Hardy Euphorbia are one of my loves in the plant world. The foliage, form and flowers are all soul satisfying. As Dan Hinkley of Heronswood Nursery said, "you can't have too many euphorbs".
Hardy Euphorbia (spurges) are part of the huge Euphorbiaceae family, consisting of more than seven thousand species in three hundred genera. Most are annual, biennial or perennial herbs; some are succulents and there are a few tropical shrubs and trees. Most of the hardy species are native to southern Europe and the Mediterranean area. Some are considered rather noxious weeds. Many of the most strange and wondrous are very cacti-like and aren't hardy in most temperate gardens.
Probably the most well-known member of this genus is Euphorbia Pulcherrima, the ubiquitous Poinsettia that makes its appearance in every grocery store around Christmas. Like the Poinsettia, most of the hardy euphorbs have flowers that are really showy bracts surrounding a specialized type of miniature inflorescence called a cyathium, consisting of a female flower surrounded by male flowers, enclosed in a cup-like structure containing nectaries. The nectar is attractive to various flies who pollinate the plants.
The fruit is a three sectioned drupe with three seeds. When the seeds are ripe, they are ejected, making gathering them a challenge. It seems that no matter how closely I watch, one day those pods are shut tight and the next they have split and the seeds are gone.
All are characterized by a milky, latex sap that can irritate the skin of the sensitive, much like poison ivy, so it's best to wear gloves when working around your plants.
I fell in love with hardy euphorbs many years ago and I wouldn't be without them in my USDA zone 7 garden. By and large, they are undemanding plants. Most of them need good drainage and some are rather marginally hardy for me, but those that I grow seem to be thriving in various degrees of shade. Most of them seed around freely, so if I do lose an adult plant in a very hard winter, there are always seedlings coming on to take its place. The evergreen species add tremendously to the winter garden with foliage ranging from silver to nearly black-purple.
Euphorbia amygdaloides var. robbiae (sometimes listed as 'Robbiae') is a perfect plant for the deep shade under shrubs and trees, although it will grow as well in sun. It spreads by underground roots, so in really ideal conditions (good soil, drainage and consistent moisture) it can be a fairly aggressive ground cover. In dry shade, however, it is quite well-behaved. The glossy leaves are evergreen for me, although they can get fairly ratty looking at the end of a really bad winter. Hardiness ratings vary with reference, but it should be hardy to at least USDA zone 6 and I've seen it rated for zone 5. The leaf rosettes remain compact, getting a bit over a foot (0.3 m) tall. In early spring, flower stems emerge from the center of each rosette, rising to eighteen or twenty-four inches (0.45 - 0.6 m), and producing pale green flowers. In my plants, the flower has almost no yellow tint, although I've seen it described as chartreuse, it really isn't. In the deep shade, it rather glows.
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