Acmella radicans (Jacquin) R.K. Jansen is a new invasive species recorded in Yunnan Province, China, and little is known about its allelopathic potential and allelochemicals. In this study, the allelopathic effects of the essential oil (EO) of A. radicans on seed germination and seedling growth of four common plants, Brassica napus, Brassica rapa ssp. chinensis, Digitaria sanguinalis, and Lolium multiflorum, were explored. The results showed that the seed germination index, germination rate, root length, stem length, and biomass of B. napus and B. rapa ssp. chinensis were significantly inhibited at all EO concentrations of A. radicans, but there was a ‘low-promotion and high-inhibition effect’ on the root length of D. sanguinalis and L. multiflorum at low concentrations of 0.5 μL·mL⁻¹ and 0.5–1.0 μL·mL⁻¹, respectively. With increasing concentrations of EO, the inhibition rates of seed germination and seedling growth of four common plants gradually increased, and D. sanguinalis and L. multiflorum were the most inhibited, followed by B. rapa ssp. chinensis, and the least inhibited was B. napus. Thirty-two components were identified using GC–MS, representing 99.07% of the EO in A. radicans. The major components were 2-tridecanone (30.46%), caryophyllene oxide (19.18%), 4,8,11,11-tetramethylbicyclo[7.2.0]undec-3-en-5-ol (7.84%), β-caryophyllene (7.67%), and widdrol (4.7%). Among the compounds we identified, (E,E)-2,4-decadienal, 2-tridecanone, γ-cadinene, δ-cadinene, (E)-α-cadinol, spathulenol, caryophyllene oxide, and widdrol have been previously reported as having possible allelopathic effects. Our study was the first to show that A. radicans could potentially release allelochemicals to influence neighboring plants during its invasion and expansion.

To examine the effects of the recent Acmella radicans invasion on plant community and diversity in invaded habitats, the composition, density, species richness, diversity indices, and evenness index of the soil seed bank community of two different habitats (wasteland and cultivated land) in Yunnan Province, China, were analyzed through field sampling and greenhouse germination tests. A total of 28 species of plants belonging to 15 families and 28 genera, all annual herbs, were found in the soil seed bank. Seed densities and species number in the seed bank tended to be greater in April than in October; cultivated land also featured higher seed densities and species numbers compared to wasteland. With increased A. radicans cover, the seed bank population of A. radicans also significantly increased, but the seed bank populations of many other dominant species (e.g., Ageratum conyzoides and Gamochaeta pensylvanica) and native species (e.g., Laggera crispata and Poa annua) clearly declined. The germination of A. radicans seeds was concentrated during the period from the 4th to the 5th weeks. Vertically, the seed number of A. radicans was significantly different among the 0–5 cm, 5–10 cm and 10–20 cm layers that accounted for 80.7–90.6%, 9.4–16.1% and 0.0–3.2% of the total seed density in wasteland, respectively; and in cultivated land, A. radicans accounted for 56.8–64.9%, 26.7–31.8% and 8.1–13.5% of the total seed density, respectively. With reduced A. radicans cover, the species richness, Simpson index, Shannon–Wiener index, and Pielou indices of the weed community generally increased, and most diversity indices of weed communities in cultivated land were lower than in wasteland under the same cover of A. radicans. The results indicate that the invasion of A. radicans has negatively affected local weed community composition and reduced weed community diversity, and that these negative impacts in cultivated land may be enhanced by human disturbance. Our study was the first to elucidate the influence of A. radicans invasion on soil seed bank community characteristics in invaded habitats, providing a better understanding of its invasion and spread mechanisms in order to aid in developing a scientific basis for the prevention and control of this invader.

Acmella radicans (Jacquin) R.K.Jansen is an annual herb native to Central America. In China, it is becoming increasingly invasive and often co-occurs with the native congener A. paniculata (Wall. ex DC.) R.K.Jansen in some habitats. In order to understand the invasion mechanism of A. radicans, we investigated the growth parameters of both the invasive A. radicans and the native congener, A. paniculata, under different light conditions (5%, 25%, 50%, 75%, and 100% of light availability) using potted plants in a glasshouse. Light level, plant species, and their interaction were significant, with plant species generally having a greater effect than light level. Acmella radicans and A. paniculata showed great phenotypic plasticity to various light intensities and had a similar trend with increased shade. The plasticity indices of all parameters of A. radicans, except for branch length and inflorescence number, were greater than those of A. paniculata under the same light intensity. The physiological parameters for A. radicans under both favorable (high light intensity) and unfavorable (low light intensity) conditions showed less inhibition than those of A. paniculata. All these responses indicated that A. radicans had greater phenotypic plasticity and higher adaptability to low light, which may contribute to its invasion success.