The Hidden Life of Parrotfeather: Growth Patterns You Should Know

Myriophyllum aquaticum (parrotfeather) is a member of the Haloragaceae family and is characterised by its feathery, whorled leaves and ability to grow both submerged and emergent forms. Originally native to South America, it has spread worldwide, often outcompeting native aquatic vegetation and disrupting ecosystem functions.

2. Taxonomy and Morphology

• Scientific Name: Myriophyllum aquaticum
• Common Names: Parrotfeather, Parrot feather watermilfoil
• Family: Haloragaceae
• Growth Habit: Perennial, aquatic, with both submerged and emergent forms. Emergent stems can extend up to 30 cm above the water surface.
• Leaf Structure: Deeply cut, feathery leaves arranged in whorls.

3. Environmental Factors Affecting Growth

3.1 Water Temperature

• Optimal Range: 16–23°C. Growth is significantly reduced at low temperatures (0–5°C), with cold stress negatively impacting most growth parameters except stem diameter .
• Climate Change Implications: Rising temperatures may facilitate further spread in temperate regions.

3.2 Light Intensity

• Adaptability: Thrives in full sunlight but can tolerate up to 70% shade. Under reduced light, the plant increases its shoot length to reach the surface.
• Light Quality: Recent studies show a preference for blue light, which may confer a competitive advantage in certain aquatic environments.

3.3 Water Chemistry

• pH: Prefers slightly alkaline conditions (pH 6.8–8.0) and tolerates a range of water hardness.
• Nutrient Availability: Growth is enhanced in nutrient-rich (eutrophic) waters. The plant is efficient at removing nutrients and pollutants, such as phosphorus and heavy metals.

3.4 Water Level and Hydrology

• Hydrological Flexibility: Tolerates water level fluctuations and can grow in both flooded and exposed conditions. Capable of root and shoot development in deeper water.

3.5 Salinity and Water Flow

• Salt Tolerance: Can survive some saltwater inundation, indicating moderate salinity tolerance.
• Water Flow: Prefers slow-moving or still waters; less common in fast-flowing environments.

4. Growth Patterns and Physiological Adaptations

4.1 Vegetative Reproduction

• Primary Mode: In non-native habitats, reproduction is almost exclusively vegetative, via fragmentation and clonal growth. Fragments readily root and establish new colonies .

4.2 Structural and Photosynthetic Adaptations

• Height and Morphology: Artificially increased plant height leads to greater above-water height, enhanced photosynthetic activity, and more robust root systems. However, excessive height can result in water transport limitations .
• Plasticity: The plant exhibits significant morphological and physiological plasticity, allowing it to adapt to varying light, nutrient, and hydrological conditions .

4.3 Comparative Growth

• Invasive vs. Native Species: In controlled studies, invasive M. aquaticum showed higher fresh biomass but fewer stem nodes compared to native M. spicatum, indicating a growth strategy focused on rapid biomass accumulation .

5. Recent Research Developments (2019–2025)

• Soil Nutrient Heterogeneity: Patchy nutrient distribution increases shoot proliferation, especially in low-density stands, potentially aiding invasion in disturbed habitats .
• Combined Effects of Light and Nutrients: Both high nutrient availability and increased light intensity promote growth and biomass accumulation, while shading and nutrient limitation suppress growth .
• Growth Inhibition and Risk Assessment: M. aquaticum is used in ecotoxicological testing for herbicide impacts, demonstrating sensitivity comparable to standard test species .

6. Invasive Growth and Management

6.1 Spread and Impact

• Mechanisms: Spreads primarily through vegetative fragmentation, facilitated by water currents, waterfowl, and human activities (e.g., boating) .
• Ecological Impact: Forms dense mats that outcompete native vegetation, clog waterways, and alter ecosystem structure .

6.2 Management Strategies

• Prevention: Best achieved through public education, cleaning of boats/equipment, and regulation of plant trade .
• Mechanical Control: Hand-pulling and mechanical harvesting are often counterproductive due to fragmentation .
• Chemical Control: Herbicides (e.g., 2,4-D, imazapyr, glyphosate) are commonly used but require repeat applications and careful management to avoid non-target impacts .
• Biological Control: Limited success; some beetles show promise, but no widely effective agents are available .
• Integrated Management: Combining methods and ongoing monitoring is essential for long-term control .

7. Conclusion

Myriophyllum aquaticum is a highly adaptable and invasive aquatic plant whose growth is influenced by a complex interplay of environmental factors. Its ability to thrive in a wide range of conditions, coupled with efficient vegetative reproduction and physiological plasticity, underpins its success as an invader. Effective management requires a nuanced understanding of its growth ecology, early detection, and integrated control strategies.

1. Water Parameters

• Temperature:
• The optimal temperature range for growth is 20-25 °C and 23–28°C (73.4–82.4°F) for aquarium use. M. aquaticum prefers temperatures between 16-23°C.
• It can survive and thrive at temperatures ranging from at least 0 °C to 30 °C.
• Growth is inhibited at 0–5°C, but plants do not die. Growth can begin when water temperatures reach 7°C.
• While not seriously affected by frost, a hard or extended period of frost may kill emergent shoots in northern latitudes.
• High water temperatures (30 °C) can inhibit growth, leading to fewer and shorter branches, as plants expend more energy for metabolism. Temperatures approaching or exceeding 32°C can cause water stress, reducing photosynthesis and increasing photorespiration, leading to overall reduced growth.
• pH:
• A stable pH between 6.0 and 8.0 is ideal for its growth.
• It displays photosynthetic activity at pH levels of 6 to 8.5 and thrives under slightly alkaline conditions (pH range 6.8-8.0).
• Hardness (if relevant):
• Recommended water hardness is 8–16 dGH (142.86–285.71 ppm).
• It can withstand a water hardness level between 50-200 ppm.

2. Light & CO₂

• Light requirement:
• Myriophyllum aquaticum requires bright lighting for optimal growth in aquariums and thrives in good light. Light is considered the most important factor limiting the growth of aquatic macrophytes.
• Intermediate light availability (e.g., 30% or 50% shade) is suggested as optimal for its growth, rather than full sunlight.
• Reduced biomass and shoot length have been observed in full sunlight compared to 30% shade, potentially due to correlated increases in water temperature causing water stress.
• The plant can survive in 70% shade, but its total biomass is reduced. In shaded conditions, submersed shoots will elongate to reach the water surface to maximize light acquisition.
• It requires rooting in bottom sediments where light can penetrate for growth and colonization.
• CO₂ (optional / recommended / essential):
• CO₂ supplementation is not required, but it is recommended as it can significantly enhance growth and improve foliage density and color. CO2 availability can also influence growth and phenotypical responses.

3. Planting Method

• Soil/Substrate setup:
• Myriophyllum aquaticum requires rooting in bottom sediments and grows best when rooted in shallow water.
• The recommended substrate is fine gravel.
• For planting, a healthy stem cutting should have its lower leaves removed and then be carefully inserted into the substrate.
• Experimental setups have used a mixture of top soil, loam, and sand (3:2:1) amended with fertilizer.
• Hydroponic / substrate-less setup:
• While typically rooted in sediment, it has been known to occur as a floating plant in the deep waters of nutrient-enriched lakes.

4. Growth & Maintenance

• Growth speed:
• It is a fast-growing aquatic plant. It grows rapidly during the first 20 days.
• Shoots grow rapidly from overwintering rhizomes as water temperatures increase.
• Trimming/pruning:
• Regular trimming helps maintain its form and prevents excessive overgrowth, especially in smaller tanks. This is crucial as its rapid growth and ability to spread through cuttings or broken stems can make it invasive in outdoor settings.
• Water change needs:
• Not explicitly stated, but consistent water quality and nutrient management are implied for healthy growth, as for most aquatic plants. Preventing overabundant growth by limiting external nutrient input can help control it.

5. Fertilization

• Root vs. water column feeding:
• Myriophyllum aquaticum demonstrates luxury uptake of nitrogen and phosphorus from sediments. It is characterized as a highly competitive plant adapted to productive environments.
• Type of fertilizer (liquid / root tab / iron, etc.):
• Liquid fertilization can improve growth and coloration.
• Regular fertilization is essential to prevent leaves from turning brown or falling off due to nutrient deficiencies.
• It does not store phosphorus or carbon in its rhizomes, which can affect its ability to survive severe winters in certain areas.

6. Propagation

• Method (cuttings, runners, rhizome division, etc.):
• The plant is most reliably propagated by stem cuttings.
• In North America and much of its introduced range, reproduction is exclusively vegetative due to the scarcity or absence of male plants; seed production is not known to occur.
• In nature, it spreads effectively by stem fragmentation, where broken off pieces readily root in new areas or moist soil to establish new colonies.
• Rhizomes and stolons are critical for its vegetative spread, providing support for adventitious roots and buoyancy for emergent growth. Stolons also serve functions like nutrient storage and dispersal in the absence of tubers.

7. Common Problems & Fixes

• Mechanical Damage: The plant is sensitive to mechanical damage, particularly from rough handling or digging fish, which can uproot stems. Stems are brittle and can easily fragment, leading to unwanted spread. Cutting or chopping is generally ineffective and can accelerate its spread.
• Fix: Plant in areas of low current and avoid keeping with fish species known for uprooting plants.
• Leaves Browning/Falling Off: This can be a sign of insufficient light or nutrient deficiencies.
• Fix: Ensure bright lighting and regular fertilization.
• Overgrowth and Invasive Spread: M. aquaticum is a globally invasive species known for rapid growth and forming dense mats. These mats can clog waterways, impede recreation, reduce water quality, and provide breeding grounds for mosquitoes.
• Fix: Regular trimming and proper disposal of trimmings are essential to prevent unwanted spread. Mechanical removal methods for large infestations are often counterproductive due to fragmentation. Prevention of dispersal through human activities (e.g., cleaning boats and gear) is crucial.
• Herbicide Control Challenges: The plant is difficult to control with herbicides due to a thick waxy cuticle on emergent leaves, which repels many chemicals. The weight of the spray can also cause emergent vegetation to collapse, washing off the herbicide.
• Fix: A surfactant (wetting agent) is often required with herbicides to penetrate the cuticle. Herbicides are most effective when applied to young, actively growing plants, and repeat applications are often needed for the best results. Imazapyr combined with a non-ionic surfactant has shown high success in mesocosm studies, though field efficacy can vary. Careful consideration is needed to mitigate harm to non-target native species.
• Low Dissolved Oxygen: Dense mats can reduce dissolved oxygen levels in the water column below, which can be detrimental to fish and create "dead zones".
• Fix: Control infestations to prevent the formation of dense mats and subsequent decomposition.

8. Aquascape Placement

• Foreground / Midground / Background:
• With a potential height of 15–30cm (5.91–11.81in), Myriophyllum aquaticum is most suitable as a background plant in aquariums.
• It can also be used as a decorative edge plant in ponds.
• Its dense growth serves as excellent cover and a safe hiding place for young fish in fry tanks.

9. Pro Tips

• Morphological Plasticity (Heterophylly): M. aquaticum is a heterophyllous plant, capable of developing two distinct leaf forms: submersed and emergent. Emergent leaves are typically stiff, feather-like, greyish green, with stomata and a thick waxy cuticle, resembling small fir trees above water. Submersed leaves are usually orange to red, lack stomata and cuticle, and are more filamentous. This ability to rapidly change growth forms allows it to adapt to varying water levels and light conditions.
• High Adaptability & Invasive Nature: This plant exhibits a robust capacity for adaptation to various environmental conditions, including a wide range of temperatures, water levels, and nutrient availability. It can even grow slowly while frozen at 0-5°C and overwinter with its roots submerged in mud. This high adaptability contributes significantly to its success as an invasive species globally, capable of colonising a wide range of habitats. Many regions, including several US states and the European Union, have banned its sale and distribution due to its invasive potential.
• Ecosystem Services: Despite its invasive tendencies, M. aquaticum can provide some ecosystem benefits. It has been observed to enhance microbial communities involved in the nitrogen cycle in constructed wetlands, contributing to wastewater remediation. It is also highly efficient in removing pollutants such as phosphorus, tetracycline, and heavy metals from water. Furthermore, it can produce allelopathic chemicals that inhibit blue-green algae growth.
• Biological Control with Beavers: In areas where beavers are present, they have been observed to significantly reduce M. aquaticum biomass (up to 90%) through herbivory. Coexistence with native beavers can therefore serve as a preventative measure against excessive parrot feather growth.
• Identification Note: M. aquaticum can be confused with other milfoils or similar aquatic plants like hornworts and bladderworts. Its stiff, finely-divided, feather-like emergent leaves are a distinctive identifying feature. Distinguishing it from Myriophyllum spicatum (Eurasian watermilfoil) can be particularly difficult, sometimes requiring DNA testing.