photo by Bruce Brethauer
Possibly one of the most fascinating groups of plants are the so-called "Ant Plants" - plants which have developed a symbiotic relationship with ants, and which produce modified parts to serve as living spaces (domatia) for ant colonies. The number of plants which have developed such modified structures (which includes modified stems, rhizomes, leaves and even thorns) is quite amazing, with representative plants from such varied groups as ferns, tillandsias, orchids, acacias, hoyas and dischidias. But when people speak about the so-called ant house plants, they are usually referring to plants of the genera Hydnophytum and Myrmecodia. Both of these genera are members of the Rubiaceae - the coffee family, and are widely distributed in tropical Asia, Malaysia, the Philippines, New Guinea (and Irian Jawa) and into northeastern Australia. These plants produce a large caudex - which in some species can exceed 2 feet in length and one foot across, and provides ants with a labyrinth of internal tunnels and chambers to serve as living quarters for ant colonies. The complexity of these tunnels can be appreciated by examining MRIs and photographs of sectioned caudexes. Amazingly, these chambers are formed by the plants themselves - and are produced when specialized tissues cut off the flow of nutrients to regions within the tissues of the caudex - after which, the hollows are produced as starved cells atrophy away. The colonizing ants may also subsequently expand and modify these chambers. Most of the species of both genera are exclusively epiphytic, growing attached to trunks and large branches of trees where their stems typically cascade or jut horizontally from the caudex - and only rarely grow in the upright position usually seen in cultivated plants. In habitat, these plants typically grow in regions of high rainfall, and low soil fertility. The ant colony which is housed within their modified stems provide fertilizer to the plant in the from of detritus which accumulates in the chambers, and is absorbed through specialized root-like tissues produced in some of the internal chambers. The plants also capture CO2 exhaled from the ants, and usually receive protection from a variety of browsing insects, and even from larger creatures, as many of the ant species which occupy these plants also carry a powerful sting. The plants provide dry living quarters for the ant colony and some species may provide a few additional incentives in the form of protein bodies, and nectar producing glands from specialized tissues on the stems to provide snacks for ants in the colony.
Hydnophytum moseleyanum is an especially attractive species with large, somewhat irregular, and comparatively smooth caudexes - which may grow to as much as 10 inches long in cultivated plants. The color of the caudex may vary from greenish, to brownish to silvery grey, and in some plants may be similar in appearance to Pachypodium brevicauli. Entrance holes to the internal chambers can be seen near the base of the caudex, but in some other species of Hydnophytum, the entrance holes are located beneath the caudex, and are seldom visible under typical viewing conditions. Plants produce multiple thin branching stems. The branching stems are fairly long - sometimes to several feet, and can develop a rather leggy, and scrambling looking plant. By clipping away leggy looking stems a more compact, and bushier plant can be produced. Opposite, barely succulent oval to lance shaped leaves are produced at fairly long intervals, as the stems have long internodes - usually 1 or more inches apart. Tiny, whitish, waxy looking flowers are produced in the leaf axils - while the flowers are insignificant, these are followed by small fleshy berries which ripen to a yellowish/orange to an orange/scarlet color, adding an attractive splash of color to the mature plants. When growing conditions are favorable - flowers and fruits may be produced throughout the year.
I have found that the secret to long term success with these plants remains somewhat elusive for me. There are a number of authors who offer advice on how to grow this species - but some differ, and sometimes rather drastically, in their details. But in general, these plants will fare best when provided with warm and humid conditions, very bright but diffuse light, and will probably thrive best when given regular fertilization - perhaps as frequently as every other watering. In the greenhouse, - especially in collections of orchids, ferns, and other tropicals which requiring warm and moist condition, these prove to be easy plants - but in trying to scale down to an aquarium, or terrarium, it is sometimes difficult to provide both high humidity and good air circulation - which I suspect is necessary for long term success in growing these plants. Conditions which are too humid may leave these plants prone to rot, while conditions which are not quite humid enough may result in plants becoming dormant, loosing many of their leaves and (in severe drought) roots. The plants appear to prefer warm conditions - in the ballpark of 75 to 85 degrees. Unusually warm conditions will tend to induce dormancy in this species, and (in my experience) extended exposure to direct sunlight tends to bleach or even scorch the leaves of my plants when I moved them outdoors in summer. If you are in the habit of moving your plants outdoors in summer during the summer months, a spot with bright light with some dapple shade - especially in the late afternoon may be best. This species will tolerate some rather cold conditions in winter provided that it is kept relatively dry (but never absolutely dry): but even so, it tends to fail if it is given extended periods of chill - and should never be subjected to freezing conditions as this will likely kill the plant outright. Winter temperatures in the range of the lower 60's to mid 70's seems to be sufficiently warm to keep it going well, but somewhat warmer temperatures are probably preferable. I grow my plants in a sealed 55 gallon aquarium with a substrate of milled coir and peat moss which is kept quite moist to provide conditions of high humidity. My seedlings are planted in pots with a variety mediums used primarily with orchids; - some with a blend of cork chips and chipped coconut husk, and others in a blend of fir bark and chipped coconut husk. Some people can grow this species on a cork bark mount - but grown on a mount, such plants may require more frequent watering and misting. When watered, these plants should probably be drenched with rather copious quantities of water, but then should be permitted to become nearly dry before being watered again. It has been my experience that these plants should never be permitted to become completely dry for extended periods - once the roots have died back, I have found it difficult to re-establish the lost root system. I suspect that it would be best to fertilize this species rather frequently - especially during warmer periods when its rate of growth increases. Plants grown in an orchid potting mix of chipped bark should probably be given an orchid fertilizer at every second or third watering. Orchid fertilizers tend to have much higher levels of Nitrogen than is recommend for most other types of succulents, but is probably necessary for most plants grown in an orchid mix. If you grow your plant in a different medium, a fertilizer with a lower percentage of nitrogen may be more suitable. In either case, the fertilizer is probably best diluted to one half to one third of the recommended concentration. I should also be very clear that while plants in the wild generally benefit from the presence of colonizing ants, the presence of ants are not at all necessary to grow this plant.
It is remarkably easy to propagate plants from seed - if you can find fresh fruits and seed. The seed loose viability rapidly with time, and will seldom germinate if the seed has had time to dry out. It is best to locate someone who has grown a plant to maturity, and beg a few ripe fruits from him or her. Each fruit produces 2 or more seed, which is surrounded by a gelatinous, sticky pulp. The seeds can be harvested by squeezing them from the berries. The seeds can be further cleaned by rinsing them under a water faucet in a tea strainer, which tends to remove the greater portion of the mucilage. The cleaned seeds should be planted immediately - do not let them dry out. They should be surface sewn on any sterile seedling mix, and kept very moist and warm (75 to 85 degrees) under bright but diffuse light. Following germination, the initial growth may be somewhat slow, but once the caudex reaches the size of a marble, growth accelerates, and the seedling begins to take on a bit of character - even at a very young age, seedlings will develop the beginnings of their first chambers - some of my seedlings have produced their first chamber when the caudex was only slightly larger than 1/4 inch across. I have heard that seedlings can begin to flower when they are about 1 year old, but in my experience- it usually takes longer - perhaps two years before they flower their first time- after about 4 or 5 years, the caudex can measure 6 or more inches across. These plants have rather shallow roots, and the roots tend to be rather brittle, so frequent repotting is not recommended. I have actually seen a number of plants where the caudex eventually spilled over the sides of the pots so that the diameter of the caudex exceeded the size of the put by several inches: even so, the plants did not seem to show any signs of stress: and of course, mounted plants should never require repotting, although it may be necessary to find ever larger mounts as the plant increases in size.
OK, this plant may not be everyone's cup of tea, but it does produce a very attractive caudex, and no-one can deny that its symbiotic relationship makes it one of the more curious plants in a world full of unusual plants. If you already grow orchids, or tropical ferns and are interested in trying something completely different, this may well be the plant for you