by Bruce Brethauer
I'm a big fan of the various species of the so called "ant plants"; plants which have developed symbiotic relationships with ants. In some species (such as the Myrmecodias and Hydnophytums) the plants produce highly modified stems which naturally develop hollow internal chambers which provide living spaces for ant colonies. The ants in turn benefit the plants by protecting their host from insect pests and providing nutrients derived from decomposing detritus from the ant colony. In addition to the previous examples there are a few members of the greater milkweed family which produce modified leaves which also provide sheltered sites for ants to establish their colonies. Some species, such as Dischidia pectinoides and D. major produce modified pouch-like leaves which serve as nesting sites. Other species produce large leaves which provide shallow, dome-like enclosures between the leaf and the substrate which can serve as a living site for ant colonies.
Hoya imbricata is one particularly attractive example of this last type of plant. It is an epiphytic plant with long, thin climbing stems which cling to tree trunks and branches, and bear very large succulent, plate-like leaves (reputedly measuring from about 2 inches, to nearly 10 inches in diameter in some varieties), which clasp the vertical surfaces upon which they grow. These leaves typically grow rather close together, slightly overlapping one another like roofing shingles or fish scales (the specific epithet "imbricata" alludes to this similarity to roofing tiles). Ants colonize the spaces beneath these leaves, often using adjacent leaves to serve as "nurseries", food storage and other specialized rooms or chambers for the ant colony. The spaces beneath the overlapping leaves may also serve as a protected highway, by which ants can travel from the ground to the upper branches of forest trees. This Hoya produces roots all along the length of the stems - those which are located just beneath the leaves will absorb nutrients from the detritus from the ant colony - providing the plant with a significant portion of its fertilization. The plant may also absorb a significant percentage of the carbon dioxide exhaled by the ants - providing the plant with vital carbon necessary in the production of sugars, proteins, and lipids.
Mature plants can grow many yards in length, and will branch and re-branch to produce intricate networks giving its host tree the appearance of being covered with shingles, or giant fish scales.
There are a number of varieties of this species in the wild, but the specific traits which distinguish the different varieties are not entirely clear to me - nor have I been able to find a listing of all of the recognized varieties in my research. Some varieties have closely spaced leaves which overlap, blanketing the trunks upon which they grow, while at least one variety is said to have long internodes with more widely spaced (non-overlapping) leaves. Most have comparatively small leaves (2 to 5 inches in diameter), while at least one variety produces leaves to about 10 inches across. In some, the leaves are of a uniform green coloration, but in others, the leaves are a dark green and are attractively marbled in pale greenish/grey tones. The leaf undersides of all varieties bear magenta to purplish pigments - which in many other plant species, is usually an adaptation to lower light levels - the purplish undersides to the leaf act as an accessory pigment to chlorophyll, which enables the plant to make use of additional wavelengths of light.
The flowers are produced in loose dangling umbels, which in my plant, measured to about 2 inches across. Larger, more mature plants will probably produce larger umbels with more flowers than this. The flowers are quite attractive, bearing "furry" greenish/cream colored petals. Other portions of the flower are of the same coloration, but are glossy and polished looking, earning them the common name for the genus, "Wax Flowers". While the flowers of other Hoya species can be highly fragrant, to my nose, the scent of this species is extremely faint: it is slightly sweet, with a trace of a musty under-tone. My plant has only flowered once: I am uncertain what combination of cooler temperatures, reduced light intensity, shorter daylight hours and less humid conditions may have helped initiate the formation of flower buds, but my plant flowered in November, about 2 months after I brought it indoors for the winter.
Hoya imbricata is not the easiest plant to maintain under typical household conditions. In my 19 months of growing this plant, I have struggled to discover which conditions best suits it: in summer, my plant usually produces a modest flush of growth, but it remains dormant through much of the other 9 months. It responds well to the increased light levels and higher temperatures of summer, especially when I move it into my unheated greenhouse in late spring. This species requires warm temperatures, bright but diffuse light, and quite humid conditions. Without high humidity, my plant languishes in a sort of persistent dormancy, and in winter, it has the tendency to loose moisture from its leaves and abort roots and young stems until humid conditions are restored. It is only when humidity exceeds about 60% that my plant even begins to show signs of growth: at levels closer to 90%, it seems to produce its most rapid growth. I am presently growing cuttings in a sealed 2 liter soda bottle with a soil-less mixture of peat moss and vermiculite watered with a weak solution of Miracle Gro fertilizer. This terrarium is kept just below two 40-watt fluorescent lights (the bulbs actually resting on the surface of the bottle). Because of the proximity of the lights, the temperature inside of the terrarium can rise to as much as 95 degrees Fahrenheit by day. At night (when the lights are off), temperatures typically fall to about 72 degrees. Conditions are very moist, so the sides of the container are perpetually drenched in condensation. This combination of warmth and moisture would rot practically any other plant, but my plant seems to thrive under these conditions, quickly responding with renewed, vigorous growth. After just a few weeks, one small cutting has produced 4 new stems, and the beginnings of at least 2 new leaves. Following this initial success, I started another cutting (a single leaf with several branching stems) under similar conditions. This cutting had been dormant for nearly one year - but within one week of this treatment, I observed the initiation of new growth at two nodes - probably the beginnings of two new vines; about a week later, it is producing the beginnings of new roots. Larger plants can be grown in a sort of mini greenhouse - I am growing my "main" plant horizontally in a long plastic storage container (the type designed for under-the-bed storage) with a pane of glass placed over the top to provide a more or less sealed environment (to ensure high humidity levels). I place fluorescent tubes on top of this (with the tubes resting just a few inches above the plant), and maintain light for approximately 14 hours a day. Even though I grow my plants on the basement floor (the coolest location in the house), temperatures inside of this container will rise to approximately 80 degrees by day, and cools to about 68 degrees at night (conditions which are probably a bit cooler than optimum). It would probably be best to place a 1 inch layer of very moist blend of Vermiculite/Perlite on the bottom of the container to provide adequate humidity, but any moisture retentive medium (such as peat-moss, or sterilized potting soil) will do.
In spite of the purple/magenta pigments on the underside of its leaves, (which is usually an adaptation to low light levels), Hoya imbricata seems to thrive when provided with bright but indirect light. When grown outdoors, bright dapple shade is probably best, but indoors, plants should be maintained just a few inches beneath fluorescent lights. Extended exposure to direct sunlight will tend to bleach and scorch its leaves.
Hoya imbricata requires a good support and a more or less solid surface upon which to grow in order to assure typical growth, otherwise the leaves of unsupported vines tend to roll in upon themselves (imagine a paper plate rolled into a cylinder). Cork-bark slabs, osmunda fiber slabs and posts, even long sections of logs and thick tree branches are good supports. For my own plant, I take two sections of black plastic mesh "gutter guards", and sew these along their sides and bottom to produce a long "sock". I fill this with an orchid potting mix consisting mostly of chipped coconut husk and cork bark. This mix retains moisture much better than cork-bark slabs, and may provide more humid conditions under the leaves than cork slab would alone. This support is rigid enough for the leaves to "clasp" normally, although I have found that it is best to wire new growth against it to assure good contact with the growing medium, at least until roots become established enough to hold the new leaves in place.
As with all Hoyas, this species requires warm temperatures to thrive: 80 to 90 degrees seems to be best, although it will tolerate higher temperatures than this: Extended periods of cooler temperatures (68 to 52 degrees) will tend to send plants into dormancy, and freezing temperatures will practically kill it instantly. While I have not tested its ultimate tolerances to cold, it will likely die if exposed to temperature in the 40's for any more than a few days, so if you do move your plants outdoors in summer, be prepared to bring it back indoors at the first predictions of cool weather.
It seems that only a few conservatories, and dedicated hobbyists grow Hoya imbricata here in the United States. Exceedingly few nurseries stock any of the varieties of this species, so it may sometimes be easier to acquire cuttings from other growers than it is to find in trade. My plant (Hoya imbricata var. basirotunda), for example, was originally acquired as cuttings generously provided by Myron Kimnach. The scarcity of this species in the trade is unfortunate, as this is an exceptionally interesting, and (in my humble opinion) one of the most attractive Hoya species that I know of. Perhaps its reputation as an "ant plant" works against it. While plants which are grown outdoors in the tropics and subtropics may sometimes become colonized by ants, it has been my experience that plants grown in more temperate climates do not attract ants, and can be grown without the presence of ants without ill effects. In nature such symbiotic relationships tend to be fairly specific, and usually involve a relatively few ant species; most ant species from northern latitudes would not colonize this plant. Grown indoors, particularly when grown in a more or less sealed environment, the chances of ants colonizing this species are virtually nil.
The specialized growing needs of Hoya imbricata will probably forever relegate this plant to dedicated growers only, particularly those from non-tropical climates. But for those growers who are not daunted by the challenges of providing year-round warm temperatures, high humidity and bright light, this species may very well be the plant for you. Its distinctive growth habit, attractive foliage (particularly those varieties with attractively marbled leaves), attractive "furry" flowers, and its fascinating adaptations to live symbiotically with ants will make it a standout in any collection. And it is unquestionably the most attractive "ant plant" which I have ever grown.
Green: Plant Research
Additional information may be found through the International Hoya Association