africa, and eastward to Madagascar and India. Even among this subset of the genus, there are many different groups whose appearance varies radically. One group of horticultural interest is known as “crown-of-thorns”, consisting of Euphorbia milii and its close relatives. These natives of Madagascar have very spiny succulent stems and non-succulent leaves.
Euphorbias have a unique cup-like structure which holds the flowers and also has glands and appendages which are of great use in differentiating similar species. This flower-bearing structure is called a cyathium, and though each one is not large, they can be showy if they are present in large numbers. In some species, the prominence of the flowers is greatly enhanced by colorful bracts (actually modified leaves surrounding the flowers), and the poinsettia is a well-known example of this. A crown-of-thorns also has showy bracts, and though these are not as large as those of a poinsettia they are similarly colored in shades of red, pink, white, pale yellow and salmon.
Euphorbia milii is a woody shrub, and different forms of it vary in size from a foot tall to over 4 feet (30 cm to well over a meter). In nature, the usual color for the pair of rounded bracts around the flowers is vivid red, but selection and hybridization have resulted in a range of different colors on cultivated plants. Though this species is not very cold-tolerant, we are able to grow it in the ground at the Ruth Bancroft Garden with the help of a protective cover placed over it in winter. The flowers with their scarlet bracts are very freely produced, and are present for much of the year.