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Korean J. Pl. Taxon > Volume 53(1); 2023 > Article
JUNG, NA, LEE, CHOI, CHO, and HYUN: New records of two alien plants, Juncus torreyi (Juncaceae) and Egeria densa (Hydrocharitaceae) in Korea


Naturalized populations of two alien plants were newly found, and we describe their morphological characteristics and habitats with photographs. One is a member of Juncaceae, Juncus torreyi Coville, and was newly found at a pool of a beach in Gangwon-do. This rush is native to North America and belongs to the sect. Ozophyllum (subgen. Juncus) according to certain morphological characteristics, such as its racemose inflorescence, the absence of floral bracteole, and unitubular leaves with perfect septa. J. torreyi is easily distinguishable from Korean rushes by its long rhizomes with swollen nodes and globular head with 25–100 flowers. Its introduction into Japan and Europe was reported, but the ecological risk associated with its over-dispersal is not known. The other alien plant is a submerged plant, Egeria densa Planch. (Hydrocharitaceae), which was found in streams in Gyeongsangbuk-do and ditches in the Busan-si area, both of which being in the watershed of the Nakdong River. Egeria densa is similar to Hydrilla verticillata (L.f.) Royle, which is native to Korea. However, it is distinguished from H. verticillata by its larger flowers and lack of overwintering organs. This alien plant is native to South America and was introduced for aquarium gardening and naturalized around the world. Egeria densa is treated as a malignant weed due to its asexual reproduction and rapid growth. Size changes and the number of populations of E. densa must be investigated.


Foreign plants continuously flow into Korea by means of artificial introduction for use or unintended introduction, and a few species have become naturalized. The alien plants are increasing in terms of the number of species and the sizes of populations, thus far 617 alien plants are listed (Kang et al., 2020). In the last two years, 23 taxa were newly recorded in Korea according to two journals (Korean Journal of Plant Taxonomy and Korean Journal of Plant Resources) and 12 taxa of them are alien plants, such as Verbena bracteata Cav. ex Lag. & Rodr (Kim et al., 2022) and Brassica tournefortii Gouan (Kang et al., 2022). We found naturalized populations of two alien plants, Juncus torreyi Coville (Juncaceae) and Egeria densa Planch. (Hydrocharitaceae), during floristic surveys and report them here.
The family Juncaceae Juss. includes seven genera of ca. 440 species, and most species belong to two large genera, Juncus L. and Luzula DC. (Kirschner et al., 2002). However, the number of genera is not yet fixed as Juncus is paraphyletic (Drábková and Kirschner, 2013). An intrageneric system of Juncus consisting of two subgenera and ten sections was proposed by Kirschner et al. (2002). In Korea, 19 native species of Juncus have been reported, and their morphological and anatomical characteristics have been analyzed (Kim and Kim, 2013; Jang and Oh, 2016; Gil et al., 2019). The alien plant J. torreyi was found in Gangwon-do, and we provide its morphological characteristics with photographs and an identification key.
Hydrocharitaceae Juss. is one of the largest families, consisting only of aquatic plants, and 18 genera and 100 species are distributed worldwide (Les et al., 2006). The plants of this family grow in freshwater or marine habitats and exhibit a great diversity of life forms, morphologies, sexual systems, and pollination mechanisms (Cook, 1996). The native plants of Hydrocharitaceae in Korea consist of seven genera and fifteen species (Na, 2010). Most of them are submerged (except Hydrocharis dubia (Blume) Backer) and grow in freshwater habitats (except Halophila nipponica J. Kuo) (Na, 2010). An alien plant found in the Nakdong River basin, E. densa, is similar to Hydrilla verticillata (L. f.) Royle in terms of its morphology. The morphological characteristics of E. densa are described and compared to a related species, Hydrilla verticillate.


1. Juncus torreyi Coville, Bull. Torrey Bot. Club 22: 303, 1895; Juncus nodosus L. var. megacephalus Torr. in Fl. New York 2: 326, 1843; Juncus megacephalus (Torr.) Alph. Wood, Class-book Bot. 724, 1861, nom. illeg., non. M. A. Curtis, 1835.—TYPE: U.S.A. shore of Lake Ontario, Aug 1834, Illegible collector name s.n. (holotype: NY, photo!).
Juncus torreyi f. brevipes Farw., Pap. Michigan Acad. Sci. 1: 92, 1923.
Juncus torreyi var. globularis Farw., Pap. Michigan Acad. Sci. 1: 92, 1923.
Juncus torreyi f. longipes Farw., Pap. Michigan Acad. Sci. 1: 91, 1923.
Juncus torreyi var. paniculatus Farw., Pap. Michigan Acad. Sci. 1: 92, 1923.
Juncus torreyi var. prolifer Lunell, Amer. Midl. Naturalist 4: 239, 1915.
Korean name: Buk-mi-gol-pul (북미골풀).
Herbs, perennial, 30–60 cm. Rhizomes 1–3 mm diam., with swollen nodes; nodes 5–8 mm diam. Culms erect, terete, 1– 5 mm diam., smooth, hollow. Leaves basal and cauline, lowest leaf sheath-like or with short blade, cauline leaves 2– 5; sheath open, 1–9 cm long; auricle 1–3 mm long, apex rounded, membranous; blade green, terete, linear, 5–30 cm × 1–5 mm, smooth, unitubulose, perfectly septate. Inflorescences terminal, with clusters of 1–10 (20) heads, 1–6 cm long; primary bract erect to ascending, linear, 1–5 cm long; heads 25–100-flowered, spherical, 10–15 mm diam. Flowers bisexual; tepals 6, light green, occasionally red or brown, margins white, lanceolate, narrowly acute; outer tepals 3, 3.8– 5 × 0.3–0.5 mm; inner tepals 3, 3.2–4.4 × 0.3–0.6 mm; stamens 6, filaments ca. 0.8 mm long, anthers ca. 0.4 mm long; style 3-fid. Capsules equaling tepals or slightly exserted, brown, 1-locular, lanceolate, trigonous, 3.9–5 mm long. Seeds ellipsoid, ca. 0.4–0.5 mm long, brown, surface reticulate (Fig. 1).
Flowering and fruiting: Aug. to Oct.
Distribution and habitat: A population of ca. 300 individuals was found at Donghae-si, Gangwon-do, in Korea, growing together with Phragmites australis (Cav.) Trin. ex Steud., Typha angustifolia L., and Juncus papillosus Franch. & Sav. in a freshwater pool along the seashore. The pool was temporarily created by rainwater after the reclamation of the lagoon. In the surrounding sandy ground, Linaria japonica Miq., Vitex rotundifolia L. f., Lespedeza cuneata (Dum. Cours.) G. Don., and Juncus effusus L. var. decipiens Buchenau grow. It is native to North America (most states in the USA and to Canada and Mexico) and was introduced into the Netherlands and Japan.
Specimens examined: KOREA. Gangwon-do: Donghae-si, Mangsang-dong, 1 Aug 2022, Kyu Song Lee s.n. (KB, KH, NNH, HIBR), 5 Aug 2022, Jongduk Jung 202208001 (KB, KH, NNH, HIBR), 202208002 (KB, KH).
Several varieties and forms within Juncus torreyi have been published, but all of them were synonymized by Kirschner et al. (2002). The Korean name is based on the fact that it is widely and naturally distributed in North America. J. torreyi belongs to the sect. Ozophyllum (subgen. Juncus) according to morphological characteristics such as its racemose inflorescence, absence of floral bracteole, and unitubular leaves with perfect septa. Therefore, the newly recorded plant was compared with three Korean native species of sect. Ozophyllum (J. wallichianus Laharpe, J. papillosus Franch. & Sav., and J. krameri Franch. & Sav.) in the key. The distinctive features of J. torreyi are long rhizomes with swollen nodes and spherical heads with many flowers. The molecular similarities relative to related taxa were compared based on the DNA sequences of plastid trnL-F and nuclear internal transcribed spacer (ITS) regions. There was no Korean rush showing significant similarity. The previously reported trnL-F sequence (GenBank accession no. MZ665441) of J. torreyi differs from that in this study (OQ435282) by only one substitution of 930 bp. The DNA sequence (MZ665440) of trnL-F isolated from J. texanus showed high similarity by two substitutions of 930 bp. However, J. texanus and J. torreyi are different in terms of the length ratio of the inner/outer tepals and length of the capsules. The nuclear ITS sequence of J. torreyi was not registered at GenBank before this study (OQ429293). The most similar nuclear ITS sequence is that of J. wallichianus (AB985731), but 21 mismatches in 655 bp exist between the two sequences. This rush is native to and commonly found North America (Brooks and Clements, 2000). Although a few records have been collected from other countries, such as Japan in 1942 and 1950 (Shimizu, 2003) and the Netherlands in 1982 and 1985 (Bijmoer et al., 2022), over-dispersal in this case remains unknown.

Key to Juncus sect. Ozophyllum in Korea

  • 1. Cespitose; rhizomes inconspicuous

    • 2. Stoloniferous, with turions; leaves subterete ············· ············································ J. papillosus 청비녀골풀

    • 2. Stolons and turions absent; leaves terete ···················· ········································ J. wallichianus 눈비녀골풀

  • 1. Scattered; rhizomes long

    • 3. Nodes of rhizomes swollen; heads (cluster of flowers) spherical, 25–100 flowered ······· J. torreyi 북미골풀

    • 3. Nodes of rhizomes not swollen; heads obconic–hemispherical, 4–10 flowered ···· J. krameri 비녀골풀

2. Egeria densa Planch., Ann. Sci. Nat., Bot., Sér. 3. 11: 80, 1849; Elodea densa (Planch.) Casp. Monatsber. Königl. Preuss. Akad. Wiss. Berlin 1857: 48, 1857; Anacharis densa (Planch.) Vict., Contr. Lab. Bot. Univ. Montréal 18: 41, 1931; Philotria densa (Planch.) Small, Man. S.E. Fl. 1503. 1933.— TYPE: ARGENTINA. Buenos Aires, ? Tweedie s.n. (holotype: K, photo!)
Submerged herbs, annual or perennial, dioecious. Roots adventitious, unbranched. Stems elongate, branched, 1.5– 3.5 mm in diam. and grow to over 1 m long; internode 2– 25 mm long. Leaves mostly in whorls of 4 at nodes, spreading or recurved, sessile, linear to lanceolate, 10–35 mm long, 2– 4 mm wide, margin finely serrulate, apex acute. Inflorescences axillary; spathe sessile. Male flowers 1–3 in each spathe, held above the water surface; pedicels 10–70 mm long and 0.5– 0.8 mm in diam.; sepals 3, ovate, 2–4 mm long, 1–3 mm wide, light green; petals 3, obovate to suborbicular, 6–10 mm long, 4–8 mm wide, white; stamens 9, filaments 1–3 mm long, yellow (Fig. 2).
Korean name: Brazil-geom-jeong-mal (브라질검정말).
Flowering: Jul. to Oct. (all year round in aquaria with water temperatures exceeding 20oC.)
Distribution and habitat: Gunwi-gun and Busan-si in Korea, belonging to the watershed of the Nakdong River. It grows together with Trapa japonica Flerow, Nymphoides peltata (S. G. Gmel.) Kuntze, Typha angustifolia L. and Zizania latifolia (Griseb.) Turcz. ex Stapf in stream and ditches with water depths of 0.1–1.2 m (Fig. 2). Native to South America (Brazil, Argentina, Uruguay, and Chile) and introduced into North and Central America, Europe, Africa, Asia, and Australia.
Specimen examined: KOREA. Gyeongsangbuk-do: Gunwi-gun, Gunwi-eup, Naeryang-ri, Wicheon Stream, 3 Jul 2019, H.R. Na 19070321 ♂ (KB, KH, NNH, HIBR); Busan-si: Gangseo-gu, Bongrim-dong, Dunchi-do, 27 Sep 2020, H.R. Na 20092701 ♂ (KB, KH, NNH, HIBR).
The exotic aquatic plant Egeria densa can be confused morphologically with the native plant Hydrilla verticillata. The distinctive features of E. densa are the flower size and stamen number. This species has a much larger and more conspicuous male flower attached to the pedicel, and lacks morphologically differentiated overwintering organs such as turions and tubers compared to H. verticillata (Fig. 2, Table 1). The two regions where this species was discovered are Wicheon Stream, a tributary of the middle stream of the Nakdong River, and Dunchi-do, a lower region of the Nakdong River. Although the introduction routes for each plant remain unclear, it is believed that individuals cultivated and distributed for horticultural purposes have emerged. Only male plants were identified in both regions, and only male flowers were observed even when grown indoors for two years (Fig. 2). It is known that no females have been identified outside the native range of this plant (Cook and Urmi Koenig, 1984). It has no overwintering organs but is known to be mainly distributed in warm temperate and cool subtropical regions. It is found at high altitudes or in cold water springs in tropical or subtropical regions (Cook and Urmi Koenig, 1984). It is treated as a noxious weed in the United States and Japan owing to its dominant vegetative propagation and rapid growth rate and is listed in the World Invasive Species Database (Global Invasive Species Database, 2023). In Korea, the distribution range is likely to expand around the Nakdong River water system, and closer investigations and continuous observations are required for its removal and management.


The authors declare that there are no conflicts of interest.

Fig. 1.
Photographs of Juncus torreyi Coville. A. Whole plant. B. Rhizome with swollen node. C. Culms. D. Leaf sheaths with auricle. E. Leaves in cross (left) and longitudinal (right) sections. F. Inflorescences. G. Flower. H. Stamen. I. Pistil. J. Outer tepals. K. Inner tepals. L. Capsules. M. Seeds. N. Population. O. Habitat. Scale bars = 10 cm (A), 1 cm (B), 5 mm (C–F), 2 mm (G, J–M), 1 mm (H, I).
Fig. 2.
Photographs of Egeria densa Planch. A. Whole plant. B. Roots from nodes. C. Stem and leaves. D. Upper leaves. E. Plants cultivated indoors. F. Axillary inflorescence, male flowers on pedicel. G. Male flower. H. Submerged habit. I. Population. J, K. Habitat (A, H, and J: Gunwi-gun, 3 Jul 2019; BD, F, G, I, and K: Busan-si, 27 Sep 2020; E: Hanam-si, 24 Mar 2021, originated from Busan-si). Scale bars = 5 cm (A), 1 cm (D, F), 5 mm (G).
Table 1.
Comparison between Egeria densa and Hydrilla verticillate
Characters Egeria densa Hydrilla verticillata
Sexual type Dioecious Dioecious, monoecious
Pollination Entomophilous Hydrophilous or anemophilous
Overwintering organs Absent Turions, tubers
Stem thickness (mm) 1.5–3.5 0.5–2.5
No. of leaves in whorls 3–6, mostly 4 3–9, mostly 5
Leaf length (mm) 10–35 6–20
Leaf margin Serrate Serrulate
Squamulae intravaginales Entire, translucent Fringed, brown
Male flower at anthesis Above the water surface, attached to pedicel On the water surface, detached from spathe
Male flower diameter (mm) 10–18 2–3
Stamens number 9 3
Female flower diameter (mm) (Not found in Korea) 3–6


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