163

Alien Aquatic Plants Naturalized in Japan:
History and Present Status
Yasuro KADONO
Department of Biology, Faculty of Science, Kobe University
Nada, Kobe 657-8501, Japane-mail: kadono@kobe-u.ac.jp

Abstract
Introduction of alien aquatic plants started in the Meiji Era (late 1800s) and some of them havebeen naturalized and caused serious weed problems in Japan. Eichhornia crassipes, Elodea nuttallii,
Egeria densa and Pistia stratiotes have proven to be typical noxious aquatic weeds. The history of suchinvasive alien aquatic plants is briefly reviewed. Recently, more and more alien aquatic plants have been
introduced due to a boom in aquarium and water gardening. Some of the newly introduced alien aquaticplants have begun to spread quickly. To date over 40 species have escaped and become naturalized inJapan. They threaten the biodiversity of native aquatic plants in various ways. The social backgroundsof the spread of alien aquatic plants are surveyed and the need for social education as well as legal regulation to discourage destructive activities are emphasized.
Key words: alien aquatic plants, aquarium plants, naturalization, vegetative propagation, weed
problems

1. Introduction
Some alien aquatic plants introduced from theirnative localities have spread explosively and caused
serious socio-economic problems in various parts of
the world as exemplified, for example, by Eichhornia
crassipes (water hyacinth) and Salvinia molesta (=S.
auriculata?) (Sculthorpe, 1967; Gopal & Sharma,
1981; Cook, 1990). They have also had serious impacts on regional biodiversity and functions of freshwater ecosystems. Cook (1985) enumerated many species which have enlarged their distribution area as
alien aquatic plants worldwide, although the documentation on the spread of aquatic plants into newareas was assumed to be unsatisfactory. Generally
speaking, means of vegetative reproduction are welldeveloped in aquatic plants and, once introduced, they
can propagate vigorously in suitable habitats. The uniformity and mildness of aquatic environments as compared with terrestrial ones are also factors which enable the spread of many alien aquatic plants over awide range.
In Japan, alien aquatic plants have been introduced
since the Meiji Era (late 1800s). For example,
Eichhornia crassipes was introduced as an ornamental
plant in 1884 or earlier (Kadono, 1996a). Similarly,
Myriophyllum aquaticum (formerly M. brasiliense; aGlobal Environmental Research8(2)/2004: 163-169
printed in Japan

species of parrot feather), Cabomba caroliniana, andIris pseudoacorus were among the alien aquatic plants
which were introduced as decorative plants around1900 (Ohtaki & Ishido, 1980). Egeria densa (formerly
often described as Elodea densa) was introduced in
the 1920s as an experimental botanical material. Actually several scientific papers have been published on
its physiology and cytology in Japanese journals sincethe 1930s. These plants escaped into the wild andhave become naturalized, spreading their distribution.
Some of them are truly invasive weeds and have become serious pests in many places (Kadono, 1996b,
2001).
In the past few decades, more and more alienaquatic plants have been newly introduced and commercially sold by various routes according to booming
demand for aquarium and water gardening. Some ofthese have escaped into the wild and started to spread
rapidly (Kadono, 2004). Their vigorous spread has
threatened the biodiversity of native freshwater ecosystems. Although much money has been spent tocontrol alien aquatic plant infestations, in some cases,
eradication of such plants has proven to be difficult.
In this paper, the history of representative alienaquatic plants in Japan is briefly reviewed and the present status and future scope are discussed.
©2004 AIRIES164

Y. KADONO

2. Weed Problems Caused by Alien Aquatic
Plants: a Historical Review
In this section some representative examples of thenoxious alien aquatic plants which have caused serious weed problems in the past in Japan are described.
2.1 Eichhornia crassipes (water hyacinth)Eichorrnia crassipes, native to South America, is afree-floating aquatic plant and well known as the mosttroublesome aquatic weed in tropical and subtropicalregions of the world (Gopal & Sharma, 1981).
Eichhornia crassipes has been introduced to many
countries as an ornamental plant for its beautiful flowers. However, once escaped, it spreads vigorously byvegetative means and chokes the surface of waters
such as rivers, lakes and reservoirs in a short time.
In Japan, in so far as records are available, it was
introduced via North America in 1884 (early Meiji
Era) as an ornamental plant (Kadono, 1996a). Afterthat it escaped into natural waters and had alreadyspread to rice fields and creeks in southwestern Japanby the early 1900s (Ueki et al., 1976). According tosome fragmentary information, the plant seemed common in the creeks of Kyushu at that time. Hakushu
Kitahara, a poet who was born in Yanagawa, an areawith many creeks in Kyushu, described the scenery of
the creeks choked by E. crassipes in a poem in the
1920s. An explosive expansion of E. crassipes wasreported thereafter in some waters in Kyushu, but it
was since the 1960s that the weed problems caused by
E. crassipes have attracted social attention. The development of urban areas and industrialization in Japan
has accelerated eutrophication of water in many lakes,
rivers and ponds. As a result E. crassipes has spread
explosively in many eutrophicated waters in southwestern to central Japan.
The dense floating mats of E. crassipes cover thewhole surface of waters and cause some problems likethe choking of irrigation and drainage ditches, interference with boating and fishing, unpleasant odors andflavors, water enrichment by dead plants in winter,and so on (Ueki et al., 1976). Because of complete
coverage of the water surface by pure stands of E.
crassipes, native aquatic plants have lost their nicheand been expelled. Depletion of dissolved oxygen in
the water occurrs under the dense mats of the plants
and its adverse influence on ecosystem function isserious. Although much money has been spent to control the E. crassipes infestation, it is still one of themost troublesome alien aquatic weeds in Japan.
2.2 Elodea nuttallii and Egeria densa
Elodea nuttallii is a submerged plant native to
North America. It was found in the northern part ofLake Biwa, the largest lake in Japan, for the first timein 1961 (at that time it was identified as Elodea
occidentalis; Ikusima & Kabaya, 1965). Withinseveral years the plants spread over the whole area ofthe lake and became the most dominant plant in thelake. Almost all the littoral regions were occupied by
the plant, which developed into dense pure stands at
2-5 m depth (Ikusima, 1984). At the same time its distribution expanded into other waters over wide areasof Japan. It is said that fragments of the plant weretransported together with the fry of Plecoglossus
altivelis (Ayu) from Lake Biwa to the rivers of various
regions in Japan (Ikusima, 1980). Infestations occurred also in some other lakes, e.g. Lake Oze-numa(Kurita & Minemura, 1985a; Nohara, 1988) and Lake
Haruna (Kurita & Minemura, 1985b). Because of
the explosive expansion of E. nuttallii plants, thenative flora of the lakes has faced a crisis of extinction.
Elodea nuttallii of North American origin can tolerate cool water, so its distribution has now expanded
throughout Japan including Hokkaido. It also grows
well in spring-fed clear, oligotrophic water. There it
competes with Ranunculus nipponicus and other species adapted to spring-fed water and threatens theunique ecosystems of Japan.
Egeria densa (formerly often described as Elodea
densa) is also a submerged plant native to SouthAmerica. Since its introduction occurred in the 1920s,
it has longer history of naturalization in Japan than
Elodea nuttallii. Kadono (1994) presumed that its
naturalization occurred at the latest in the 1940s,
based on herbal specimens collected in 1941 in
Yamaguchi, southwestern Japan. Most probably theplant started to expand its distribution in southwesternJapan at that time. However, few reports are available
about its exact status till the 1980s. However, in
1980, it underwent an explosive expansion in Lake
Biwa and became the dominant plant in place of
Elodea nuttallii. The event attracted social attention
as a serious weed problem because its dense communities hindered ship navigation and fisheries in thelake. Of course its effect on the ecosystem of the lake
itself was considered serious. Because Egeria densa
as well as Elodea nuttallii are evergreen plants, they
have a competitive advantage over the native aquaticplants. They cover the bottom of the lake from winterto spring and hinder the sprouting and early growth ofnative plants which are dormant in winter. Actually
the biomass of the native plants declined drasticallyafter the invasion of Elodea nuttallii and Egeria densa(Ikusima, 1991).
In the 1980s, Egeria densa became the second mostcommon aquatic plant species following Potamogeton
crispus in the rivers and streams of southwesternJapan (Kadono, 1988). Egeria densa was shown togrow well above 15℃ (Haramoto & Ikusima, 1988)
and has extended its distribution mainly in central tosouthwestern Japan, but even in northern Japan it has
invaded some waters under the influence of hot
springs or other kinds of warm discharge. Recently,
dominance of E. densa communities has been
observed in many rivers. This may be associated with

Alien Aquatic Plants in Japanthe increase of water temperature caused by global
warming and constant supply of nutrients.
Although the reason is still not known, populationsof both Elodea nuttallii and Egeria densa have been
observed declining or completely disappearing after
their explosive expansion. A similar case is wellknown regarding Elodea canadensis in the late 1800s
in Europe (Arber, 1920). As to the causes of the decline in the populations, some hypotheses have been
proposed: e.g., genetic explanations such as the loss of
viability owing to lack of sexual reproduction; anddepletion of some nutrients (for review, see Hutchinson,1975). Kadono et al. (1997) suggested vulnerability
to attacks by pathogens or fungi because of genetic
uniformity of the population. The involvement of suchorganisms as a possible cause of the decline of
populations should also be explored in future studies.
In case of Lake Biwa, the biomass of Elodea
nuttallii and Egeria densa decreased as compared withthat in the 1970s and 1980s and the populations ofrepresentative native species have recovered well
(Hamabata, 1991). However, the biomasses of both
species are still considerable. Large masses of floatingmats of E. nuttallii detach from the bottom and drift
up to the lake shores every year.
2.3 Pistia stratiotes (water lettuce)
Pistia stratiotes (Fig.1) is a free-floating plant
which is now widely distributed in tropical andsub-tropical regions of the world. The species wasFig. 1 Pistia stratiotes invasion in a Salvinia natans
community. (Kakogawa City, Hyogo Pref.)Fig. 2 Pistia stratiotes on sale in a gardening shop. The
attached label says that the plant is a ‘biotope plant’useful for water purification. (Kobe City, Hyogo Pref.)165

recorded as early as the 1930s in the Okinawa Islands,
the southernmost islands of Japan (Makino & Nemoto,1931), but its naturalization in other regions was notreported till the 1990s. Since the early 1990s an explosive expansion of P. stratiotes has been reported inmany waters mainly in southwestern Japan (e.g.,
Shimoda, 1982; Kamiaka, 1999; Mochida & Miura,
2001). Kadono (2001) suggested that the northward
shift of the distributional areas of the species may be
associated with increasing temperatures due to global
warming.
The rate of propagation of P. stratiotes is comparable to that of Eichhornia crassipes. In the case that P.
stratiotes and E. crassipes flourish together in thesame water, P. stratiotes is more dominant and difficult to control than E. crassipes.
Pistia stratiotes can not survive Japanese wintersexposed and its dominance may be only for one year,
although a case of over-wintering in ponds with warm
water discharge has been reported (Kamiya & Kunii,
1995). Nevertheless, new populations are found inother waters the following year. The reason is that P.
stratiotes has become a popular and common plant for
cultivation in private ponds, aquaria, or ‘biotopes’ andis available at low prices in many gardening shops andeven at so-called ‘home centers’ (Fig. 2). The plants
are often thrown away when they cover the wholewater surface by clonal growth under cultivation.
They become a source of vigorous spread in suitable
habitats. Successful seed production and overwintering by seeds as reported by Yamamoto and Fujii(1996) may be a potential menace which makes it
more difficult to eradicate the populations.
2.4 Other species naturalized for a long time inJapan
In addition to the above-mentioned species, someother alien species have established themselves inJapanese waters for a long time. Myriophyllum
aquaticum (=M. brasiliense), a parrot feather, wasintroduced as a decorative plant in the 1910s. No accurate history of its naturalization or expansion is
known but it has now become a nuisance plant covering whole water surfaces in some places (Fig.3).

Fig. 3 Myriophyllum aquaticum invading a community of
Nuphar japonica. The Nuphar japonica plants were
threatened by M. aquaticum. (Akou City, Hyogo Pref.)166

Y. KADONO

Cabomba caroliniana is a popular aquarium plant. Its
first record in Japan was in 1929 (Masamune, 1931).
Though it has not caused such serious weed problems
in our country, it is now distributed all across Japan.
Iris pseudacorus was introduced from Europe in the
1890s (Ohtaki & Ishido, 1980). At first it was a horticultural plant but now it has become naturalized inmany lakes, ponds and rivers throughout Japan.
In Table 1, alien aquatic plants which have been reported to occur in the wild are listed, including recently naturalized ones.

3. The Recent Increase in Newly Introduced
Alien Aquatic Plants: Its Background andPresent Status
Recently more and more alien aquatic plants havebeen introduced into Japan and sold in aquarium
shops and through the Internet. The cultivation of
aquatic plants in aquaria and in the water gardening
ponds is in fashion and has many semi-professional
and amateur enthusiasts. Now anyone can get alienaquatic plants easily by various means. This is why

Table 1 A list of alien aquatic plants naturalized in Japan*.

-------------------------------------------------------------------------------------------------------------------1. Widespread and completely naturalized species
Eichhornia crassipes (Mart.) Solms-Laub.
Egeria densa Planch.
Elodea nutallii (Planch.) St. John
Iris pseudacorus L.
Paspalum distichum L. var. distichum
Paspalum distichum L. var. indutum Shinners
Spirodela puncutata (G.F.W.) Thompson
Wolffia globosa (Roxb.) Hartog & Plas
Cabomba caroliniana A.Gray
Nasturtium officinale R. Br.
Myriophyllum aquaticum (Velloso) Verdc.
Nymphaea spp. (various cultivars and unidentified species are included)
2. Species with limited distribution
Sagittaria graminea Michx.
Vallisneria gigantea Graebn.
Heteranthera limosa (Sw.) Willd.
Bacopa rotundifolia (Michx.) Wettst.
Lemna gibba L.
Trapa bispinosa Roxb.
Utricularia gibba L.
Utricularia inflata Walter
3. Species which have increased rapidly in recent years
Azolla spp. (Several species and hybrids were introduced in rice fields to feed ducks in
anti-herbicide agriculture)
Pistia stratiotes L.
Alternanthera philoxeroides Griesb.
Veronica anagallis-aquatica L.
Gymnocoronis spilanthoides DC.
Hydrocotyle verticillata Thunb. var. triradiata Fern.
Hydrocotyle ranunculoides L.f.
Ludwigia repens Forster
4. Species which have been observed in the wild but may be of temporal occurrence
Salvinia mollesta Mitchell
Limnobium laevigatum Heine
Eichhornia azurea (Sw.) Kunth
Pontederia cordata L.
Thalia dealbata Fraser
Heteranthera zosterafolia Mart.
Lagarosiphon major (Ridl.) Moss
Hydrocleis nymphoides Buchen.
Neputunia oleracea Lour.
Nymphoides aquatica (Walt.) O. Kuntze-------------------------------------------------------------------------------------------------------------------* This list was compiled based on published records and other reliable information on naturalized plants. Some otherunidentified alien aquatic species have been reported, including Alismataceae, Poaceae, Cyperaceae, Juncaceae, etc.

Alien Aquatic Plants in Japan

167

more and more new alien aquatic plants are being
introduced and circulated.
Some horticultural and landscape planning companies also deal with alien aquatic plants commerciallyas ‘biotope plants,’ or plants serving as water purifiers.
These plants are planted in the construction of‘biotopes’ and also in a number of public ‘restoration’works without correct recognition of the concept of
restoration. Such cultivated plants become a sourceof escapees into natural waters. In some cases they areplanted intentionally, although some of them are truly
invasive.
For example, Gymnocoronis spilanthoides (Fig.4),
a Senegal tea plant of South American origin, is rapidly expanding its distribution. The flowers of theplant attract many insects including a beautiful butterfly species as pollinators so some amateur entomologists are interested in the plant. It is said that at one
time they did plant G. spilanthoides in order to collectthe butterfly. The species is also commercially dealt
with as a ‘water purification plant’ with no such scientific evidence. Once established in the wild, it
spreads vigorously throughout the area (Suyama &Fujiwara, 2003). Up to now it has been reported that G.
spilanthoides occurs from Kyushu to central Japan.
Most of the populations have spread from artificiallytransplanted sites.
Veronica anagallis-aquatica of European origin

has become a very common plant in some areas insouthwestern Japan (Tanaka & Noguchi, 1994). It
forms large stands especially in habitats disturbed byriver construction works. In addition it hybridized
with a related native species Veronica undulata(Tanaka, 1995), and the populations of V. undulata are
now endangered due to the spread of V. anagallisaquatica genetically as well as ecologically.
Alternanthera philoxeroides (Fig.5), a species ofalligator weed, is a notorious aquatic weed around theworld (Spencer & Coulson, 1976). Still now, its distribution is rather sporadic but the populations are increasing rapidly around Lake Inba-numa, Chiba Prefecture, central Japan (Sugiyama & Kuramoto, 2004)
and in some localities in southwestern Japan.
Hydrocotyle ranunculoides (Figs.6 & 7) has invaded the Kikuchi River and its tributaries in Kyushu
and shown an explosive expansion (Ito, 2001).
Despite efforts to control the weed by local governments, it is again spreading, forming many dense
stands in the area. Its growth potential in relation to
the nutrient status of water was studied and its invasiveness was predicted (Hayashida et al., 2003). It isalso one of the most invasive alien plants in Japan.
The common features of the invasive alien aquaticplants mentioned so far are strong ability to regeneratefrom the plant fragments and vigorous vegetative
propagation thereafter. Thus, once established, theyFig. 4 Gymnocoronis spilanthoides stand invading a river. In
this river the plants spread widely in a few years.
(Takatsuki City, Osaka Pref.)Fig. 6 Hydrocotyle ranunculoides community in a tributaryof the Kikuchi River. (Shisui Town, Kumamoto Pref.)Fig. 5 Alternanthera philoxeroides spreading in the ‘Wando(stagnant pools)’ of the Yodo River. (Osaka City,
Osaka Pref.)Fig. 7 Hydrocotyle ranunculoides elongating using creeping
stolons.

168

Y. KADONO

spread very quickly in suitable habitats. From theviewpoint of ecological risk assessment we mustevaluate such traits accurately.
Many other alien aquatic plants have become naturalized and spread to various degrees. Some are weedsof rice fields and introduced unintentionally. Heteranthera limosa and Bacopa rotundifolia are among theexamples of these. Species which have been cultivated in aquaria and ponds such as Limnobium
laevigatum and Hydrocleis nymphoides have sometimes been observed to grow in the wild. They musthave been transplanted intentionally by some aquarium enthusiasts. It is, however, doubtful if they will
spread in the future. They may be of temporaloccurrence, but some plants which were transplanted
to the wild have established themselves. We often encounter alien insectivorous plants which the enthusiasts transplanted to natural wetlands. Figure 8 shows
Utricularia gibba growing in a species-rich natural
pond. Here the species has become one of thedominant species in the flora of the pond. It was anintentional introduction by an enthusiast.

4. Concluding Remarks
More than 300 aquatic plants are available on themarket as aquarium plants (Kobayashi, 1995) andmany of these are alien. In addition newly introduced
alien aquatic plant species are increasing from year toyear. Among them, over 40 species have already become naturalized in Japan. In addition to the species
listed in Table 1, there are some more species whichhave not yet been identified. Of course, some of them
are limited in distribution and may not be invasive,
but we have inadequate knowledge of their ecologicaltraits and suitable habitat conditions. We should assume that many of them may be invasive. Nevertheless, the introduction and circulation of alien aquaticplants have not been regulated legally as of this time.
Besides the hobby of aquarium fans, another big
problem is that some alien aquatic plants are planted
in nature conservation and restoration projects by administrations and some citizens groups in good will.
They think simply that the recovery of ‘greenery’ is a

Fig. 8 Utiricularia gibba naturalized in a pond. (Nara City,
Nara Pref.)favorable thing. Such mistaken activities should be
stopped as soon as possible. It is necessary to havepeople understand the importance of the conservationof biodiversity of locally unique nature. We shouldmake all efforts to prevent the escape of alien aquaticplants. To achieve this purpose, both social education
to promote the public awareness of the ecologicalrisks of alien plants and legal regulation of alienaquatic plants will be equally important and indispensable.
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