Myriophyllum aquaticum (Used synonyms include: Myriophyllum Brasiliense, Myriophyllum proserpinacoides and Enydria aquatica)
Parrot's Feather, Diamond Milfoil, Parrot Feather Watermilfoil, Brazilian Watermilfoil.
Originally native to the waterways of the Amazon Basin, the plant has been distributed to many parts of the world, as an indoor or outdoor aquatic cultivar, for over 100 years. Accidental escapes from cultivation have led to the establishment of wild populations, which are becoming problem invaders in southern and seaboard USA, Australia, New Zealand, Tasmania, South Africa, parts of Europe and the UK.
No firm date for first cultivation in Great Britain has been established, but the earliest naturalisation record here dates from about 1960. Since that time, spread has been comparatively slow, with still or slow-moving water bodies being preferred, and eutrophic conditions always a pre-disposing factor. Certainly, over 300 sites in the UK are now affected, and the rate of spread seems to be increasing.
The problems produced by Parrot's Feather have much in common with both Floating Pennywort and Swamp Stonecrop. All produce thick rafts of tangled vegetation, which block out light and markedly reduce dissolved oxygen. Many native plants are out competed, and fish and invertebrates may die through lack of oxygen. The base of the food-pyramid is also disrupted as colonies of uni-cellular algae are shaded out. The problems in the UK are mainly those of damage to bio-diversity, as most of the infestations are found in ponds. Canals, gravel pits, reservoirs, drainage ditches and slow-flowing streams are also being affected, so it may be that some of the problems experienced in the USA and southern Africa will occur here, as infestations spread to more substantial bodies of water. These include hindrance to navigation and damage to recreation; clogging of drainage channels leading to flooding, blocking of irrigation inlets and providing ideal breeding habitat for malarial mosquitos.
Parrot’s Feather is an aquatic, sometimes semi-terrestrial, rhizomatous perennial. Its stems reach several metres long, not branching, and are rooted from the lower nodes. It grows emergent stems measuring up to 30cm, giving an almost conifer-like appearance. The leaves appear in whorls of 4 to 6, slightly dimorphic between submerged and emerged stages. Submerged examples of the pectinate leaf measure 3.5 to 4 cm long and 0.8 to 1.2 cm wide. Its leaves are pinnae linear and number 25 to 30, growing up to 0.7cm long. Lower submerged leaves usually decay rapidly.
Emergent leaves en-masse appear bright green, but on individual inspection are glaucous. They are arranged in whorls of 4 to 6 and are pectinate, measuring 2.5 to 3.5cm long and 0.7 to 0.8cm wide. They are pinnae linear and are 18 to 36 in number: concentrated at the distal end of the leaf. The lower 5 to 7cm of leaf rachis. It bears naked flowers, which are female in the UK and USA, and are solitary and axillary. Bracts are pinnatifid. Sepals can appear as 4 triangular to denticulate, and up to 2mm long. There are 4 styles with petals and stamens being absent from female flowers.
Only female plants are found in the UK, so dispersal is by spread of the persistent rhizome system, or by fragmentation of rhizomes or stems. There are no specialised organs for vegetative reproduction, but the stems are brittle, and pieces may be carried by water currents or cling to waterfowl. Unlike the non-invasive native members of the genus (the Water Milfoils) Parrot's Feather does not self-fragment in order to assist dispersal.
In a similar manner to Swamp Stonecrop, a stem section with one node only and length of 5mm is sufficient to root in sediment and start the growth of rhizomes, submersed stems and adventitious roots. Rooting depths may be as great as 2 metres, and well-anchored colonies may spread over many metres of the water surface as floating vegetable mats. Any mechanical method of control carries the risk of fragmentation and further spread of viable material.
Though frosts of -3°C or lower are fatal to top growth, the submersed stems and rhizomes are protected from climate extremes. The USA range, in consequence, includes Zone 6 habitats, where winter minimums may be lower than -23°C.
Myriophyllum aquaticum has another marked difference from the native Water Milfoils, in that it is able to become semi-terrestrial and advance away from the shoreline, or survive where its presence has helped precipitate the drying out of the infested pond. It is understood, to be growing well at one arid location in Cornwall.
Parrot's feather is capable of rooting in the marginal mud of watercourses, also in water depths of up to two metres. It seems likely that spread will be limited to still or slow moving water, as there seems to be a self-limiting factor where restriction of water flow produces an increase in flow speed, resulting in the channel remaining at least partly open, where the flow rate impedes the establishment of the plant. Colonies well rooted in the substrate succeed best and are capable of spreading considerably over the surface of deeper water. Floating deep-water colonies are being observed on nutrient enriched lakes in the USA.
Reproduction and dispersal in the UK is only by fragmented rhizome or stem rooting in substrate where deposited. As with other problem aquatic weeds, so little is needed to make a viable propagule that spread on boots, tyres, ducks' feet etc. is inevitable. Detached fragments soon sink, then root and spread as rhizomes through the sediment, eventually extending upwards to produce the familiar tangle of stems.
Research into biological methods of control may yet yield results for employment in the UK, though insect species found to damage Myriophyllum in its native range and in Florida might not be adapted to survival here even during the growing season, and it would take much research to ascertain whether such non-specific plant feeding insects might not, if released, have damaging environmental effects themselves.
The following insects are the subject of research in the USA and South Africa:
Lysathia flavipes (a Flea Beetle) causes moderate damage in the USA, but has become widely established in South Africa where it causes defoliation which can result in death.
Listronotus marginicollis (a stem-boring weevil) has limited effects.* Both the above species are found in Parrot Feather's South American native range.
Lysathia ludoviciana (a Flea-Beetle) is found in the larval stage on laboratory grown Myriophyllum.
Argyrotaenia alvana (a Tortrix Moth) feeds on the plant as a caterpillar.
Choristoneura parallela (a Tortrix Moth) behaves similarly.
Paraponyx alhonealis (a Leaf-mining Moth) tunnels the plant as a caterpillar.
All three of the above are from Florida, and are yet of unestablished potential as control agents.
An isolate from the pathogenic fungus, Pythium carolinianum, has been found to greatly suppress growth when used as an innoculant.
In New Zealand, a powdery mildew, Microsphaera alphitoides, has recently been noted there as a pathogen of trees of the genus Platanus, and also on populations of Myriophyllum. In the UK, this species of mildew is indigenous and has serious effects on seedling oaks and young trees. Commonly known as Oak Mildew, it also affects Sweet Chestnut. Research on its pathogenic propensity on Parrot's feather must surely be worthwhile.
Mechanical methods such as cutting, pulling or dredging can be practised where access permits. As with all easily fragmented water-weeds, mesh should be placed downstream of any operation to prevent further spread. Draglines and rototillers are also used in the USA, though most mechanical controls are reserved for areas where the Parrot's Feather can spread no further. Control by mechanical methods does not equal eradication.
Light deprivation, provided by floating opaque film applied in Spring, can be very effective if kept up for twelve months.
Measures to reduce the nutrient loading in eutrophic waters can reduce the plants-invasive capacity. Most infestations in the UK are tackled by means of herbicides.
The residual herbicide Dichlobenil, applied as granules in April, whilst bound to basal and marginal substrates, can be effective.
2,4-D, applied as a spray to emerged foliage is also effective, though use of a surfactant is necessary to penetrate the leaf-cuticle. Unfortunately, the effects of the surfactant and the weight of herbicide-wetted foliage can cause emergent growth to collapse into the water, thus much reducing efficacy.
Glyphosate can be useful in repeat applications or late in the season, though it tends to blacken the foliage with only partial kill.
Other chemicals, not permitted for use in the UK are employed in the USA. These include Endothall, Diquat and Complexed Copper. back to top
*Another Listronotus species, Listronotus setosipeanis, is useful in the control of Parthenium Ragweed.