Invasive Plants of California's Wildland

Parrot’s feather (Myriophyllum aquaticum) is a stout aquatic perennial that forms dense mats of intertwined brownish stems (rhizomes) in water. These stems grow to six and a half feet in length and resemble bright green bottlebrushes emerging from the water. The bottlebrush appearance results from the fact that the leaves appear in whorls of four to six at each node and each leaf is feather-like, the blade divided into twenty-four to thirty-six thread-like segments. Upon close inspection the leaves look gray-green. Parrot’s feather also has leaves below the water surface, appearing reddish, feathery, and limp. Unlike other milfoils (Myriophyllum spp.), parrot’s feather stems may grow as much as eight inches above the water surface (Orchard 1981, Wester-dahl and Getsinger 1988).

Haloragaceae. Parrot’s feather is a perennial, submersed/immersed aquatic plant. Stems: long and unbranched, rooting freely from lower nodes. Leaves: in whorls and slightly dimorphic. Submerged leaves in whorls of 4-6, oblong in appearance, 1.4-1.6 in (3.5-4.0 cm) long, 0.3-0.5 in (0.8-1.2 cm) wide; pectinate with 25-30 linear pinnae to 0.27 in (0.7 cm) long. Lower leaves usually deteriorate rapidly. Emergent leaves glaucous, in whorls of 4-6, oblong, 1-1.4 in (2.5-3.5 cm) long, 0.27-0.31 in (0.7-0.8 cm) wide; pectinate with 18-36 linear pinnae in the upper 80 percent of leaf. Lower 2-2.75 in (5-7 cm) of leaf rachis naked.

Inflorescence: indeterminate spike with flowers singly in axils of upper emergent leaves. Flowers: only female flowers known from parrot’s feather plants in the United States. Female flowers on pedicel 0.008-0.02 in (0.2-0.4 mm) long; 4 white sepals, 0.02 in (0.5 mm) long, 0.012 in. (0.3 mm) wide, denticulate with one to several small teeth on each margin; no petals or stamens, 4 clavate styles 0.004-0.008 in (0.1-0.2 mm) long, stigmas white and densely fimbriate; ovary pyriform, 0.02-0.03 in (0.6-0.7 mm) long, 0.02 in. (0.6 mm) wide; 4-ribbed longitudinally between sepals. Fruit: none in United States (Orchard 1981).

Both parrot’s feather and spike watermilfoil can be found in freshwater lakes, ponds, and canals with slow-moving waters in northern and central California (Anderson 1990). Parrot’s feather can be found throughout much of the United States from New England to Florida and westward to California and Washington. Typically, it is found rooted at depths to 6.5 feet (2 m), but emergent stems may elongate and spread over deeper waters or to pond edges.

Parrot’s feather is native to South America and was introduced into the United States in the late 1800s for use in aquaria and water gardens (Kane et al. 1991). It was first collected in the United States near Washington D.C. in 1890. It was reported from South Africa in 1918 or 1919, Japan in 1920, New Zealand in 1929, Australia in the 1960s, and England in the 1970s. A population was reported in western Washington in 1944 (Washington Water Quality Program 1998).

Parrot’s feather is capable of sexual reproduction in its native range, but the spread of parrot’s feather in the United States results solely from vegetative reproduction. The stems of parrot’s feather are brittle and fragment easily. These fragments settle in sediments and produce new plants (Orchard 1981, Kane et al. 1991). Fragments can be spread by boats, trailers, and by dumping aquarium plants in waterways. They can also be spread by waterfowl and other wildlife, as well as by moving water.

Parrot’s feather may compete with native aquatic plants, eliminating them or reducing their numbers in infested sites. It forms dense mats that can entirely cover the surface of the water in shallow lakes and other waterways. These mats clog waterways, making them unusable for navigation or recreation and causing flooding out of the channel. It can block irrigation pumps and water intakes, and it provides optimal habitat for mosquitoes (Orr and Resh 1989, Systma and Anderson 1990; Parsons 1992). In California this species is becoming an increasing problem in irrigation and drainage canals. A 1985 survey of irrigation, mosquito abatement, flood control, and reclamation agencies in California indicated that parrot’s feather infested nearly 600 miles of waterways and over 500 surface acres (Washington Water Quality Program 1998).

While parrot’s feather may provide cover for some aquatic organisms, it can significantly alter the physical and chemical characteristics of lakes and streams. Infestations can alter aquatic ecosystems by shading out algae in the water column that serve as the basis of the aquatic food web. It also alters habitats for aquatic organisms, waterfowl, and other wildlife.

Reproduction of parrot’s feather in the United States is believed to be entirely by vegetative means, resulting from stem fragmentation and/or regrowth from sections of rhizomes (underground stems) (Jacot Guillarmod 1979, Kane et al. 1991). Even in South America, virtually all parrot’s feather plants are female. Male plants are unknown outside South America, so no seeds are produced in North American populations.  

With its tough rhizomes, parrot’s feather can be transported long distances on boat trailers. Any rhizome or stem sections with at least one node, even as small as 0.2 inch (5mm) long, can root and establish new plants. Rhizomes stored under moist conditions in a refrigerator survived for one year. Once rooted, these new plants produce rhizomes that spread through sediments and stems that grow until they reach the water surface (Orchard 1981). The result is a dense, tangled mass of parrot’s feather in the water column.

 Growth is most rapid from March until September. In spring shoots begin to grow rapidly from overwintering rhizomes as water temperature increases. Rhizomes function as a support structure for adventitious roots and provide buoyancy for emergent growth in summer. Emergent stems and leaves extend from a few inches to over one foot above the water surface. Underwater leaves tend to senesce as the season advances. Plants usually flower in spring, but some plants may also flower in fall. The inconspicuous flowers form where emergent leaves attach to the stem.

In fall plants typically die back to the rhizomes. In some areas, parrot’s feather may maintain considerable winter biomass. Because the plant lacks tubers, turions, and winterbuds, rhizomes serve all those functions. Parrot’s feather does not store phosphorus or carbon in its rhizomes, and this may explain its failure to invade areas with severe winters.

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Parrot’s feather is difficult to remove from an aquatic system, so it is best to prevent it from establishing in the first place. The public must be made aware of the problems caused by parrot’s feather and how it can be spread by dumping unwanted plants from water gardens or aquaria or by boats, trailers, and fishing equipment that are not cleaned before being moved to a new waterway. If parrot’s feather becomes established, only chemical and mechanical control methods are available.

Mechanical methods: Parrot’s feather can be removed by mechanical harvesters. In Washington, workers use a dragline to remove parrot’s feather plants. A truck-mounted crane with a special attachment plucks weeds out of the ditch. The dragline operation is conducted annually from August to December, with control generally lasting for one growing season (Washington Water Quality Program 1998). Care must be taken to ensure removal of all plant parts during harvest, since even tiny stem or rhizome fragments can root and establish new plants. Because of this, mechanical harvesting often results in the spread of parrot’s feather rather than its elimination or suppression.

Parrot’s feather has a high tannin content, so most grazers, including grass carp (Ctenopharyngodon idella), find it unpalatable. Grass carp also prefer soft plants, such as Elodea canadensis, and the tough, woody parrot’s feather stems are avoided. USDA approved biological control agents are not currently available. Potential agents do exist, but they have yet be tested for host specificity. A complex of insects feed on parrot’s feather in its native habitat. Lysathia flavipes, a flea beetle found on parrot’s feather in Argentina, causes moderate damage under field conditions. Also found in Argentina is a weevil, Listronotus marginicollis, that apparently feeds only on parrot’s feather in its native range. Other insects have been found on parrot’s feather in Florida. Lysathia ludoviciana, a flea beetle native to the southern United States and the Caribbean, uses parrot’s feather as a host plant for larvae under laboratory conditions. However, the flea beetle is not often found on parrot’s feather in the field. Two members of the Tortricidae family, Argyrotaenia ivana and Choristoneura parallela, have also been found on parrot’s feather in Florida, but their effect on the plant is unknown. In addition, larvae of the caterpillar, Parapoynx allionealis, mine parrot’s feather leaves, but the impact of these larvae is unknown.

Fungal control options exist as well. An isolate of Pythium carolinianum collected in California has shown some promise as a potential biocontrol agent. Parrot’s feather stems experimentally inoculated with this fungus produced significantly less growth than control plants (Washington Water Quality Program 1998).

The underwater and above-water foliage of parrot’s feather make herbicides difficult to deliver effectively. Emergent stems and leaves have a thick, waxy cuticle that inhibits herbicide uptake, and a wetting agent is required to penetrate it. Often the weight of the spray will cause emergent vegetation to collapse into the water, where the herbicide is washed off before it can be translocated throughout the plant. The most recent version of an herbicide label will give recommended rates and information about whether the compound is registered for use in specific situations. Herbicide use is more highly regulated in aquatic systems than in terrestrial systems.

Westerdahl and Getsinger (1988) report excellent control of parrot’s feather with 2,4-D, diquat, diquat and complexed copper, endothall dipotassium salt, fluridone, and endothall and complexed copper. Diquat is used on emergent parrot’s feather, as well as in the water to kill rhizomes. Copper complexes are used only on submersed plants. Diquat is not legal for use in aquatic systems in California. Fair control was obtained with acrolein and glyphosate. Acrolein is used only in non-fisheries water, and glyphosate, formulated as Rodeo, is used only on emergent parrot’s feather. The Monsanto Company suggested that applying a 1.75 percent solution of Rodeo® with surfactant to the plants in summer or fall when water levels are low would give about 95 percent control. Control of parrot’s feather may be achieved with low-volatility ester of 2,4-D at 4.4-8.9 kg/ha, sprayed onto emergent foliage. The granular formulation of 2,4-D was needed to control parrot’s feather for periods greater than twelve months. It is more effective when applied to young, actively growing plants (Washington Water Quality Program 1998).

In practice, weed control efforts report little success with herbicides to control parrot’s feather. Glyphosate causes emergent vegetation to turn black, but within two weeks the plants have recovered. An experimental fall application of triclopyr also proved ineffective (Washington Water Quality Program 1998).