by Bruce Brethauer
The genus Plectranthus belongs to the mint family; approximately 350 species have been recognized, with species widely distributed through Africa, Madagascar, Asia, Austlralasia, and the Pacific Islands, and includes annuals, perennials, semi-succulents, and shrubs. The semi-succulent species includes many succulent-leaved species which are common in cultivation. Some are remarkably fragrant plants with scents which can range (depending upon species and variety) from menthol, to citrus, and even turpentine; Plectranthus tomentosa has the strong smell of Vicks. At least one species, Plectranthus amboinicus is used as an herb, and another species, Plectranthus caninus, is recommended as a dog and cat repellant.
Plectranthus ernstii, while having mildly spice-scented leaves, is more memorable for its large and attractive caudex which (in the plants which I have seen) can exceed 1 inch in diameter, but may reach about 2 inches across in very old plants. In younger plants, the caudex shows some minor bulging between the nodes, but in very old plants, there are conspicuous constrictions at the nodes of the stems and main branches. While the caudex and main branches will eventually take on the appearance of woodiness, it is my understanding that these always remain succulent, and do not produce any significant wood in their tissues. The leaves are only slightly succulent, and are covered with a minute (and nearly invisible) layer of "fuzz". As in virtually all members of the mint family, the leaves are produced in opposite pairs. Small salvia-like spikes of lavender-blue flowers, or white flowers with lavender details are produced in the summer and into fall, providing some late season interest. Plants are generally well branched and rather compact, typically growing to about a foot or so high. The succulent 'trunk" and main branches of this plant makes it natural bonsai.
Plectranthus ernstii is native to Natal, South Africa, and it was only described as recently as 1982; it is therefore relatively new to cultivation. Nevertheless, it has proven to be a popular plant amongst cactophiles, and bonsai enthusiasts. It is easy to grow, a fast grower, and easy to propagate from stem cuttings. Even young plants will quickly grow a caudex - including plants which have been grown from cuttings (as a rule, caudex plants usually grow their best caudexes when grown from seed).
This plant responds well to my guidelines on growing cacti and other succulents, with a few minor exceptions: While it is very tolerant of drought, I have found that this plant grows best when it is not subjected to extended drought during its growing season. Plants require very bright light - ranging from very bright indirect light to (when properly acclimatized) direct sunlight, but it will also tolerate dapple shade. Because this is a faster growing plant, it will appreciate more frequent fertilization during its growing season than most other succulent plants (about once every 2 to 3 weeks in the spring and summer). Plants are drought and heat tolerant: my plant has shown no limits to its heat tolerance here in Ohio (it will readily tolerate temperatures to at least the mid 90's, and I suspect that it will easily tolerate temperatures to 100 degrees or more). I cannot guess its ultimate heat tolerance, but I suspect that it will suffer from the extreme temperatures of Phoenix and Tucson. It probably will not tolerate frosts (most of the Plectranthus species which I have grown are killed outright by light to moderate frosts), and it likely does best when temperatures are maintained at 50 degrees or higher during its winter dormancy. During its winter dormancy, this plant will tolerate drought, but should probably be watered more frequently than most other succulent plants (perhaps once every three weeks is best). Also, if you keep its roots constrained in a small bonsai pot, this plant should probably be repotted every year to renew its potting medium.
Also remember that the stems and caudex are succulent, but not woody, so care should be taken when moving, or transporting plants as the stems are rather brittle; the appearance of a mature plant can be seriously marred by any damage caused by mishandling. I am not aware of any significant issues with insects, but keep an eye out for aphids and mealy bugs just in case.
The attractive appearance of this plant, and its tendency to produce a "natural bonsai" makes Plectranthus ernstii a popular plant. Its attractive (albeit small) spikes of lavender-blue flowers are just the "icing on the cake". Its succulent stems gives it a good degree of drought resistance, making it more forgiving than many of the other bonsai plants. Its compact size makes it suitable for comparatively small spaces, and it lacks the spines and toxins that some other succulents may produce, making this plant better suited for households with inquisitive children and pets. While this plant is widely known and grown, it has not yet become "Mainstream" (I have never seen it offered in the "Big Box" nurseries); even so, I expect that it is frequently offered at the sales of many area cactus societies - and through many of the mail order nurseries which specialize in cacti and other succulents. If you are interested in starting a collection of "natural bonsai", this plant is a great candidate.