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Plant Highlight: Sedum dendroideum
In botany, as in zoology, classifiers seek to erect genera (and families as well) which are monophyletic. This means that all members of the group descend from a common ancestor and are more closely related to each other than they are to members of other groups. However, discovering how things are related – and constructing a “family tree” to show this – has not always been easy. Recently, DNA evidence has emerged as a powerful new tool to help in sorting things out. As more work is done in the Crassulaceae (or stonecrop family, the group in which Sedum is placed), the genus Sedum is sure come under attack. It has long been the “garbage can” of the family, the place where new species are dumped when they don’t fit anywhere else. This has resulted in a large and unwieldy grouping which everyone realizes is not monophyletic. Once the true relationships are sorted out, multiple genera will inevitably have to be created to accommodate all the disparate plants now placed in Sedum.
Within the genus, our featured plant belongs to the group Pachysedum. This is a Mexican group which tends to have relatively large, conspicuously succulent leaves and woody stems. It seems likely that these plants are more closely related to the Echeverias and Graptopetalums than they are to other Sedum groups. As evidence of this, there are many hybrids between Pachysedum species and both Echeverias and Graptopetalums, and these are often seen in cultivation. Included in the group are some of the largest species in Sedum.
S. dendroideum is closely related to the commonly-cultivated Sedum praealtum, sometimes treated as a subspecies (Sedum dendroideum ssp. praealtum) and sometimes considered a separate species. S. praealtum is quite common in California gardens, and has escaped and naturalized in some locations. The two species both have a shrubby habit of growth and bright-yellow flowers, but shrubs of S. praealtum are more sprawling and tangled while those of S. dendroideum tend to be more upright-growing and neater. There are also differences in the leaves, which tend to be somewhat pointier in S. praealtum and more blunt in S. dendroideum. A key distinction is the characteristic row of glands visible along the edges of the leaves in S. dendroideum. These redden in response to high levels of light, so that the plants typically show bright-red margins. Plants of S. praealtum may have some red at the leaf-tips as well, but not to the degree displayed by S. dendroideum.
The general appearance of a plant of Sedum dendroideum is somewhat similar to that of a jade tree (Crassula ovata), and casual observers sometimes mistakenly take it for this, but the jade tree develops a thicker trunk and has pink flowers. There are some forms of S. dendroideum which reach only a foot or 2 in height, but the form we have at RBG is larger than this, and has reached over 3 ft. (one meter) tall. One population has been reported in southern Mexico with plants up to 9 ft. high (nearly 3 meters). It is easy to grow, roots readily from cuttings, and makes a very attractive garden subject. Of course, it is especially showy when the flowers provide a burst of intense yellow.