Floating Pennywort

Sharing with Japanese Knotweed the ability to reproduce itself from the tiniest plant fragment and thereafter to grow with alarming rapidity, it is likely that further spreading will occur at a substantial speed. In 1999, there were only 35 reports of infestations, mainly in the southern counties of England and Wales. The number of sites by 2003 had increased to over 90, and there is no reason to suppose that such a rate of increase is not continuing.

Rooting in the marginal mud of a watercourse, a single node of a single stem is enough to start into growth a vast mat of tangled vegetation. Water-borne seeds are also a significant vector for propagation. This floating mass, from the top of the leaf canopy to the base of the floating root-system can be up to one metre in depth. Shoot extension during the season has been reported at 30cm daily. The overall mass can spread out from the margin of the infested watercourse by as much as 15m in a summer.

Such rapid growth and the sheer plant bulk gives rise to many problems. Water deoxygenation results in the death of fish and invertebrates. Our native floating and submerged water plants are simply out-competed. Floating islands of vegetation break off and can block sluices, causing flooding.

Fishing, or other recreational uses of infested bodies of water are also hampered. There have also been reports of cattle drowning when they have strayed from grazing-marshes on to the apparently solid surface of the Pennywort.

It is of great concern when this invasive alien plant causes degradation of important wetland sites, such as the Exminster Marshes and the Pevensey Levels. Here, despite control measures, species diversity, and indeed, the whole ecosystems have been damaged.

Control is by mechanical or chemical methods, either alone or in combination. Eradication is elusive, owing to the ability of the plant to regenerate from the smallest fragment and its resistance or idiosyncratic response to herbicides.

Japanese Knotweed took a hundred years to become an invasive weed of any note in these islands. Floating Pennywort, Hydrocotyle ranunculoides, since first reported naturalised in 1991, is already doing serious environmental damage and hampering water management in many tens, or more, sites in southern England.

Sharing with Japanese Knotweed the ability to reproduce itself from the tiniest plant fragment and thereafter to grow with alarming rapidity, it is likely that further spreading will occur at a substantial speed. In 1999, there were only 35 reports of infestations, mainly in the southern counties of England and Wales. The number of sites by 2003 had increased to over 90, and there is no reason to suppose that such a rate of increase is not continuing.

Floating Pennywort is a native of North America and is found there in hardiness zones as low as 6, which indicates frost tolerance down to at least -23°C. It has spread to Central and South America, where it may be considered as indigenous, and to Australia, where it has become a serious nuisance. From this naturalisation range alone it may be deduced that it is equally untroubled by high temperatures and insolation. In Australia indeed, infestations readily double their biomass every three days. It is found frequently throughout southern Europe, the Netherlands, and, of course, the UK.

Introduction here was in the 1980's through the nursery trade for water garden planting, sometimes erroneously sold as the native Marsh Pennywort, Hydrocotyle vulgaris. Despite its invasive propensities, it ihas only just been listed under Schedule 9, section 14 of the Wildlife and Countryside Act and will be banned from sale from October 2010. These provisions have made it illegal to plant or cause these plants to grow in the wild.

Floating Pennywort is an herbaceous, hardy, perennial semi-aquatic or aquatic plant, which is found in still or slow-moving, eutrophic bodies of water. Distribution has mainly been the southern parts of England and Wales, though the range of colonies is spreading.

Propagation in the UK is primarily vegetative, by introduction of pieces of stem with at least one node. Stem fragments initially root in the marginal substrate of the water body, plants sometimes spreading inland where suitably damp soil is available. More readily, stems become floating, below water level, and spread across the water.

Seed is produced at the end of the growing season, and propagation by seed also occurs.

Rooting of stems is impeded where there is a marginal shelf, and the speed of colonization is reduced. From their marginal anchorage, the plants rapidly grow into the water, rooting frequently at nodes. The shoot tips are at first submerged, becoming emergent and forming an occlusive canopy of leaves.

Maximum growth occurs at the end of the summer when water temperatures are at their highest, and the flower stems emerge from the water. Pollination is by insects. As the fruits ripen, the flower stems turn downwards towards the water-surface, in readiness for seed dispersal.

In winter, the herbaceous stems die back, and the plants survive at the water's edge as stem bases with smaller leaves.

Areas of water receiving substantial shading, as from buildings or established trees, rarely become dominated.

This weed is a floating or creeping, mat-forming perennial. Its stems are fleshy, slender to stout; floating or creeping. It has many roots that are hair-like and fibrous; emerging from stems at nodes spaced between 4 and 6 cm. The leaves are typically from 2 to 8cm in diameter; sometimes up to 18cm. The shape: reniform to suborbicular, with basal sinus to petiole with 3 to 7 lobes, crenate or lobed. The petiole is l-40cm, slender to stout, glabrous. Flowers in 5 to10 flowered umbels; small, white to greenish yellow, with 5 tiny petals; arising from the leaf base on a l to 5cm inflorescence stalk*. Pedicels to 2mm. Fruits are l-3mm, divided into 2 halves, suborbicular; mericarps flattened, with faint lateral ribs.

*Features of some extant descriptions suggest to me that some UK populations may be a related North American Species, Hydrocotyle umbellata, which is mainly distinguishable in having the inflorescence stalk as long as the petiole.