Naturalized exotic weed Tagetes minuta: A potentially invasive species in Nepal
Article information
Abstract
The exotic herbaceous plant Tagetes minuta L., native to South America, has been reported as a naturalized species in Nepal. The plant is listed among the invasive species due to its threat to natural communities in many countries, including India and China (countries that neighbor Nepal). Plant characteristics such as small, light, and relatively numerous seeds, rapid seed germination, drought tolerance, and the potential to harm neighboring plants are the characters of this plant that make it invasive. In Nepal, the species is reported from different parts of the country. The weed has been found densely colonized along roadsides and agriculture fields around Tachai village in Manang district, a Trans-Himalayan region in central Nepal. An ecological assessment revealed its significant presence in the field. Local people consider the plant to be detrimental to crops and native species in their locality. Analyses indicate that T. minuta is problematic in the locality. Hence, control and management of this weed are urgent.
INTRODUCTION
Tagetes minuta L. belongs to one of the largest families of vascular plants, Asteraceae. It is native to the temperate grassland of Southern America (Holm et al., 1997; Jingsheng et al., 2023). The common name of the plant is Stinking Roger. In Nepal, it is called as Jangali Sayapatri (meaning wild Marigold). The species is an annual aromatic herb having woody, ribbed, and branched stems. Unipinnate leaves are opposite (upper leaves alternate), having elliptic leaflets with serrate margin and acute apex. The inflorescence is terminal corymb, and flowers are yellow-green, having fused petals, 2–3 ray florets, and 4–7 disk florets. Seeds are achenes black, ellipsoid, pappus of 1 or 2 setae, apices ciliate (Soule, 1994).
It is a weed plant spread out from its native land to Europe, Africa, Australia, and Asian continents (Stadler et al., 1998; Babu and Kaul, 2007; Min and Tsering, 2015; Qi et al., 2022). The plant is reported as an invasive weed across many countries including tropics, subtropics, and temperate regions (Holm et al., 1997; Hulina, 2008; Byers et al., 2012; Jingsheng et al., 2023). The weed generally thrives in diverse habitats adapting to varying environmental conditions including roadsides, poorly managed fields, and neglected rangelands (Jingsheng et al., 2023). Light seeds and large number of seeds enable easy spread of T. minuta and fast-growing seedlings can establish quickly in novel areas contribute to its invasion (Kohli et al., 2009; Batish et al., 2011; Jingsheng et al., 2023).
In case of Nepal, it was reported during 1988 by N.P. Manandhar from Dolakha district, central Nepal (Herbarium number: KATH060582). It is then subsequently reported by Press et al. (2000) and categorized as a naturalized species. On the other hand, the plant has been listed as an important herb for commercial production of essential oil (DPR, 2012). Some studies have highlighted its antimicrobial properties and ethnomedicinal values (Joshi et al., 2014; Khakurel et al., 2014; Paudyal et al., 2021).
Invasion of alien plants is recognized to be a major threat to biodiversity and ecosystem services (Rai and Singh, 2020). The process of plant invasion involves introduction, establishment, naturalization, spread and then ecological impact (Richardson et al., 2000). It is unpredictable that how long plant takes to become naturalized or become invasive. Even after naturalization, the plant can become invasive anytime due to the influence of environmental factors (Dai et al., 2016). Moreover, the impacts and invasive nature of T. minuta have not yet been adequately monitored and assessed in Nepal but reports from neighboring countries of Nepal (China, India, Bhutan, and Pakistan) have included the plant in the list of invasive species (Batish et al., 2011; Qureshi et al., 2014; Jingsheng et al., 2023; Thinley et al., 2022).
Early detection of invasion and potential impact of T. minuta is instrumental in controlling further spread of the weed and formulating management intervention strategies (Brooks and Klinger, 2009; Blackburn et al., 2014). There is a need of keeping regular monitoring of any naturalized plants that have increased chances of becoming invasive species. Therefore, this study aims to identify potential invasiveness of T. minuta in the country and provide a baseline status for its emerging threats.
MATERIALS AND METHODS
Survey site
A field survey was conducted during October, 2023 in Tachai village, Manang district of central Nepal (Fig. 1). The village lies between 28°32′12″N and 84°20′27″E, at 2,200 m above sea level (asl.), near a tourist area of Nason Rural Municipality, Dharapani on the bank of Marshyangdi River (one of the major rivers of Nepal). A graveled road connects Dharapani to Tachai village which passes across the river through a forest, landslide area, and agriculture fields. Along the roadside and agriculture fields in this area, a dense population of T. minuta was observed (Fig. 2).
Map of survey site. A: Nepal map with Manang district, B: Manang district showing Nason Rural Municipality, C and D: Nason Rural Municipality showing the site of occurrence of T. minuta.
Tagetes minuta growing in agriculture field, Tachai village, Manang.
Ecological assessment
To conduct an ecological assessment of T. minuta, we employed a quadrat sampling method in agricultural fields near the Tachai village, Manang. We randomly selected sampling points within the fields, at a distance of 200 meters between two points. At each point, we sampled a 1 × 1 m2 quadrat and recorded the abundance of T. minuta and other herbs. A total of 20 quadrats were sampled across the agricultural fields. We calculated the frequency, relative frequency, density, relative density, abundance, relative abundance, and importance value index (IVI) of T. minuta and other species present in the quadrats (Mishra, 1968). The plants were identified following literatures and with the help of experts. Herbarium specimens are deposited in the Tribhuvan University Central Herbarium (TUCH), Central Department of Botany, Tribhuvan University, Nepal.
Impact survey
The impact of T. minuta was assessed through interviews and focus group discussions with the community people of the Tachai village. Household heads (17 households) and knowledgeable people participated in interviews and group discussions. During interview and group discussion, open-ended questions were presented to gather their knowledge and perspectives on the plant’s arrival, spread, and potential effects on the ecosystem. Participants were asked about the initial introduction of T. minuta to the village, how they first noticed it, and how its distribution has changed over time. The participants were also asked about uses or practices associated with the plant and whether they have observed any ecological changes due to its presence, use (if any), and management efforts that have been undertaken to control or manage T. minuta. Informed consent was obtained from all the participants prior to the interview and group discussion.
Secondary data and distribution map
In addition to the field survey, occurrence data for T. minuta in Nepal was obtained from a digital database of the National Herbarium and Plant Laboratories (KATH, 2019; http://plantdatabase.kath.gov.np/). The occurrence data were also extracted from the database of the Global Biodiversity Information Facility (GBIF) (https://www.gbif.org/). A total of 22 occurrence points were collected. A distribution map was prepared using QGIS software (QGIS.org, 2024). To know the invasive status of T. minuta, we conducted a review. We searched Google Scholar and CABI using the search phrase: Tagetes minuta along with the terms invasive, exotic, non-native, and naturalized. This search identified published studies on the invasiveness of T. minuta in neighboring countries and other regions.
RESULTS
Spatial distribution of T. minuta
Tagetes minuta has a global distribution to 77 regions across Asia, Africa, North America, Europe, and Australia (Holm, 1997; CABI, 2021). In the Hindukush Himalayan region, it has been reported from India, Bhutan, Nepal, China and Pakistan. Currently, no evidence suggests that T. minuta has been reported in Myanmar and Bangladesh.
There are six records of occurrence of T. minuta from the National Herbarium and Plant Laboratories (KATH) database that are from Dolakha, Bhajhang, Mugu, Jumla, Sindhuli, and Myagdi districts from elevation range 1,540–2,806 m asl. A total of 22 records of occurrence are reported from the GBIF database. These occurrence points represent all seven provinces of Nepal (Fig. 4). Current study recorded the species from 2,200 m asl. in the Manag district of Nepal (Fig. 2, 3).
Spatial distribution of T. minuta in Nepal (Data source for species occurrence: GBIF, https://www.gbif.org/).
Tagetes minuta. A: Habit, B, C, D: Inflorescence and flowers. Scale bars = 1 cm.
Invasion status of T. minuta
Tagetes minuta was highlighted as the invasive species in two South Asian countries, India and Pakistan, a decade ago and recently in Bhutan and China (Table 1). At the global level, it is listed as an invasive species in 14 countries (CABI, 2021).
Status of T. minuta in the countries of Hindukush Himalayan region.
Current status of T. minuta in study locality
Among the plots sampled in the study site (buckwheat and wheat fields), T. minuta showed its frequency of 45%. Similarly, the relative abundance was 2.4 and a relative density was 1.38 (On-line supplemental Table S1). On comparing the IVI of this species with the most frequent invasive species Bidens pilosa and Galinsoga quadriradiata in the study field, it ranked after B. pilosa (Table 2).
Importance Value Index (IVI) of T. minuta along with other invasive plants in the study area.
People’s perceptions
Local people named T. minuta as Jangali Sayapatri (wild marigold). In response to the first question about the initial appearance of T. minuta in the study site, local people reported that the species first emerged along the roadsides near the Dharapani area (a tourist destination near the village) approximately a decade ago (Table 3). Initially, only a few yellow flowering plants appeared, having a similar fragrance of marigold flowers. The marigold is commonly used in religious rituals in Nepal. Wondered by the similarity in fragrance, the local community assumed the plants to have a connection with marigold. Over time, the plant gradually spread along the roadsides, particularly following the gravel road from Dharapani to Tachai village. This expansion ultimately led to the introduction into adjacent agricultural lands. T. minuta continues to spread and proliferate.
People’s perception on Tagetes minuta.
Regarding the utilization and management of T. minuta, participants of focus group discussion concluded that the plant has no use. In contrast to traditional marigolds, its flowers are unsuitable for crafting garlands (Mala in Nepali) and are excluded from ritual practices also. Additionally, they mentioned that T. minuta does not serve as animal fodder. When asked about the infestation of T. minuta in agricultural fields, they expressed concerns about its invasive nature, referring it as a problematic weed that grows extensively in road margins, agricultural fields, and fallow lands. During the summer months, T. minuta proliferates into dense, monotypic stands. Its introduction has minimized the population of native plants. The people noticed a reduced growth and yield of crop plants associated with T. minuta introduction. Labor work and weeding cost have been increased since its infestation into the agriculture land.
Local people have tried to manage the plant by uprooting from agricultural fields. Despite efforts to manage the plant, they highlighted the challenge, as it tends to regrow persistently. Its presence has negatively impacted agricultural productivity posing challenges for effective control measures (Table 3).
DISCUSSION
This study identifies T. minuta as a problematic alien plant species in the study area. This research was conducted to evaluate the potential of T. minuta to be an invasive species based on published papers, ecological assessment, and people’s perceptions. We find that it has the potential to be invasive because of its inclusion as an invasive species in neighboring countries with similar climatic conditions (Table 1). Our ecological assessment also indicates a significant presence in agricultural land, evidenced by its 45% frequency in sampling. The weed exhibits the relative frequency and density, with IVI value after other invasive species in crop fields such as Bidens pilosa and Galinsoga quadriradiata (Table 2). This suggests that T. minuta has become a significant contributor to the local ecosystem, along with other highly problematic invasive species. This species has the potential to outcompete native plants and crops, which may disrupt the overall ecological balance of the local ecosystem.
Tagetes minuta bears light seeds in enormous numbers with awns. Studies highlighted that the seed germination rate is also high, and the plant can be adapted to drought and intense light conditions (Jingsheng et al., 2023). Also, the plant produces volatile oil, which has allelopathic potential on neighboring plants, insects, and microbes (Arora et al., 2015). These traits of the plant that were observed in the field signify the invasive nature like other invasive species in Nepal, such as Ageratina adenophora, Chromolaena odorata, and Mesosphaerum suaveolens (Gulzar et al., 2024).
Regarding the global distribution of the plant, it has been reported in many countries, including Hindu Kush Himalayan region, such as Nepal. It is reported as an invasive species in many countries worldwide, including South Asian nations like India, Pakistan, Bhutan, and China (Table 1), however, Gulzar et al. (2024) have excluded this weed as invasive species in Bhutan from their inventory. In Nepal, its distribution from east to west and from lowland to high mountains (Fig. 4) indicates that the plant has established in diverse geographical regions, suggesting a greater potential for invasiveness.
According to the local people, T. minuta appeared first along the roadside of a tourist area-Dharapani of Manang district. It implies that the plant might have been introduced through foreigners traveling to the high mountain areas of Annapurna trek. The fact that the plant migrates towards agricultural fields through newly constructed roads indicates that the plant thrives well in disturbed sites. The spread of the plant into agricultural lands, people’s perception of the detrimental nature of the plant to crops and native species, and the need for intensification of weeding or high weeding labor costs establish that T. minuta is emerging as one of the threatening weeds in the locality. Besides, frequent regeneration after uprooting the plant, no known use of the plant, and disliking by animals indicate that there will be further enhancement of colonization of the weed in other land types. This evidence clearly supports that T. minuta is being problematic in the area studied.
This study highlights the urgent need for assessing the spread and impact of the weed. Regular monitoring in areas where the weed is found should be conducted to mitigate its effects and prevent further proliferation; once the weed becomes invasive, control and management will become challenging. Early intervention is essential to prevent its spread and reduce long-term ecological impacts (Epanchin-Niell and Liebhold, 2015). Therefore, taking immediate action to manage T. minuta during its early stages of spreading and being problematic is crucial.
The findings strongly indicate that this weed has begun to proliferate from roadsides to agricultural ecosystem. This highlights that the plant is likely invasive in the site of the Trans-Himalayan region of Nepal. Farmers and responsible authorities should now be aware of the situation and implement effective management of T. minuta. Furthermore, regular monitoring should be initiated in all parts of the country where T. minuta has been observed.
SUPPLEMENTARY MATERIAL
On-line Supplemental Data Table S1 is available at https://doi.org/10.11110/kjpt.2025.55.1.36
Notes
ACKNOWLEDGMENTS
Research Division, Rector Office, Tribhuvan University, Kathmandu, Nepal is acknowledged for the research grant (Grant No. TU-NPAR-077/78-ERG 09). Department of National Parks and Wildlife Conservation, Ministry of Forests and Environment, Government of Nepal and Annapurna Conservation Area Project (ACAP), Pokhara, Nepal are acknowledged for study permission. We are thankful to the local people of Tachai village, Manang for support and information.
CONFLICTS OF INTEREST
The authors declare that there are no conflicts of interest.