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Plant Highlight: Aloe tongaensis (Aloidendron tongaense)
By Brian Kemble
About the name and area of occurrence
A number of Aloe species develop trunks and grow tall enough to be referred to as “tree aloes”, but many of these are single-headed. However, there is a group that is characterized by dichotomous branching (forking of the growing tip), including the tallest of all the aloes, Aloe barberae. This group has been put into a separate genus named Aloidendron, but this new classification has not yet gained wide acceptance, and in most books these plants are listed under Aloe. Of the group, the most recently named is Aloe tongaensis, published in the journal Aloe by Ernst van Jaarsveld in 2010. The name tongaensis refers to Tongaland, also known as Maputaland, which is an area in the northeastern corner of KwaZulu-Natal Province in South Africa and adjacent parts of eastern Eswatini and the southern tip of Mozambique. Prior to being named as a species, this plant was considered to be a form of Aloe barberae, and it was sometimes sold under the name Aloe ‘Medusa’.
About the plant
At maturity, Aloe tongaensis is a much-branched tree with a rounded crown and a height of up to 26 feet (8 m) and a trunk diameter of up to 32 inches (80 cm) at the base. Its appearance is similar to the related Aloe barberae, but with more branching and the branches a little less stout. Like A. barberae, its old leaves are soon shed, so that the tannish-gray trunk and branches are on full display. On average, there are about 20 smooth green sword-like leaves at the end of each branch, and these are about 18 inches long (45 cm). The leaves are strongly channeled on the upper side, and rounded below, with small teeth along the margins. The teeth are .08 inch long (2 mm) and spaced about .2 to .4 inch apart (5 to 10 mm).
About the flowers
The inflorescence of Aloe tongaensis is not very tall, with a height of up to 14 inches (35 cm), but it has multiple branches bearing rounded heads of showy flowers. The floral branches first extend outward, then curve upward like the arms of a menorah so that the racemes are held erectly. An inflorescence may have 10 or more racemes, though usually less than this. The yellow to yellow-orange buds are initially upward-pointing, then pivoting to point first outward and then downward by the time they open. The curved tubular flowers are orange, yellow-orange, or salmon-orange, with a length of 1.85 inches to 2 inches (47 to 50 mm) and a diameter of .315 inch (8 mm). The pedicels holding the flowers are .4 to .55 inch long (10 to 14 mm). The tepals* are fused into a tube for most of their length, separating into 6 round-tipped segments at the tip. The anthers with their yellow pollen protrude slightly from the mouth. This is an autumn-flowering species, with its peak bloom in November and December at the Ruth Bancroft Garden, and in April and May in its Southern Hemisphere home.
About the fruits
After pollination, the ovary at the flower’s base swells into an oval seed capsule. Within the capsule are 3 chambers holding stacks of brown seeds, these having small papery “wings” around their rims. When ripe, the capsule browns and splits open, enabling the seeds to spill out.
Plants in cultivation
The area where Aloe tongaensis occurs, in southern Mozambique and northeastern South Africa, is warm and tropical, so plants in habitat do not experience freezing temperatures. In cultivation, it can tolerate brief overnight lows a little below freezing, but not sustained freezes. At the Ruth Bancroft Garden, we grow it where some protection is provided by a high tree canopy above, but it still would not be safe in a very cold winter. In frost-free locations, such as San Francisco or coastal Southern California, it grows rapidly and presents no difficulties. Although it comes from a region of summer rainfall and dry winters, it does not mind the winter rains experienced in California or other Mediterranean climates. It can be grown in a container, but this will limit its size unless a very large container is used.
* The term “tepals” applies to flowers where the distinction between sepals and petals is ambiguous.
