tcm (r&d) : Floating pennywort

Floating pennywort

Hydrocotyle ranunculoides
 

• Species and Names
• Introductory History
• Taxonomy
• Identification
• Scope
• Control Methods

Species and Names

Species
Hydrocotyle ranunculoides

Family
Historically, Hydrocotylaceae; then Umbellifcrac; then Apiaccae. Following DNA analysis, now placed in Araliaccae

Common names
Floating Pennywort, Water Pennywort

Introductory History

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 is still available from suppliers.  Moves are being made, and Parliamentary lobbying is active for it to be classified under Schedule 9, section 14 of the Wildlife and Countryside Act. Thus Floating Pennywort may well join the rogues gallery of Japanese Knotweed and Giant Hogweed, in the near future. These provisions will make it illegal to plant or cause these plants to grow in the wild.

Taxonomy

Description

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.

Biology

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.

Identification

The weed consumes bodies of water, grows in thick mats

A large stand of floating pennywort Image courtesy of Richard Torrens

 

 

The underwater root system of the plant Image courtesy of Richard Torrens

Small green/white flowers, pale reddish stems, circuar, rounded edged leaves Image courtesy of Richard Torrens

Scope

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. The recent de-listing of products containing Diquat Dibromide has, furthermore, removed an important tool from the kit of the specialist weed manager.

Control Methods

The science and methodology for controlling Floating Pennywort are still in development, owing to the very recent emergence of this pest. It is not currently possible to achieve eradication as the result of a single action. Whatever method or methods of control are chosen, provision must be made for follow-up monitoring and repeat treatments.

Chemical Controls

International experience so far has found a small range of chemicals effective, with the most important being 2,4-D amine and Diquat Dibromide. Glyphosate is also employed despite limited activity, though it would not be considered as a chemical of choice. Diquat Dibromide  has further been used in the USA in mixes with the algicide/herbicide Endothall, to good effect.

In the UK, Diquat no longer has approval, and Endothall is likewise not listed. For all practical purposes, amongst foliar applied chemicals, only 2,4-D amine remains viable.

2,4-D amine is usually applied at the rate of 4.5kg amine salt per 200 litres of water per hectare. In the USA, up to 50% more product may be employed. Treatment is usually carried out at the end of the growing season when all submerged tips have emerged and are producing aerial leaves, as it appears that the chemical is not translocated to the submerged tips, which remain unaffected. There are also problems caused by the density of the canopy, as unsprayed leaves also remain alive. Although submerged tips and lower leaves may not all belong to the plant individuals whose leaves have been sprayed, it is surprising that translocation seems so ineffective in this case. 2,4-D amine, which translocates in both xylem and phloem tissues, is usually considered an efficient translocator.

Foliar effects from 2,4-D amine, sprayed at these concentrations, are very rapid; often within 24 hours. It is possible that over-rapid aerial tissue death is impeding translocation to those parts of the plants not touched by the spray. Research on repeat applications at lower dose levels might prove valuable.

Floating Pennywort, in large colonies, is known to cause oxygen depletion, with resultant death of fish and invertebrates. It is especially important, therefore, to restrict the area sprayed at any one time, to avoid further depletion, caused by plant material killed by the herbicide application, decaying en masse.

Consideration should also be given to the factors affecting speed of dissipation in an aquatic environment; also to the potential of the chemical to bio-accumulate in both invertebrate and vertebrate aquatic species.

As it has been found that a repeat application of 2,4-D amine after three weeks increases control, staggered spraying should be reasonably practical to achieve.

Experimentation with the residual herbicide, Dichlobenil, applied to the water margin in winter; over the area where the semi-dormant Pennywort is rooted, is likely to meet with success.

Dichlobenil tends to be a particularly potent residual herbicide where the target species retains any degree of metabolic activity during winter months.

It might be hoped that the scale of reduction of spring growth might thus be so much reduced as to make a subsequent clean up with 2,4-D amine comparatively simple.

Possible problems with erosion have to be considered before using Dichlobenil, because it is non selective and highly persistent, suppressing most non-woody vegetation for up to a full growing season. It could be possible to create a marginal strip, above and below the waterline, unprotected by any kind of vegetation, which would be easily washed away.

Mechanical Control

Scrupulous removal of all plant material, by dredging, digging and final finger-weeding can be an eventual path to the eradication of smaller infestations.

In other cases, cutting, dredging or pulling may used to help gain access prior to chemical treatments, to remove moribund material after such treatments; or to keep infestations in check, where chemical means are proscribed.

In all cases where mechanical treatments are to be carried out in flowing water or any water body with an outflow (however small) downstream barriers must be installed to prevent propagating the problem. It is also necessary to remove all fragments of plant material for the same reason.

Previously floating mats of Pennywort may be safely stacked and composted after removal. The soil-rooted parts of the plants are better spread, well away from water, and left to dry.

Disposal of herbicide treated matter is much more problematical, as 2,4-D amine remains intact in treated plants until released during the decay process.