This article followed all ethical standards for research without direct contact with human or animal subjects.

A list of invasive alien plant species occurring in Lesotho is presented in Table 1. Author citations are provided here and will not be repeated henceforth. A wide range of ethnobotanical uses of the IAPs is provided, ranging from food, fodder, through firewood, building material and timber production, to medicine and pesticides. In addition, information on their distribution and origin, as well as potential risks, is also given. In its report, NES (2005) documented a total of 31 invasive aliens comprising 15 trees and shrubs, 12 herbaceous and succulent weeds, 3 aquatic weeds and 1 grass species. A more comprehensive evaluation, produced 2 years later (NES 2007), listed a total of 54 invasive alien plant species (51 terrestrial and 3 aquatic) representing 25 families of angiosperms and 1 pteridophyte, with the largest number of species recorded in Asteraceae (7) followed by Cactaceae (5). That study also added information about species habitats, uses, distribution maps, pathways of introduction and invasive traits to the previous data. In the current study, a total of 58 species is recorded, comprising 1 pteridophyte, 1 gymnosperm and 56 flowering plants (52 dicotyledons and 4 monocotyledons). These numbers exclude 11 species (marked with † in Table 1) not listed by NES (2007) as invasive, but warrant further analyses to determine their status in the country (as discussed in subsequent sections). The largest number of species is recorded in the family Asteraceae (9), followed by Fabaceae (7) and Cactaceae (6). This is not surprising as Asteraceae is the largest flowering plant family, while Fabaceae is the third largest.

As Lesotho is completely surrounded by South Africa, we make a comparison with neighbouring areas of South Africa with regard to status, occurrence and distribution patterns of IAPs, particularly because ‘invasiveness elsewhere is one of the most reliable indicators of invasion risk’ (Early et al. 2016:5). To this end, it is important to note that nine species recorded by NES (2007) as IAS in Lesotho are not declared as such by the National Environmental Management: Biodiversity Act (No. 10 of 2004) (NEMBA) (https://invasives.org.za/legislation/what-does-the-law-say) in South Africa (highlighted in bold in Table 1). These are Avena fatua, Cosmos bipinnatus, Erigeron bonariensis, E. canadensis, E. sumatrensis, Foeniculum vulgare, Glandularia aristigera, Polygonum aviculare and Populus nigra var. italica. However, it is possible for a species to be invasive in one ecosystem but not in another. On the other hand, 11 other species (Acacia baileyana, Agrimonia procera, Eucalyptus camaldulensis, Hypericum perforatum, Ligustrum japonicum, L. lucidum, Morus alba, Pennisetum villosum, Sorghum halepense, Verbena brasiliensis and V. rigida) known to occur in Lesotho, and reported to be invasive in South Africa (https://invasives.org.za/resources/national-status-reports), are not recorded as invasive in Lesotho. However, it is important to note that South Africa is much larger and more diverse than Lesotho, with many different ecological conditions such as habitats, climate zones, soil types, etc., which may be suitable for the plants to become invasive. A search on the Plants of Southern Africa website (https://newposa.sanbi.org) for occurrence records of these plants in Lesotho returned the following results (number of specimens shown in brackets): A. baileyana (0), Agrimonia procera (1), H. perforatum (1), L. japonicum (0), L. lucidum (0), M. alba (0), P. villosum (1), S. halepense (3), V. brasiliensis (3) and V. rigida (3). A preliminary checklist of Lesotho plants (Kobisi 2005) listed all these species, with an exception of L. japonicum, as occurring in Lesotho. According to Henderson (2001), there were some records for A. baileyana, H. perforatum, the two species of Ligustrum and M. alba, while there were none for P. villosum and S. halepense occurring in Lesotho. Although Bromilow (2010) cautions that the distribution maps provided in his book are not a ‘definitive indication of the actual distribution of a plant’, his maps suggest that all these species (with the exception of H. perforatum) do occur in Lesotho. Of these species, the National University of Lesotho herbarium (ROML) has collections of only S. halepense and V. brasiliensis; however this should not be seen as an indication that the other species do not occur in the country, and it is possible that the country is under-reported. The existence of these species in Lesotho and their invasive status in South Africa may not necessarily suggest that they are invasive in Lesotho; however, these conflicting distribution records have revealed the dire need for a detailed assessment of their potential invasiveness or invasive status in Lesotho. It is surprising to note that NES (2007) did not list E. camaldulensis as an invasive plant in Lesotho, although this plant occurs widely in the country. The reason for this could be that the plant is naturalised in the country and considered by most people to be a very useful naturalised plant, instead of an IAP. A more detailed assessment is required to determine whether the distribution of E. camaldulensis fits Pyšek et al.’s (2004) definition of naturalised plants.

Despite being invasive, some of the plant species are utilised for a variety of purposes ranging from food (vegetables, fruits), medical conditions (skin, reproductive, digestive and respiratory problems) to functional uses such as building, poles, rafters, firewood and sleighs. For example, Nasturtium officinale and Hypochaeris radicata are consumed as leafy vegetables, whereas Opuntia ficus-indica, Gleditsia triacanthos, Pyracantha angustifolia, Rosa rubiginosa and Rubus cuneifolius are consumed for their edible fruits, with the last two species also used in jam making. In addition, Convolvulus arvensis is used by bees for producing honey. Animal feeds include such species as O. ficus-indica, Pennisetum clandestinum, Salsola kali and S. halepense. The tubers of Cyperus esculentus are eaten raw or roasted (pers. obs.).

A total of 17 species is used for medicinal purposes. Four of these (Agave americana, Datura stramonium, S. halepense and Tagetes minuta) are used for a variety of skin problems, namely bruises, blisters, sore feet, pimples, swelling and boils (Jacot Guillarmod 1971; Moffett 2010; Moteetee & Van Wyk 2011; NES 2007; Seleteng-Kose et al. 2015). Interestingly, the leaves of A. americana are used in the making of petroleum jelly in Lesotho. Other plants of medicinal importance include those used in the treatment of digestive ailments such as dysentery and constipation, and these include: A. dealbata (Moffett 2010), A. mearnsii De Wild. (Jacot Guillarmod 1971; Moffett 2010; NES 2007), O. ficus-indica (Moteetee & Van Wyk 2006; NES 2007; Schmitz 1982; Seleteng-Kose et al. 2015; Van Wyk & Gericke 2000) and S. halepense (http://www.invasives.org.za/component/k2/item/341-johnson-grass-sorghum-halepense). Three species (Argemone ochroleuca Sweet, S. halepense and D. stramonium) are used for respiratory ailments, including asthma and fever. Moreover, D. stramonium is also used to treat headache (Maliehe 1997; Moffett 2010; NES 2007; Phillips 1917; Schmitz 1982; Van Wyk, Van Oudtshoorn & Gericke 2009). Three species are used for the treatment of reproductive problems: Xanthium spinosum (Moffett 2010; Moteetee & Van Wyk 2011; Phillips 1917; Shale, Stirk & Van Staden 1999) and Populus x canescens (Moffett 2010; NES 2007; Seleteng-Kose et al. 2015) are used specifically for sexually transmitted infections (STIs) and A. ochroleuca is used by women for controlling menstrual flow and increasing breast milk. In addition, A. ochroleuca is utilised for the treatment of sore eyes (Maliehe 1997; Moffett 2010; Moteetee & Van Wyk 2011). Other medicinally important species include S. halepense, which is used for the treatment of rheumatism, and A. americana and T. minuta, which are ground into powder and taken as snuff (Jacot Guillarmod 1971; Moffett 2010; Moteetee & Van Wyk 2011; Seleteng-Kose et al. 2015).

Functional uses (such as poles, rafters, hedges and sleighs) are observed in several Acacia species, such as A. dealbata, A. decurrens, A. mearnsi, as well as Populus x canescens and A. americana. Leaves of A. americana are also used to make fibres used in floor mats (Moteetee et al. 2019). The importance of trees such as A. decurrens and Pinus halepensis Mill. as sources of firewood is highly significant in remote areas where electricity and other sources of fuel are scarce. Other species used for making fire include D. stramonium, G. triacanthos, P. angustifolia, R. rubiginosa and T. minuta. Some species are used as ornaments, namely Ailanthus altissima, Arundo donax, Cereus jamacaru, Echinopsis spachiana, G. triacanthos, Melia azedarach, M. aquaticum, O. humifusa and Robinia pseudoacacia. Women and girls use C. esculentus for making chaplets and necklets. In addition, they use pollen from A. baileyana as powder. Other useful species include T. minuta, which is used as a fumigant and in the formulation of pesticides as well as perfume. In addition, it is placed under bedding to deter bed bugs. Nicotiana glauca is used as a rat or cockroach poison, whereas M. aquaticum is used in aquarium tanks for aesthetic purposes. Many of the plants (18 in total) are used for ornamental purposes. Interestingly, in a study by Weber, Sun and Li (2008), the use of AIPs for ornamental purposes in China was identified as the most frequent economic use, with medicinal uses second. This seems to be the case for Lesotho as well.

On the other hand, some of the IAPs seem to be making notable impact in the commercial arena, for example A. americana and R. rubiginosa have entered local and international biotrade industries for making useful products for the food, pharmaceutical and cosmetic industries (Department of Environment 2014). In fact, Lesotho is currently exporting R. rubiginosa fruits to Germany (through the Rosehip Company) for making tea and jam, as well as for the production of essential oils used in the cosmetics industry. In addition, the remaining residue is reported to induce fertility in animals. To this end, it is projected that by 2040 the country may witness depleting number and abundance of commercially useful native species and increasing number and abundance of commercially used IAS (Department of Environment 2014).

A majority of the plants have originated from Europe (16), the Americas (North America = 8; Central and South America = 20) and Asia (16), with a few species from the rest of Africa (8) and China (Northern, Western) (5). Lesotho is divided into four agro-ecological zones, namely Lowlands, Foothills, Mountains and Senqu Valley, most of the recorded species have spread throughout the country across the four zones. The plants include Acacia species, A. americana, A. ochroleuca subsp. ochroleuca, Cirsium vulgare, Populus x canescens and R. rubiginosa. It is worth noting that an estimation of IAP infestation for Lesotho by Kotze et al. (2004) showed that most parts of the country, including all the four zones, have infestations ≥ 40%. On the other hand, a limited number of species are confined to specific zones because of varying altitudinal and climatic preferences which are different in the four zones. For example, A. filiculoides, G. triacanthos, M. azedarach and P. halepensis are confined to the lowlands, which is characterised by warmer temperatures and low altitude (below 1820 m.a.s.l.). However, the distribution of C. esculentus, E. spachiana, Cylindropuntia imbricata and H. radicata also extends to the Foothills and Senqu Valley, with the former having altitude of more than 1820 m.a.s.l. and the latter being the most degraded and driest zone. Only one species, C. jamacaru, is confined to the mountain zone; it is worth noting that in its natural habitat in eastern and north-eastern Brazil, the species occurs in dry, open forest areas and prefers less dry areas (Tropical Plants Database, Ken Fern. tropical.theferns.info.; tropical.theferns.info/viewtropical.php?id=Cereus+jamacaru).

Some of the plants are reported to be unsafe. For example, the fruits of P. octandra are said to be poisonous and capable of causing skin irritation, T. minuta causes irritation of the skin or photodermatitis yet it is used as a snuff (Jacot Guillarmod 1971), Ricinus communis and Sesbania punicea are extremely poisonous and the seeds are lethal. In fact, R. communis is regarded as the most poisonous plant for humans in the world, although it is used for medicinal purposes in many parts of Africa (Kuete 2014). In addition, A. ochroleuca subsp. ochroleuca is reported to be poisonous and an irritant, while the seeds can contaminate sheep’s wool (Bosch 2007). Although O. indica has many uses, the cladodes are reported to be poisonous when fed to livestock in large quantities. Young seedlings of X. spinosum and X. strumarium are toxic to livestock and burs may irritate the skin. Similarly, R. pseudoacacia (leaves, seeds and inner back) and D. stramonium (seeds) are poisonous to both animals and humans (NES 2007). Species of Datura including D. stramonium are used as narcotics in many parts of the world (https://www.cabi.org/ISC/datasheet/18006).

If properly planned and executed, many IAPs can be controlled through three primary methods of control: mechanical, chemical or biological. However, we are not aware of any control or eradication programmes in Lesotho, and we are also not aware of any early detection programmes. Mechanical methods may include physical removal of rhizomes, roots, cultivation and repeated cutting of the stem (Bromilow 2010). The removal can be done by hand-pulling, using hand-held tools or even bulldozers, and can be done in combination with burning (Van Wilgen et al. 2001). However, many species are difficult to control because they have extensive underground systems and produce large quantities of seeds and therefore regenerate vigorously. Furthermore, if the area is heavily invaded, mechanical controls can be laborious and therefore costly. Other mechanical methods include ring-barking (which can be used to kill large trees such as Populus x canescens) as well as cut-stumping and frilling of small trees such as A. altissima and R. pseudoacacia. In the case of trees, these methods should be followed by an application of herbicides as unremoved fragments may re-sprout and grow after the initial cutting. Herbicides can also be applied to kill seedlings of targeted IAPs, for example D. stramonium is susceptible to a wide range of herbicides that can be applied to the soil or leaves, including acifluorfen, bentazone, atrazine, cyanazine, simazine, bromoxynil, metolachlor and 2,4-D (https://www.cabi.org/ISC/datasheet/18006). However, there are concerns regarding the use of herbicides because of their negative impact on the environment (Van Wilgen et al. 2001). Biological control is preferred over mechanical and chemical controls as it is regarded to be more cost-effective and safer. In South Africa, the programme goes as far back as 1913 and since then 63 control agents have been successfully released (Zimmerman, Moran & Hoffman 2004). For example, the release of the mealybug (Hypogeococcus pungens Granara de Willink) is reported to have been successful in curbing the spread of C. jamacaru across most parts of the country (Paterson et al. 2011).

Legal instruments have also been instituted to control and eradicate invasive species in Lesotho, namely Laws of Lerotholi (1959), Weeds Eradication Act 18 of 1969, as well as the Environmental Act (2001). However, the biggest challenge is the establishment of prevention programmes for minimising further damage of ecosystems by IAPs. Several initiatives have been undertaken, such as manual removal of the species in some areas. Although the Lesotho Environment Act of 2001 prohibits the introduction of invasive alien species into ecosystems, ‘there are no provisions for the necessary instruments to prevent introductions, spread and management of invasions, both for alien and indigenous species’ (NES 2007). Furthermore, unlike in the neighbouring South Africa, where a national strategy for dealing with biological invasions has been developed (National Strategy 2014), no such legislation or guidelines exist in Lesotho. In addition, there is no national list of invasive species or their categorisation based on their level of impact or risk as is the case for South Africa (http://invasives.org.za/resources/national-status-reports).

According to Shackleton et al. (2007), the reliance on IAS by rural livelihoods is often not considered when control measures are put in place. While we did not consider the possible effects of the regulation and/or eradication of IAPs to the livelihoods of the rural communities in the current study, it can be inferred from the previous sections that despite the invasive nature of these plants, many of them have numerous uses in Lesotho. In a country with high unemployment rates between 24% and 28% and 53.7% of the population living below the poverty line (https://www.worldbank.org/en/country/lesotho/overview), the importance of these plants cannot be overemphasised. For example, we (all three authors) have personally observed scores of people (both men and women, young and old) collecting bucket loads of R. rubiginosa fruits, most likely to supply the export market. In this instance, the resource use benefit could outweigh its threat to biodiversity and the ecosystem; however, this warrants a more detailed analysis.