BIODIVERSITAS ISSN: 1412-033X Volume 21, Number 9, September 2020 E-ISSN: 2085-4722 Pages: 4376-4383 DOI: 10.13057/biodiv/d210957 Population density, multiple harvesting, and ability of Ipomoea reptans to compete with native weeds at tropical wetlands BENYAMIN LAKITAN 1,2,♥ , KARTIKA KARTIKA 1 1 Faculty of Agriculture, Universitas Sriwijaya. Jl. Raya Palembang-Prabumulih Km 32, Indralaya, Ogan Ilir 30662, South Sumatra, Indonesia. Tel.: +62-711-580169, Fax.: +62-711-580276, ♥ email: blakitan60@unsri.ac.id 2 Research Center for Sub-optimal Lands, Universitas Sriwijaya. Jl. Padang Selasa No. 524, Palembang 30139, South Sumatra, Indonesia Manuscript received: 18 August 2020. Revision accepted: 28 August 2020. Abstract. Lakitan B, Kartika K. 2020. Population density, multiple harvesting, and ability of Ipomoea reptans to compete with native weeds at tropical wetlands. Biodiversitas 21: 4376-4383. Despite as a nutritious, fast-growing, and well-adapted leafy vegetable at tropical wetlands; Ipomoea reptans has not been intensively cultivated yet. This study was designed for increasing productivity of this vegetable by optimizing population density, extending harvesting period, and its ability to compete with native weeds at tropical wetlands. Bottom wet culture system (BWCS) was implemented by placing all pots within 2 m x 4 m experimental pool filled with water to 2-cm depth to make sure bottom part of the substrate within each pot was continuously water-saturated. Results of this study indicated that despite fluctuated yield at each harvest, accumulative yields after five consecutive harvests were not significantly different among population densities from 14 to 71 plants per m 2 . Yet, quality of yield in most cases was better in lower population density treatment (14 plants per m 2 ), as indicated by SPAD value and marketable size of individual plants. Frequent NPK fertilizer application was effective for increasing yield. The first harvest was done at 4 weeks after seed sowing; thereafter, the plants were routinely re- harvested at about every week. This fast-growing vegetable also exhibited ability to compete with native weeds commonly found at tropical wetlands at density up to 11.3 mg cm -2 . Keywords: bottom wet culture, Ipomoea reptans, plant-weed competition, population density Abbreviations: BWCS: bottom wet culture system; DAS: days after sowing; NPK: nitrogen-phosphorous-potassium; SPAD: index for leaf chlorophyll content; WAS: weeks after sowing INTRODUCTION Ipomoea reptans is commonly named as water spinach in English and has been cultivated as leafy vegetable in many Asian countries (Hong and Gruda 2020); however, it has not been intensively cultivated. In Southeast Asia, this plant is popularly known as kangkong. Water spinach is an amphibious plant, adapting well to wetland ecosystems. It can float on water surface, grown on muddy soils, or on non-flooded lowlands. Besides as green vegetables, water spinach has been used in pharmaceutical industry (Chen et al. 2018; Hefny-Gad et al. 2018) and for phytoremediation of contaminated soil, especially in wetland ecosystem (Chanu and Gupta 2016; Tang et al. 2017). Water spinach is rich in amino acids and vitamins. It gives benefits via antioxidant and α-glucosidase inhibitory activities (Lawal et al. 2017). Water spinach is a fast-growing leafy vegetable. This vegetable can be cultivated in many ways. Farmers in Indonesia mostly grow water spinach by directly broadcast the seeds on lengthy rectangular raised beds. The plants are harvested at once every month and then immediately replanted again. Water spinach can also be planted in wider spaced rows, in this case, the first harvest is conducted within a month after planting and thereafter re-harvested regularly every week if water and nutrients availability are well managed. The other way is by free-floating stem cuttings on water. Water spinach has hollow stems, make it float on water (Kaur et al. 2016). Water spinach has also been used for purifying water polluted with some agrochemicals (Zhang et al. 2014); before the water was used for fish cultures, such as tilapia (Guo et al. 2019) and comet goldfish (Saaid et al. 2013) in an aquaponic system. Multiple harvest system relies on rapid plant regrowth after each of previous harvest. Many crops have been cultivated with multiple harvest system. Fresh stalk biomass of sweet sorghum could be harvested three times annually, i.e. the first cut and two ratoon crops (Rolz et al. 2017). The stalk is used as raw material for ethanol production. Rice crops could be harvested twice, i.e. main harvest and ratoon crop (Wang et al. 2020). Sugarcane was commonly harvested several times over a stretch of several consecutive years (Marin et al. 2019). Multiple harvests in Gramineae genus or Cerealia crops have been more commonly practiced. Meanwhile, multiple harvests in annual vegetables are rarely exercised. Fertilizer application after harvest is expected to enhance growth of the axillary stems in water spinach. Competition between water spinach at different plant population densities and native weeds at tropical wetlands has not been intensively studied. Objectives of this study were to increase productivity of water spinach by optimizing plant population density from 1 to 5 plants per pot, equivalent to 14, 28, 42, 57, and 71