Widely distributed in almost 60 countries, the spiked water milfoil (Myriophyllum spicatum) is an aquatic plant that can be extremely invasive. Spiked water milfoil can crowd out the native plants and damage the habitats of native fish.
I. Appearance and Characteristics
Myriophyllum spicatum (Eurasian watermilfoil or spiked water-milfoil) is a submerged aquatic plant which grows in still or slow-moving water. It is native to Europe, Asia, and North Africa, but has a wide geographic and climatic distribution among some 57 countries, extending from northern Canada to South Africa. It is considered to be a highly invasive species.
Eurasian watermilfoil has slender stems up to 250 centimeters (8.2 ft) long. The submerged leaves (usually between 15–35 mm long) are borne in pinnate whorls of four, with numerous thread-like leaflets roughly 4–13 mm long. Plants are monoecious with flowers produced in the leaf axils (male above, female below) on a spike 5–15 cm long held vertically above the water surface, each flower is inconspicuous, orange-red, 4–6 mm long.
Eurasian water milfoil has 12- 21 pairs of leaflets while northern watermilfoil M. sibiricum only has 5–9 pairs. The two can hybridize and the resulting hybrid plants can cause taxonomic confusion as leaf characters are intermediate and can overlap with parent species.
II. How to Grow and Care
Sunlight
Spiked water milfoil flourishes in full-sun environments where it can receive unfiltered, natural light for most of the day. Optimal growth and health are achieved when subjected to these intense light conditions, as they fuel the plant’s photosynthetic engine and promote lush, vigorous development. Although spiked water milfoil is adaptable to varying light intensities, a lack of adequate sunlight may result in reduced growth and diminished vitality. As an aquatic plant typically found in outdoor water bodies, spiked water milfoil demonstrates resilience but its preference clearly lies in sunny areas of the ecosystem, where it can carpet the water’s surface and grow with abandon.
Watering
Thriving in its native aquatic environments, spiked water milfoil has adapted to be submersed or floating with a high tolerance for wet conditions. This species is accustomed to consistent moisture and does not fare well in drought situations, requiring a balance of hydration without being waterlogged. The watering regimen for spiked water milfoil should follow a routine of once every week to maintain its lush growth. Given that spiked water milfoil is typically found in water bodies, it’s best suited for outdoor garden ponds where it can contribute to the ecosystem by oxygenating the water and providing habitat for aquatic life.
Pruning
Regular trimming may be necessary to control its spread.
Propagation
Spiked water milfoil is an aquatic perennial herb known for its feather-like foliage and submerged flowering spikes. To propagate spiked water milfoil, effective and practical tips include taking cuttings from healthy parent plants, which should contain several nodes. These cuttings can then be planted in wet soil or shallow water, where roots will develop from the nodes immersed. It’s important to monitor water conditions, ensuring they’re conducive to root establishment and plant growth.
Transplanting
The optimal time to transplant spiked water milfoil is during the warmer embrace of late spring through mid-summer, fostering robust growth. Choose a sunny aquatic spot for consistent success, and ensure gentle handling of delicate roots to encourage establishment.
III. Invasive species
- Introduced areas
Myriophyllum spicatum was likely first introduced to North America in the 1940s where it has become an invasive species in some areas. By the mid 1970s, watermilfoil had also covered thousands of hectares in British Columbia and Ontario, Canada, and spread some 500 kilometres (310 mi) downstream via the Columbia River system into the Pacific Northwest of the United States. Eurasian watermilfoil is now found across most of Northern America where it is recognized as a noxious weed.
- Impact
In lakes or other aquatic areas where native aquatic plants are not well established, the Eurasian plant can spread quickly. It has been known to crowd out native plants and create dense mats that interfere with recreational activity. Dense growth of Eurasian milfoil can also have a negative impact on fisheries by creating microhabitats for juvenile fish and obstructing space for larger fish ultimately disrupting normal feeding patterns.
Due to the Eurasian milfoil plant’s inability to provide the same microhabitat for invertebrates as compared to native aquatic plant species, densely populated areas of Eurasian milfoil create an ecosystem with less food sources for the surrounding fish. Dense Eurasian milfoil growth can also create hypoxic zones by blocking out sun penetration to native aquatic vegetation preventing them from photosynthesizing. Eurasian watermilfoil grows primarily from broken off stems, known as shoot fragments, which increases the rate at which the plant can spread and grow. In some areas, the Eurasian Watermilfoil is an Aquatic Nuisance Species. Eurasian watermilfoil is known to hybridize with the native northern watermilfoil (M. sibiricum) and the hybrid taxon has also become invasive in North America. This hybridization has been observed across the upper midwestern United States (Indiana, Minnesota, Michigan, Wisconsin) and in the Northwest (Idaho, Washington).
- Control
The aquatic moth Acentria ephemerella, the water veneer moth, feeds upon and damages this water milfoil. It has been used as an agent of biological pest control against the plant in North America. The milfoil weevil (Euhrychiopsis lecontei) has also been used as biocontrol.
Another method for biocontrol is Grass Carp, (one of the Asian Carp species) which have been bred as sterile, and are sometimes released into affected areas, since these fish primarily feed on aquatic plants and have proven effective at controlling the spread. However, the carp prefer many native species to the milfoil and will usually decimate preferred species before eating the milfoil. In Washington State the success rate of Grass Carp has been less than expected. They were used in 98 lakes and 39 percent of them had no submerged plant life left after only a short time.
Since roughly 2000, hand-harvesting of invasive milfoils has shown much success as a management technique. Several organizations in the New England states have undertaken large scale, lake-wide hand-harvesting management programs with extremely successful results. Acknowledgment had to be made that it is impossible to completely eradicate the species once it is established.
As a result, maintenance must be done once an infestation has been reduced to affordably controlled levels. Well trained divers with proper techniques have been able to effectively control and then maintain many lakes, especially in the Adirondack Park in Northern New York where chemicals, mechanical harvesters, and other disruptive and largely unsuccessful management techniques are banned. After only three years of hand harvesting in Saranac Lake the program was able to reduce the amount harvested from over 18 tons to just 800 pounds per year.
In order to prevent damages from mass growth of the plant in lakes, the water level can be lowered. By freezing out the bottom of the bank in the winter months, the population of the plant decreases.
- Management and spread prevention
Trailering boats has proven to be a significant vector by which Eurasian milfoil is able to spread and proliferate across otherwise disconnected bodies of water. Effective methods for mitigating this spread are visual inspections with subsequent hand removal or pressure washing upon boat removal. In the Okanagan River Basin of south-central British Columbia, a specially-adapted rototiller is used to dredge shallow water to damage or destroy the root system.
