Range: Thought to be Texas to northern Mexico, potentially western Louisiana
Though the scientific epithet “louisianica” suggests origin from the state of similar name, this strange plant has an uncertain history but is thought to be native to further southwest (however has been introduced to numerous regions across North America and worldwide). Known by many names including Louisiana Devil’s Claw, Unicorn Plant, Ram’s Horn, etc., this species inhabits moist regions such as the banks of waterways, wet meadows, and waste-processing or dump areas, though can also be found growing in somewhat drier areas as well provided the soil retains moisture. Plants may grow to 60-80 m tall on slowly branching stems, and up to 100 cm in diameter, with leaves on long petioles and themselves irregularly rounded in shape and up to 30 cm across. All vegetative parts, including flower stalks and sepals, are covered in small glandular hairs that trap potential pest insects and also release acrid odors when disturbed. Inflorescences develop on the ends of each branch and may bear one to over a dozen blooms. Flowers are up to 6 cm in length and similarly wide, tubular or trumpet-shaped with the end spread into 3 large lower and 2 smaller upper lobes. Tube color is greenish to pink outside, mottled mauve and white inside, the lobes varying shades of light pink with a yellow palate stripe from the tube to the upper center of the central lower lobe. The stigma sticks out of the mouth on the top side of the corolla tube and is two-lobed; when touched (usually by a visiting pollinator) the lobes fold shut, trapping any pollen left on the surface. Pollinated flowers are replaced by an oblong fruit with an ovate base, and a long, curving “horn” (hence the names “unicorn plant” or “ram’s horn”) which together can grow up to 10-15 cm in length; the exterior is also covered in sticky glands. Mature pods drop the fleshy outer coating to reveal a woody inner layer, covered in winged protrusions and as it matures splitting down the “horn” to produce two inward-curving, hook-like adornments with wickedly sharp tips used to snare the fur or feet of passing animals for dispersal. Black seeds are released from the opening between the hooks. This species can be distinguished from relatives such as P. parviflora by the black color of the seeds, peachy pink flower color, and often smaller size.
Cultivation: Sow seeds an inch or so deep in a rich, well-draining but moisture-retaining soil, and keep warm throughout the growing season. Fertilize occasionally if in a pot, and manually pollinate flowers to produce seeds. Old seeds may require scarification to assist germination; best results are with seeds less than 3 years old. Grow in partial shade to full sun.
Lifespan and reproduction: Annual. Reproduces solely through seeds; cuttings are not known to be successful.
The large pale pink flower of P. louisianica. Flowers may at times have a strong aroma, sometimes sweet and other times mixed with something reminiscent of the acrid smell the rest of the plant gives off.
Some additional fun facts about this species: like others in the genus, and a few related genera, the young green pods can actually be picked and either pickled or roasted much like okra and eaten. So if you have a lot of plants and a lot of pods, there's another option besides letting them go to seed.
Additionally, the mature pods have been used in Amerindian basket weaving and other crafts, both the whole pods themselves and also sometimes the fibers that make up the rigid, woody structure.
It's also sometimes speculated that these plants might be para-carnivorous, the insects trapped on the leaves sometimes being described as looking empty after a short period. However no studies have been able to reliably show digestive enzyme activity or absorption of materials from captured insects. In another fashion, it's thought that the wicked hooks on the seedpods may sometimes ensnare and kill small animals, or result in crippling wounds on large hooved creatures in particular, resulting in death; the seeds may then benefit from the nutrients released by the decomposing corpse, thereby indirectly benefiting from being a "murderous plant." Little hard proof has yet been given for these claims however. But, there is little doubt that the seedpod claws are sharp and strong enough to inflict serious injury, and the pods have an odd "dual" internal seed chamber structure, wherein the first seeds are readily released as they fall out the opening, but a second set is held behind a thin wall in the sides of the pod that are only easily released if that wall is damaged or decomposes, suggesting they are meant to be dispersed after a great period of time and possibly only after the pod is subject to significant damage, weathering, or flooding.
On the right: a newly mature pod, before the prongs fully dry out and split apart.
On the left: a fully dry seedpod, with prongs fully functional.
A closer look at the tips of the seedpod hooks. Each tip comes to a razor point and held rigid by a strong woody internal structure.