The Origin, Discovery, and Demise (Hopefully) of a Population of a Non-native Invasive Plant
Abstract: A newly-discovered population of the non-native Ruellia dipteracanthus probably had its origin as hidden rootstock in a gallon-container of a different wetland plant purchased at a local nursery. In the three years since its apparent planting, the species has become well-established in the immediate area. Efforts to remove the species have been initiated.
On 31 August 2021, I found a small population of the non-native Ruellia dipteracanthus (Nees) Hemsl. in the creek bed near my house on Salton Drive, Austin, TX.
https://www.inaturalist.org/observations/93431167
@centratex was the first to suggest this ID and it was subsequently confirmed by @eric_keith. This was the 2nd report of the species in the Austin area and the 3rd Texas location of this potentially invasive species (Keith et al. 2017). The species is native to Brazil and established in the wild in Mexico (Mowat 2017, Nees 1847, cited in Keith et al. 2017). I had not previously noticed the species even through I work regularly in this stretch of the creek bed removing other invasive species such as Ligustrum spp. and Chinese Tallow Trees (Triadica sebifera). Another non-native species, Mexican Ruellia (R. simplex) is common along the creek banks in this watershed, and it was only because I noticed a difference in flower color, leaf shape, and pubescence that I realized it was something different (and non-native). The clusters of R. dipteracanthus are in a mixed native herbaceous groundcover on a sand and gravel bar just a foot or so above the level of the adjacent perennial creek.
I wondered about the origin of this non-native species and expected that the source would be from some suburban yard in the watershed upstream of the location of the plants. On 4-5 September 2021, I criss-crossed the neighborhood streets flanking this reach of the creek, studying flower beds in the front yards of homes in the area. I covered an area of about 65 acres (26 hectares) including most of the immediate watershed of the creek as far as 0.5 mi (0.85 km) upstream of the population of plants. This is only a small portion of the 500-acre watershed upstream of the plants, but I found no other plantings of the species.
Returning to the plants with the intent of collecting a few voucher specimens, I noticed that the clusters of the species were in the same area as some Smooth Horsetail (Equisetum laevigatum) that I had planted on the creek bank about 3 years prior.
https://www.inaturalist.org/observations/93802354
Then it dawned on me: The new Ruellia plants were almost perfectly coincident with those clusters of horsetails, with just a few small clusters immediately downstream where they had probably recently spread. My theory now is that even though I had only purchased one 1-gallon container of horsetails to plant, the pot may have contained some unnoticed rootstock of R. dipteracanthus, and thus I inadvertently planted both species at the site. Moreover, I had split up the horsetails into about 3 or 4 small clusters to plant in an area of about 10 m x 20 m, and each of those horsetail clusters now has a vigorous associated colony of the Ruellia.
That realization changed my goal: I immediately made plans to begin the process of removing the non-native Ruellia from the location. In that effort, I soon found that the species grows in dense clusters with vigorous rhizomes, a deep root system, and that stems bent over by floodwater were also commonly rooting at the nodes. Aboveground stems easily broke off at ground level nodes, a strategy which would benefit the plants and make them efficient colonizers under disturbance regimes like flood events.
https://www.inaturalist.org/observations/93802355
The effort to remove the easily reachable clusters of plants will take several days of digging in the sand, gravel, and cobble of the creek bed. It is likely that regrowth of some plants will occur in future growing seasons and that retreatment and vigilance will be necessary to ensure that all material has been removed. This is not a new type of effort for me. I previous undertook the removal of invasive Elephant Ears or Taro (Colocasia esculenta—not originating from me!) along this same stretch of creek and that effort took a few years to complete and requires continuing monitoring.
https://www.inaturalist.org/observations/1914012
https://www.inaturalist.org/observations/19376164
https://www.inaturalist.org/observations/19686222
https://www.inaturalist.org/observations/2116918
References:
Keith, E., J. Wright, and W. Godwin. 2017. Naturalized occurrence of Ruellia dipteracanthus (Acanthaceae) in the USA. Phytoneuron 2017-57:1-3.
