Today in the FEATURED PLANT section we will talk about Kalanchoe ×houghtonii, an artificial hybrid that is currently one of the most aggressive invasive taxa on the Catalan Mediterranean coast.

Origin and entry

This taxon of hybrid origin was first created in California in the 1930s by the forced cross between two species native to Madagascar, K. tubiflora and K. daigremontiana by A.D. Houghton, an English physician and botanist living in the United States. It seems that the experiment was repeated two decades later in Portugal by F. Resende. There are no further records of the hybrid until the late 1960s, when it was first reported in Australia, this time in the wild, and a few years later in America. The first records in Europe date from the early 1990s, in the Valencia (province of Alicante). It did not arrived to Catalonia until two decades ago, in terms of confirmed presence. Even so, in just twenty years it has become one of the most common non-native species on the coast of our territory. As it is a succulent plant and theoretically cannot withstand winter frosts, it does not yet form stable populations in the interior of Catalonia.

To the left, propagules formed along the margin of the leaves of Kalanchoe ×houghtonii. To the right, “carpet” formation close to Roses.

Kalanchoe ×houghtonii is a plant that can reach (or even exceed) 1,5 m when in bloom. It has fleshy, opposite, more or less lanceolate leaves, with dark spots on the underside and on the serrated margin where the propagules (small seedlings) are arranged, which eventually detach and are responsible for vegetative/asexual reproduction. This type of reproduction is so vigorous that the plant can end up forming “carpets” which prevent other species from growing. It forms inflorescences with very showy, dark red pendulous flowers. If you want to know how to identify it in more detail, you can consult its data sheet here.

The parentals: an added problem

Kalanchoe tubiflora, in Palamós.

The hybrid differs from the parental species mainly in the morphology of the leaves: the leaves of K. daigremontiana are more triangular in shape and have a very characteristic basal fold, while those of K. tubiflora are subcylindrical and only form propagules in the most distal part.

The two parental species are also present in Catalonia, although to a lesser extent than the hybrid. Kalanchoe tubiflora is a species that is clearly expanding in Catalonia, and in some locations it is already fully naturalised; in El Vendrell, for example, there has been an established population for almost a decade. On the other hand, K. daigremontiana is, for the moment, still in a very incipient phase of entry into our territory: the first observation dates back to 2020 and has only been observed occasionally, always with populations of a few individuals. Even so, the evolution of both K. daigremontiana and K. tubiflora must be closely monitored, as both species are invasive in some parts of the world (particularly K. tubiflora), such as Australia, the United States and Cuba.

Situation on the Costa Brava: the role of the LIFE project

Large individual of Kalanchoe ×houghtonii, near Roses.

Due to its cleanly invasive character, K. × houghtonii is one of the target taxa of the LIFE medCLIFFS observer network. Only during the preparatory stage of the transects for the LIFE medCLIFFS volunteer network (primera meitat de l’any 2022),

first half of the year 2022), we were able to detect about thirty new localities on the Costa Brava, which exemplifies the increasing risk posed by this hybrid to natural coastal habitats. Of the parental K. tubiflora (you can consult its data sheet here)you can consult its file here), we were able to locate three new populations on the Costa Brava during the same period (in Blanes, Lloret de Mar and Palamós, the latter with a large number of individuals, many of them juveniles). The work of the two networks will be essential to assess the state of the invasion and predict its evolution in the coming years. We do not rule out that volunteers may also observe K. daigremontiana, detected in Blanes in 2020 within the facilities of the Marimurtra Botanical Garden.

Text: Jordi López-Pujol