Proboscidea louisianica, the "Unicorn Plant" - Cactus Club

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Proboscidea louisianica, the "Unicorn Plant"

Plant of the Month > Species O to P

by Bruce Brethauer

       Every now and then, I may discuss a plant which may not quite fit in with the general opinion of what constitutes a truly succulent plant. The "Unicorn Plant" even stretches my definition a bit, but at the joint meeting of the Midwest and Central Ohio Cactus and Succulent Society, I shared a few of the unusual seedpods and seed of this species, and only managed to give a very incomplete description of my plant. A number of people took home some of seed packets and seedpods, so I thought it would be best to discuss this species here to provide a more complete treatment of this unusual and interesting species.

       At first glance, the "Unicorn Plants" do not seem especially unusual; they have a coarse appearance, with large rounded hollyhock-like leaves held on upright and spreading succulent stems which may grow to about 2 or 3 inches in diameter. The entire plant grows to heights of about 3 feet in most plants, but may grow to about 5 feet in exceptional individuals, and can spread to 4 or 5 feet across. These plants exhibit a sort of dichotomous branching in which the stems repeatedly fork to produce two branches, with a flowering stalk produced in the axils between the two branches. The succulent stems assist the plants in storing moisture through periods of drought. It also appears to be very adaptable to soil types: I grew my plant in ordinary garden soil along with my vegetables, but in the wild, Proboscidea louisianica tends to occur in wetter habitats, and is usually associated with the moist, fertile soils along watercourses (as opposed to bogs and other wetlands with acid, nutrient poor soils). This species is widely distributed throughout the "lower 48" states of the United States, (being absent only in the states of Arizona, Montana, North Dakota, Utah, and Wisconsin). Other species of this genus tend to grow in more arid regions, mostly in the American Southwest, but also in Central and South America. The flowers are attractive and are fairly large, producing loose spikes of foxglove-like flowers with pale mauve to pink petals with bright yellow markings in the throat. After flowering, they produce large curving and tapered green fruits. At maturity, the "rind" of the fruits either splits and falls away, or rots away (depending upon weather and temperatures) to reveal the inner, woody seedpods, which also split along a portion of their length, to produce two long thin curved hooks which grapple the legs and fur of passing animals which unwittingly assist in dispersing the seed of this species. Some people use the pods in handcrafts to create unusual ornaments, and others pickle or cook the immature fruits (when they are still tender) for use in the kitchen, where they are said to be used in much the same way as Okra. The native peoples of this country are said to have ground the seeds to produce an edible meal, and some processed the pods to produce tough cordlike fibers, and were even said to have selectively bred a variety with unusually large pods to produce longer fibers. But today, most authorities consider the "Unicorn Plants" to be noxious weeds, and very few people intentionally grow this species anymore.

       Nevertheless, I find this to be such an unusual plant, with a number of odd properties, that I am more than a little fascinated by it, and suspect that a number of others (especially among the younger growers, and the young at heart) will also find it to be an interesting plant - either in spite of, or because of its more annoying attributes.

       I first became interested in growing Proboscidea louisianica when I heard about it through the International Carnivorous Plant Society: The entire plant, including the flowers and fruits, is covered with fine hair-like structures which are tipped with glands which secrete a gummy, and rather smelly liquid. The droplets which are produced are capable of trapping small insects (such as aphids and midges) which can become mired when they blunder into this plant. On close examination, each leaf may contain several dozen aphids, gnats and other small sized insects which have become trapped in the gelatinous droplets, (most of the trapped insects can be found on the leaf underside). The leaves are also frequently covered with dirt, small bits of leaf litter and other detritus which may fall or blow onto the plant and became mired in the droplets. Rainstorms tend to wash the upper surfaces of leaves and exposed stems free of trapped insects and other rubbish, but the leaf undersides may retain their victims, even through rainy periods.

       But the Unicorn Plants are not true carnivorous plants (they lack the digestive enzymes to break down insect proteins, and they do not appear to have the capacity to absorb nutrients from the insects which are captured in their "traps"; nor do they appear to lure their victims to the gummy "traps" which cover these plants). I suspect that these droplets have a completely different function, which (in my opinion) seems to act as a repellent to deter most insects. Aside from the aphids caught in the gummy droplets, none of my plants were touched by any insect pests, even though Mexican bean beetles and Japanese beetles infested plants on either side of the Unicorn plants in my garden. Even bees seemed to be deterred from visiting the flowers of this species: while they were clearly drawn to the attractively colored flowers, I have never seen a bee actually land on the flowers of my plant - they merely hovered a few inches above the flowers for a few moments, and flew off. I suspect that the scent produced by these droplets may have a direct role in repelling insects - the plants have a scent reminiscent of moldy sneakers and melted rubber. When the soil is especially moist, and the plants are heavily laden with their gummy droplets, the scent seems especially pronounced. The amount of liquid which is secreted by this plant can be rather remarkable; anyone who blunders into it will not only get covered with a generous helping of slime, they will also carry away the distinct scent of this plant - it has a real "yuck factor" which is irresistible to many young growers and (for that matter) some adults as well. One site warned that it is difficult to wash this slime from skin, fabrics and garden tools, but this does not reflect my experience - I have found that it easily washes off with soap and water.

       A curious trait which I have observed on my plants but have never read in any article on this species, is that the stigma lobes are highly sensitive, reacting rapidly whenever being touched. The stigmas end in two flattened lobes which are more or less "hinged" at their midlines (imagine an open clamshell). Whenever I touched the inner surface of these lobes, (such as when I transferred pollen to the stigmas), they snapped shut in a matter of about 2 or 3 seconds, sealing pollen between the two lobes. On a smaller scale, it was reminiscent of watching a Venus Fly Trap capturing its prey - but again, I suspect that this action has nothing to do with catching insects, but is instead, a means to secure and seal pollen between the stigma lobes. It is fascinating to watch, and I can only speculate what mechanism activates and controls this reflex.

       It is easy to grow plants of Proboscidea louisianica, they thrive under conditions of summer heat, and frequent rains, but appear to be remarkably tolerant of drought as well. Given ordinary garden soil, warm temperatures, and enough moisture to keep a tomato plant happy, the growth on my plants was nothing short of remarkable, with plants growing to heights of about 30 inches, and producing their first flowers in 4 to 5 weeks from when the seedlings first emerged from the soil. Since these plants grow so rapidly, and can grow quite large, there is little need to encourage additional vigor through fertilization - in the instructions provided by the International Carnivorous Plant Society, I was warned  "...fertilize it if you dare...", so let the grower beware.

       I have found these plants to be very drought tolerant, and while I pampered the seedlings through their first weeks, frequently watering them as needed, once these plants began flowering, there seemed little need to provide additional water - their thick stems seem to provide the necessary moisture reserves to get them through periods of drought, the infrequent rains of our summer of  2007 seemed to have been sufficient.  While I suppose that this may qualify this species as a succulent, I admit that it is more than a bit of a stretch to call it one.     

        According to my resources, germinating the seeds can be a bit of a challenge, and seeds gathered from wild plants are notoriously difficult to germinate: scarification, stratification, gibberellic acid pretreatments, and other techniques have been applied to improve germination, with limited success: the seed seems to be adapted to live dormant in soils for years before germinating. But the seeds from plants which have been in cultivation for several years are much easier, with nearly 100 percent germination rates reported in some instances. The seed which I acquired appear to have been derived from plants in cultivation, as their germination rates were in the ballpark of 50%; I imagine that the seed which I shared at the September meeting of our Societies will have similar germination rates. Even so, they are relatively slow to germinate, taking (about in my experience) 2 to 4 weeks from a late June or early July sewing. I planted seed directly in my garden without providing any pretreatments, burying the seed to a depth of about ¼ to ½ inch. Soaking the seed overnight in warm water before sewing may possibly speed germination times. The seedlings are distinct, with large fuzzy leaves which are reminiscent of a seedling hollyhock, only with more fuzz on its leaves and stems; also, the hairs on the leaves soon acquire their characteristic coating of  gummy, scented droplets, when it becomes impossible to confuse these plants with any other seedlings in the garden. Growth is rapid, and plants will soon begin to produce their first of many hundreds of attractive flowers, which appear to be produced continuously thereafter until the cooler  temperatures of autumn slows its growth to a crawl. While I initially pollinated flowers by hand to assure fruit set, this was largely unnecessary; many flowers regularly set fruits whether or not I pollinated their flowers. Fruit develops rapidly, quickly producing curved, pod-like fruits which are reminiscent of the milkweed pods - these are also covered with the characteristic slime tipped fuzz of the leaves, stems and flowers, and the fruits take about a month to six weeks to fully mature. At maturity, the pods (usually) split, to reveal the woody seedpods inside. The seedpods are long (to about 5 inches or so in length), these also split along a portion of their lengths, giving them the appearance of  the head and curved tusks of an extinct mastodon, or of the raised head and giant mandibles of a monstrous ant. The pods are adapted to catching the legs and fur of passing animals, which disperse the seed as the animals travel. Some of the seed falls out of the pods randomly, while other seeds are held tightly in inner recesses of the pods, and are only released after the pods have been trampled underfoot, or after the pods have rotted away. Each plant can produce several dozen pods, and giant plants can probably produce in excess of 100 pods - each containing 20 to 50 or more large irregular seeds.     

       While it is still too early to tell, I suspect that my plants will perish at the first frosts - already they are looking pretty world weary - they have lost most of their leaves, and are only producing a few flowers. My plants have also had a tendency to break under their own weight, splitting at the forks in their stems. While the stem splitting did not kill the plants, it did mar their appearance, and probably sapped some of their strength. I suppose that staking the plants would have prevented this, but I really did not want to get "slimed" by my plants as I staked them.     

       I suppose that this species is really a "hard sell" for most growers: its nasty scent and the slime producing glands will make it undesirable for most growers. Even so, its interesting seedpods, attractive flowers, and its unusual insect trapping properties (whether or not this plant qualifies as a "true" carnivorous plant) will make it interesting to other growers. It is a remarkably easy plant to grow - once you manage to germinate a few seedlings. If you do grow it, check out its unusual stigmas, with their "fly-trap" reflexes, I have seen nothing like it in any other unrelated plant species. Its high "yuck factor" makes it interesting to some young growers (particularly boys) and may be useful in stimulating the interests of others, who have yet to acquire an interest in growing plants. I will not go much beyond this: I can hardly recommend it to most gardeners, but there are certainly others who will, like myself, find this to be an interesting plant.


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