Lonicera japonica

Lonicera japonica is distinguished from native twining honeysuckles by the fact that members of each pair of its leaves are distinct, whereas at least some of the opposite leaves of native twining species are united (Fernald 1950).

Lonicera japonica is generally associated with disturbance and has spread to old fields, roadsides, fence rows, prairies, sand barrens and forest openings. In Pennsylvania, it is a major component of the third stage of succession in old fields, increasing after fields have been abandoned for four years (Keever 1979). Light appears to be the major limiting factor (Andrews 1919, Leatherman 1955, Thomas 1974), but recent studies suggest that Japanese honeysuckle can invade established woodlands when natural processes such as storms or Dutch elm disease create canopy openings (Thomas 1974, Slezak 1976). Invasion is particularly effective in moist woodlands and floodplain forests (Andrew 1919, Snyder pers. comm., Wistendahl 1958). Slezak (1976) found that in poorly drained areas the frequency of Japanese honeysuckle decreased with increasing canopy closure and understory closure had no significant effect. In well-drained areas, however, the frequency of Japanese honeysuckle was inversely correlated with closure of the subcanopy, but canopy coverage was not correlated with honeysuckle frequency (Slezak 1976). Japanese honeysuckle is one of the few species that can withstand pollution from heavy metals and SO2 (Caiazza and Quinn 1980).

Infestations have reached pest proportions in areas with annual precipitation of at least 100 cm and mean January temperatures of at least -1 C and freezing January night temperatures at least 5% of the nights (Leatherman 1955).

No discussion of the habitat of Japanese honeysuckle would be complete without mentioning that Lonicera japonica is still being propagated and promoted for use as a groundcover in areas where it has not reached pest proportions.

The invasive ability of Japanese honeysuckle is the result of an interplay of four biological factors: the dispersal of its seeds by birds into light gaps of otherwise closed forests; its ability to propagate by vegetative runners; its particular method of twining, which restricts it to climbing vegetation with a small diameter but prevents its climbing the boles of mature trees; and its persistent evergreen leaves that permit photosynthesis during periods when the invaded vegetation is dormant.

Japanese honeysuckle blooms most prolifically in sunny situations where the opportunity for vegetative proliferation is restricted (Andrews 1919). Although the flower appears to be specialized, the plant is believed not to be dependent on any single insect because its blooming period extends from April to December in Georgia (Andrews 1919) and late May to October in Kentucky (Sather pers. obs.).

The inconspicuous black berries each contain three seeds (USDA 1971), which are dispersed by birds from light gap to light gap in forest (Martin et al. 1951). Seedlings are believed to photosynthesize soon after germination because their food content is low (Leatherman 1955). Despite the abundance of data relating to light requirements, most experimental studies have been conducted on cuttings, with no information on the light requirements for germination. Light requirements are probably not high because seedlings are known to become established in shaded understories. Little and Somes (1967) report that seedling growth is slow for the first two years of life. Because of their slow growth rate, new plants are unable to take advantage of canopy gaps. Once established, honeysuckle can persist at low light levels without noticeable growth and respond to increased light with more vigorous runner production.

In studies designed to measure the effect of shading on forage production in Texas, Blair et al. (1983) grew honeysuckle under light-intensities of 92%, 55%, and 0% of full sunlight. Although shade did not affect the timing of phenological events, there was a significant inverse relationship between leaf dry matter and light intensity at all levels of shading. In the deepest shade, new leaders were formed but died back.

Leatherman (1955) found that about half of her experimental cuttings survived at 10% of full sunlight and that at 25% full sunlight survival was good. There was no significant difference in the dry weight production between plants grown in 25% of full sunlight and those grown in full sunlight (Leatherman 1955).

Slezak (1976) divided honeysuckle-infested plots into density and vigor classes and found that vigor (measured by the number of vegetative runners) was adversely affected by shading of less than 3% of full sunlight, but density was unaffected. In experimental studies at Theodore Roosevelt Island National Park, Thomas (1980b) demonstrated that Japanese honeysuckle produced good growth at 47% of full sun and found from winter light measurements that all closed forest in the park had light levels between 49% and 86% of full sun during the winter months. Japanese honeysuckle is thus well adapted to persist in deciduous forests at low summer light intensities and put on growth when canopy gaps occur or at winter light levels.

Japanese honeysuckle has a long photosynthetic season. In the south, the plant is evergreen. In Illinois leaves fall when winter temperatures reach around 17 centigrades (Schwegman pers. comm. 1987). In areas where it is facultatively deciduous, it is one of the first plants to leaf in the spring. In New Jersey, leaf production begins when soil temperatures are between 1 and 9 centigrades (Leatherman 1955). In Illinois, Japanese honeysuckle produces leaves with the spring ephemerals and retains its leaves through November, allowing at least three months for photosynthesis in otherwise shaded deciduous forests (Schwegman pers. comm. 1987).

Thomas (1980b) calculated that in the Washington, D.C. area there are an average of 52 days a year between first and last frost when temperature and light conditions in closed canopy forests are adequate for honeysuckle photosynthesis. The combination of honeysuckle's ability to photosynthesize at winter temperatures and light levels and its ability to persist under summer shade appear to be the major factors contributing to its destructiveness. In areas where winter temperatures fall too low for the late season photosynthesis to occur, the species is less of a problem.

Japanese honeysuckle produces long twining vegetative runners. Little (1961) found that the combined length of lateral and sublateral runners from one sprout in one year reached 15 meters. In well-lit areas, Slezak (1976) found that 7% of sampled plots contained over 50% plants with seven or more runners over 60 cm in length. These runners serve a dual function. Wherever they come in contact with the soil, runners can produce roots at the nodes, producing dense mats of plants. In addition to their ability to root, the runners create new habitat by their twining habit. Unlike Virginia creeper, which climbs its host by holdfast, or grapevine, which possesses tendrils, honeysuckle climbs by twining around its host. This twining habit limits the diameter of accessible hosts, which may nonetheless be as great as 15 centimeters (Andrews 1919). Honeysuckle is unable to climb boles of mature trees although it uses other lianas to reach the canopy where they are available (Andrews 1919).

The root system of mature Japanese honeysuckle plants has been reported to reach depths as great as a meter and horizontal lengths up to 3 meters (Leatherman 1955). Wherever aboveground runners contact the ground they will root. Once runners from a single parent plant have established roots, they will resprout as separate individual plants if their aboveground parts are severed. Little and Somes (1967) report that it took only two years for honeysuckle to reach pre-treatment densities from root sprouts after a variety of herbicide applications.

Japanese honeysuckle produces long twining vegetative runners. Wherever they come in contact with the soil, runners can produce roots at the nodes, producing dense mats of plants; once runners from a single parent plant have established roots, they will resprout as separate individual plants if their aboveground parts are severed. It blooms most prolifically in sunny situations where the opportunity for vegetative proliferation is restricted (Andrews 1919). The inconspicuous black berries each contain three seeds (USDA 1971), which are dispersed by birds from light gap to light gap in forest (Martin et al. 1951).