Lonicera japonica (Japanese honeysuckle)

L. japonica is an aggressive vine which develops into a smothering mass of belowground runners and aboveground intertwined stems that cover extensive areas of the ground or climb up trees for many metres. It interferes with forestry operations and orchards and smothers native vegetation, preventing natural successional processes by killing or weakening young trees and preventing seedling regeneration. The fleshy fruit are spread by birds and animals, and runners go for long distances underground. It cannot be controlled simply by hand, but herbicides are moderately effective.

L. japonica is considered a major pest in the United States, Australia, and New Zealand as well as in many countries in Europe and South America due to its ability to escape from cultivation and invade both disturbed and natural areas. It has a smothering habit and can engulf small shrubs and trees that it climbs. It seriously alters the understory and herbaceous layers of native plant communities such as prairies, barrens, glades, flatwoods, savannas, floodplain, wet forests, woodlands, and montane forest. It also may alter understory bird populations in forest communities. Vegetative runners are most prolific in the open sun and will re-sprout where touching the soil, forming mats of new plants. L. japonica may form a complete blanket outcompeting trees and shrubs and it is becoming an increasingly troublesome species in shrublands, forest margins, and open roadsides (Starr et al., 2003). 

The genus Lonicera L. (syn. Caprifolium Mill., Metalonicera M. Wang & A. G. Gu, Xylosteum Mill., family Caprifoliaceae) includes some 180 species of erect shrubs and twining climbers native to North America (south to Mexico) and Eurasia (south to North Africa, the Himalaya, Java, and the Philippines). The genus Lonicera has been divided into two subgenera: the subgenus Caprifolium and the subgenus Lonicera. Within the subgenus Lonicera four sections have been established: Coeloxylosteum, Isoxylosteum, Nintooa, and Isika (Theis et al., 2008). Each of these sections is marked by a unique combination of morphological characters (Theis et al., 2008). 

The genus name,

L. japonica is a perennial woody climber vine, twining or clambering, much branched, 5-8 m in length. Stems strong, flexible, cylindrical, pilose, brown, smooth. Leaves opposite; blades 4-7.5 × 1.2-3 cm, lanceolate, oblong-lanceolate, or elliptic-lanceolate, chartaceous, glabrous except for some hairs on the midvein, the apex acute or obtuse, abruptly acuminate or mucronate, the base rounded, subtruncate, or obtuse, the margins entire revolute, ciliate; upper surface dark green, dull; lower surface pale green, dull, with a prominent midvein; petioles 5-12 mm long, brown, pubescent. Flowers fragrant, sessile, in pairs at the end of short axillary branches; bracts 2, at the base of each flower, ovate, rounded, ca. 0.7 mm long, ciliate. Calyx green, tubular, ca. 3 mm long, the sepals 5, subulate, ca. 1 mm long; corolla white, turning yellowish when mature, 2.2-2.5 cm long, infundibuliform, the tube puberulous outside; the limb with 5 lobes, one of which is longer (ca. 2.2 cm) and free; stamens 4, exserted; style exserted, with the stigmatic surface claviform (Acevedo-Rodriguez, 2005). Fruits are sessile berries, 0.4-0.7 cm in diameter, hard and green when immature, and black and soft (even fluid-filled) when ripe. Fruits contain two or three seeds that are approximately 0.2 cm in diameter, ovate to oblong, with a flat to concave inner surface and three ridges on the dorsal surface.

L. japonica is native to China, Japan and Korea. Currently it is widely naturalized - mostly in  North America, Canada, Central America, parts of South America, Europe, and Australasia (see Distribution Table for details). It has only a limited presence in Africa. 

The history of the introduction and spread of L. japonica has been well documented in North America, where it is a serious weed (Andrews, 1919; Leatherman, 1955; Hardt, 1986). It was introduced into the USA in 1806, was widely cultivated by the 1860s, and has become 'oppressively established in many parts' (King, 1966). However, it did not appear in early floras, and was not noted in the wild until 1882. The first account of its spread was in 1904, and it has now spread over much of the eastern United States from Illinois and Michigan in the north to Florida in the south, and is listed as a pest in several states. Being planted for several reasons including road stabilization and food for birds encouraged its spread. In Hawaii, the first naturalized collection of L. japonica was made in 1951 (Wagner et al., 1999).

L. japonica was first discovered in Canada in southwestern Ontario woodlands in 1976, and by 2007 had been found growing without cultivation in 15 localities (Larson et al., 2007). While not occurring frequently in southern Ontario, climate change models suggest it may become more abundant in the future.

L. japonica was introduced into Australia between 1820 and 1840 (Stackhouse, 1981) and was one of the most frequently listed plants in nursery catalogues between 1840 and 1980. It was first collected growing wild in Australia in November 1919 (Ewart and Tovey, 1920). In New Zealand, L. japonica was offered for sale in 1872 (Esler, 1988), and it was first collected in the wild in 1926 (Webb et al., 1988). In the late 1980s L. japonica was described as 'abundantly naturalized in many areas but less common in the southern parts of the South Island' (Webb et al., 1988).

In the West Indies, L. japonica was first recorded in 1881 in Martinique and in 1886 in Puerto Rico (US National Herbarium). 

The risk of introduction of L. japonica is very high. This species is commonly planted as an ornamental and medicinal herb and has repeatedly escaped from cultivation. Once established it behaves as a weed mostly in disturbed and waste places, interrupting plant succession by overtopping and smothering young trees and preventing their recruitment to the overstory (Langeland et al., 2008; Randall, 2012). The risk of introduction of seeds dispersed by birds and other animals remains high principally in areas close to cultivation. Additionally, seeds and seedlings of this species are still sold online and in the nursery trade in many countries and thus the species is available for further dispersal.

A number of studies in the USA have assessed the potential for range spread of this invasive species. Wang et al. (2012) suggest that probability of invasion is correlated positively with adjacency to water bodies, temperature, site productivity, species diversity, and private land ownership. Lemke et al. (2011) suggested that disturbed forest and/or high fragmentation has a higher invasion potential and given past trends and expected continued population growth this disturbance and fragmentation will only increase. Beans et al. (2012) report that both climate change and human influence determine the distribution of L. japonica, and that despite the plants long invasion history, it is expected to spread beyond its current US range. Modeling the spread of invasive horticultural plants using climate alone risks under-predicting areas with poor climates and high human influence, and it is suggested that planting invasive horticultural species should be discouraged as even suboptimal climates may result in further range expansion.  

Natural Dispersal (Non-Biotic)

This is not a common method of dispersal, although stem fragments are carried by water in floods.

Vector Transmission (Biotic)

L. japonica spreads by seeds, with transport in the gut of birds the main disperal method. Seeds are dispersed by birds in Australia (Blood, 2001), particularly silvereyes [Zosterops lateralis]. In New Zealand, the fruit is eaten by blackbirds, song thrushes and silvereyes (Williams and Karl, 1996). Brushtail possums [Trichosurus vulpecula] will also eat the fruits, and some seeds pass through apparently undamaged (Williams et al., 2000). Grazing and ungulate mammals including deer, sheep, goats, and cattle may assist spread by transporting vegetative fragments and dispersing seeds.

In North America, fruits are eaten by a wide range of birds, from turkeys to small passerines. Because of the high water content of the fruit, the seeds pass quickly through birds, including gallinaceous species such as turkey and quail (Handley, 1945). The fruits are also eaten by mammals such as deer, but it is not known whether seeds survive passage through the gut.

The wide range of birds which eat the fruit and disperse the seeds means that the species could potentially be dispersed by the local birds of any place it was introduced to.

Agricultural Practices

Attempts to control L. japonica by mowing can result in its spread, especially if fragments of vines are caught in machinery.

Accidental Introduction

There are no reports of L. japonica being distributed accidentally and it is not a contaminant of trade.

Intentional Introduction

As a horticultural plant, L. japonica is frequently sold, or given away, and transported by people. Seeds are available via the Internet.

L. japonica smothers other plants at all growth stages, i.e. it covers bare ground (pre-emergence) right through to occupying cut-over (post harvest) forests.

L. japonica is highly distinctive, at least outside its native range, as a pest plant. In the USA, L. japonica (including the varieties) is easily distinguished from native honeysuckle vines by its upper leaves and by its berries. The uppermost pairs of leaves of L. japonica are distinctly separate, while those of North American honeysuckle vines are connate, or fused to form a single leaf through which the stem grows. L. japonica has black berries, in contrast to the red to orange berries of North American honeysuckle vines.

L. japonica can also be confused with Lonicera fragrantissima and Lonicera periclymenum. However, these species can be distinguished by the following differences:

Outside its native range in the Southern Hemisphere, L. japonica can withstand an extensive period of frost, and it grows as high as 1120 m in south-east Australia. It reaches maximum abundance in places below 500 m with a moist, warm temperate climate (Williams et al., 2001). It has some capacity to withstand seasonal drought. In New Zealand, L. japonica grows from sea-level to 743 m in the central North Island.  

In North America, L. japonica grows up to 1800 m in both open and shaded situations (Leatherman, 1955). Growth is limited in northern parts by the death of shoots from frosts, in western parts by inadequate precipitation, and in southern parts possibly by the absence of sufficiently cold temperatures to break seed dormancy (Leatherman, 1955). Infestations have reached pest proportions in areas with annual precipitation of at least 1000 mm, mean January (winter) temperatures of at least -1°C, and freezing temperatures on at least 5% of January nights. In Ukraine, L. japonica has been noted as growing until the first frosts, its hardiness apparently being due to the shoots lignifying rapidly (Panova, 1986).

L. japonica appears tolerant of a wide range of substrates derived from volcanic, sedimentary, and metamorphic rock types (Auld and Medd, 1987). It is quite tolerant of poor drainage but rarely establishes on excessively drained and drought-prone sandy or stony soil and is tolerant of moderate salt spray. In North America, L. japonica grows on a wide range of substrates, from pH 4.0 to 7.9, and spreads most rapidly on soils above pH 6.0 (Leatherman, 1955). L. japonica is one of the few species tolerant of pollution from heavy metals and sulphur dioxide (Caiazza and Quinn, 1980).

Documenting invasion of L. japonica in southern Illinois, USA, West et al. (2010) report that the plant occurred preferentially in areas of high litter cover and species richness, comparatively small trees, low PAR, low soil moisture and temperature, steep slopes, and shallow soils.

Genetics

Genetic variability within a population can be a means of adaptation to new environments. Levels of allozyme variation in populations of L. japonica (2n = 18) in the south-eastern United States are no higher than in the congeneric native L. sempervirens (2n = 36). Genetic variability thus appears less important than other life history traits in the relative success of this invasive species (Schierenbeck et al., 1995).

No hybrids have been reported, but there are several cultivated forms (Webb et al., 1988). The potential of these to produce weedy forms is unknown.

Reproductive Biology
 
Plants grown from cuttings will produce flowers after two years (Leatherman, 1955), but there appear to be no studies that have recorded the time to flowering from seed. Flowers open at dusk and are pollinated by large moths (Roberts, 1979; Miyake and Yahara, 1998).

In North America, indigenous hawk moths (Sphingidae), wax moths (Gelechiidae), syrphid flies (Diptera), and bees (Hymenoptera: Apis melifera and Bombus) have been observed extracting pollen from flowers (Starr et al., 2003).

L. japonica is generally described as producing abundant fruit in North America. Fruit production in New Zealand and Australia varies considerably from place to place and year to year, some stands producing no fruit in any one year (Williams et al., 2001). Fruit are produced most abundantly on side shoots of second-year or older wood.

Physiology and Phenology

L. japonica seeds require a period of cold temperatures to break dormancy. Germination in soil of 63% was achieved following exposure to temperatures of 5-8°C for 60 days (Leatherman, 1955). Germination occurs in spring, as soon as air temperatures exceed 10°C. Germination occurs over a wide temperature range but was greatest at a fluctuating daily range of 18-25°C (Leatherman, 1955). The occurrence in Australia of plants in some places unlikely to experience such cold conditions suggests that this germination requirement is not rigid (Williams et al., 2001).

L. japonica seeds have only a small amount of endosperm, and the cotyledons contain chlorophyll at an early stage. The oval cotyledons are small, foliar, and green. Two true leaves are produced by the time the seedling reaches 3 cm tall. The first true leaves are simple and appear several days after the cotyledons have opened. Seedling leaves are indented, with lobate margins (Buchanan, 1981). Taproot development occurs simultaneously with leaf development. Seedlings can reach 30 cm in five months from germination and the first branches appear in this period (Leatherman, 1955). Overall, seedling growth is slow for the first two years (Little and Somes, 1967). Older seedlings develop several shoots arising from the original stem base, typically growing out in all directions over the surrounding vegetation.

L. japonica has increased competitive ability in the presence of raised carbon dioxide levels (Sasek and Strain, 1991) which suggests a capacity to spread with the predicted raised levels of global carbon dioxide. It has greater plasticity (Schweitzer and Larson, 1999) and higher rates of photosynthesis than the North American native L. sempervirens (Schierenbeck and Marshall, 1993).

Associations

L. japonica is generally associated with vegetation disturbance. It invades areas of natural disturbance such as along watercourses and commonly drapes itself over shrubs and small trees at forest margins, particularly near settlements and beside roads, forming low, tangled thickets supporting few other plants, sometimes tens of metres across.

In Australia, L. japonica invades areas dominated by Eucalyptus species in disturbed bushland, riverine woodland, forests close to urban areas, disturbed drainage lines, and pine forest. In New Zealand, L. japonica has long been recognized as a weed of hedges (Gunning, 1964) and now occurs in a wide range of open habitats such as roadsides and wastelands, the margins of wetlands (including coastal wetlands), and communities with some degree of woody cover. Shrublands 4-6 m tall may be completely covered by particularly vigorous stands but in dense forest or scrub, it is often restricted to forming a curtain of growth on the outside margins. It often grows in association with other alien vines or scramblers and weedy alien shrubs and small trees.

In North America, L. japonica is a major component of succession in old fields (Keever, 1979). It can invade established woodlands, particularly deciduous woodlands, but is limited by the deep shade of evergreen forest. Woodlands are invaded when natural processes such as storms, hurricanes and landslides or human-related disturbances create canopy openings (Andrews, 1919; Wistendahl, 1958; Slezak, 1976; Thomas, 1980). 

In its adventive range, L. japonica is browsed by a wide range of ungulates including deer, sheep, goats, and cattle, while in Australia it is eaten by several marsupials according to unpublished sources quoted by Williams et al. (2001).

The species Pythium sp., Eutypella fraxinicola and Microsphaera penicillata have been listed as present on L. japonica in the USA, but these species are probably opportunist as the host is not native there (Farr et al. 1999). Appendiculella lonicerae and Cladosporium lonicericola have also been reported as parasitic on L. japonica (He and Zhang 2001; Song et al. 2002). Miranda et al. (2014) report the presence in the Brazilian states of Rio de Janeiro and Paraná of leaf spots consistently associated with a cercosporoid hyphomycete, Pseudocercospora lonicerigena. However, inoculation experiments suggested that the fungus is not highly pathogenic.

L. japonica affects forestry operations in North America by interfering with site preparation (Little and Somes, 1967, 1968) and by occupying inter-tree spaces in forests managed by selecting individual trees (Cain, 1992). It is a particularly serious pest in Virginia apple orchards, where it handicaps cultivation, dries soil, and harbours mice (King, 1966).

L. japonica competes with native plants for light and nutrients. It outcompetes native vegetation by spreading rapidly and completely covering and toppling small trees and shrubs. The species also prevents the development of understory vegetation and the germination and establishment of seedling of native trees resulting in a reduction in forest understory diversity (ISSG, 2014).

L. japonica invades open sites, including forest edges, shrublands, and moist grasslands on floodplains, gullies, or stream banks. It smothers shrubs and small trees, killing or weakening them. Its stems twine around other plants and eventually grow over their crowns, smothering the host. Wetland and riparian margins are especially vulnerable (Williams and Timmins, 1990, 1999). In eastern North America, open habitats such as low shrublands may be completely smothered, and several habitat types such as glade communities and ravines are threatened. In forest interiors, particularly of deciduous forests, L. japonica causes the collapse of the understory shrub layer and occasionally small canopy trees, preventing the establishment of new shrub populations (Robertson et al., 1994). This leads to a simplified forest structure and lower floristic diversity. 

In oldfield successions in New Jersey, L. japonica occurred in many vegetative associations, where it often inhibited later successional species (Myster and Pickett, 1992). An investigation into both above-ground and below-ground effects of the competitive process, showed that Japanese honeysuckle had a greater effect on the allocation patterns of the host tree than on its photosynthesis (Dillenburg et al., 1995). This was mediated through competition between the vine and its host for soil nitrogen (Dillenburg et al., 1993a, b).

No social impacts are reported. Because of its economic and environmental impacts, it is an offence to spread L. japonica in, for example, New Zealand (Williams et al., 2001), which means that people must forgo the pleasure of growing it.

L. japonica is a valued medicinal herb that contains anti-complementary polysaccharides (Shin et al., 1992; Van Galen, 1995). Polyphenolic compounds from it help maintain human vascular homeostasis (Chang and Hsu, 1992). It is also mixed with other plants to produce a product called Aden I which has antibiotic effects (Houghton et al., 1993). Leaves and flowers are used in the treatment of chicken pox (Luo, 1989), and may be used as a food additive to increase productivity of broiler chickens in Korea (Cho, 1992).

L. japonica is a horticultural ornamental and is widely grown for its fragrant blossom, with several cultivars being available. Its introduction and spread over more than 150 years has been well documented in North America by Leatherman (1955) and in Australia and New Zealand by Williams et al. (2001).

In North America it has also been used as a road-bank stabilizer, and as food and shelter for wildlife (Handley, 1945). It provides valued year-round browse for deer in Alabama, USA, where the use of fertilizers has been recommended to increase its production and quality (Dyess et al., 1994). Its extensive spread in eastern USA is attributed to its use as protection of rail and road embankments (King, 1966). It may also be of value to apiarists (Roberts, 1979).

It is likely to be introduced to countries where it does not already exist, although these are probably few because it has been traded internationally for so long.

Introduction

L. japonica must be controlled if spread is to be prevented, but consideration should always be given as to what might come up in its place and whether additional cultural treatment of the site is appropriate, e.g., hand planting of trees into bare ground left by the demise of L. japonica. 

The species is securely established in much of its introduced range, but Schierenbeck (2004) suggested that there may be hope for the control of local infestations and its further spread in areas that have a fairly recent introduction history. Land management principles that discourage its use and public education to prevent further dispersal are strongly recommended.

Mechanical and Cultural Control

Mechanical control is totally ineffective for stabilised populations of L. japonica because the original plants and the cut runner resprout. However, regular mowing along the edge of a colony may slow its lateral spread. Heavy grazing by stock will not eliminate existing stands of L. japonica but it will prevent lateral expansion of colonies. Hand pulling is too time-consuming and fails to remove all the underground stems. Hot fires can reduce infestations according to observations in Williams et al. (2001).

For small patches, repeated pulling of entire vines and root systems may be effective. Seedlings and young plants should be pulled when the soil is moist, holding low on the stem to remove the whole plant along with its roots. Monitor frequently and remove any new plants. Cut and remove twining vines to prevent them from girdling and killing shrubs and other plants.

An effective method for removal of patches of honeysuckle covering the ground is to lift up and hold a portion of the vine mass with a rake and have a chainsaw operator cut the stems low to the ground. Mowing large patches of honeysuckle may be useful if repeated regularly but is most effective when combined with herbicide application. Mowing should be done twice a year, first in mid-July and again in mid-September. Plants can also be grubbed, taking care to remove all roots and runners. Burning removes aboveground vegetation but does not kill the underground rhizomes, which will continue to sprout.

In certain situations, tethered goats have been used to remove honeysuckle growth, but goats must be monitored to prevent their escape to the wild where they would become an added ecological threat.

Chemical Control

Numerous formulations have been tried in North America, Australia and New Zealand. Most give externally variable results depending on the season, soil type, moisture regime, application rates and vigour of the infestation.

Glyphosate, a rate-selective herbicide (i.e., application rate can determine plant susceptibility, with broadleaf plants becoming susceptible at high rates) has universally been the most successful chemical, applied at rates of 1.0-1.5% v/v, and generally with the addition of a penetrant and a low pressure spray system. Small infestations where individual stems can be treated can be cut or scraped then painted with undiluted or diluted glyphosate. Larger infestations need to be sprayed (e.g. Yonce and Skroch, 1989; and references in Williams et al., 2001).

Good results are also achieved with clopyralid, another rate-selective herbicide that can persist in the top 5 cm of soil for up to a year after broadcast application (Rice et al., 1997). Spray is applied while the vines are actively growing, and most effectively in late summer or autumn (provided green leaves are present) when the chemical is translocated to the roots. Periods of cold or dryness when the plant is not actively growing should be avoided for spraying.

References to trials of numerous other chemicals are given in Williams et al. (1998, 2001).

Biological Control

A review of the potential for biocontrol in New Zealand (Standish, 2002) showed that no programs were operative anywhere in the world. There are no obvious insect agents but there are 14 fungal pathogens of L. japonica that are potential biological control agents (Standish, 2002). Two insect species recently found attacking L. japonica in its native range may have potential as biocontrol agents; Appendiculella lonicerae (Song et al., 2002) and Cladosporium lonicericola (He and Zhang, 2001).

A classical biological control programme was initiated in New Zealand in 2004-2005 with a survey of the natural invertebrate fauna and pathogens associated with the weed (Waipara et al., 2007).  The honeysuckle was being attacked by a diverse range of native and introduced invertebrates. But overall the damage was minimal and none of the herbivore niches on the weed were well utilised, implying that exotic agents may have a chance of inflicting additional damage that might be sufficient to control the species. In Brazil, Miranda et al. (2014) found that while Pseudocercospora lonicerigena was infecting the honeysuckle, it was not highly pathogenic. The authors suggested that the introduction of additional co-evolved natural enemies collected in Asia, perhaps piggy-backing on an ongoing project already underway in New Zealand, would be necessary to control this invasive alien weed in Brazil.

Integrated Control

There are no records of integrated control involving biocontrol, chemical, and mechanical control. Mechanical control, or even hand pulling, is often used in combination with chemical control.