Brazilian Waterweed

Egeria densa is native to temperate and subtropical regions of South America, specifically southeastern Brazil, Argentina, and Uruguay.

• Natural range includes the Río de la Plata basin and associated freshwater systems
• Thrives in slow-moving or still freshwater bodies such as ponds, lakes, ditches, and streams
• Has been introduced to every inhabited continent and is now naturalized across North America, Europe, Asia, Africa, Australia, and New Zealand
• First introduced to Europe in the early 19th century as an aquarium and botanical garden specimen
• In many regions outside its native range, it is classified as an invasive species due to its aggressive growth and ability to form dense mats that choke waterways

Egeria densa is a fully submerged aquatic perennial with a slender, branching stem that can grow up to 2–3 meters (occasionally longer) in length.

Stems:
• Slender, cylindrical, and highly branched
• Can reach lengths of 2–3 m or more under favorable conditions
• Brittle and easily fragmented — a key factor in its invasive spread
• Adventitious roots form at nodes along the stem, anchoring the plant in substrate

Leaves:
• Arranged in whorls of 4–6 (occasionally 3 or 8) around the stem
• Linear to narrowly lanceolate, approximately 1–3 cm long and 2–5 mm wide
• Bright green, translucent, with finely serrated margins (visible under magnification)
• Leaf margins bear tiny teeth (0.1–0.2 mm) — a distinguishing feature from the similar Hydrilla verticillata, which has more prominent serrations

Flowers:
• Dioecious — male and female flowers occur on separate plants
• Only female plants are known in most introduced populations outside South America
• Flowers are small (~12–20 mm across), white, with three petals, and are borne on long, thread-like hypanthia that extend to the water surface
• Male flowers detach from the plant and float to the surface to release pollen

Roots:
• Fibrous, white to pale brown adventitious roots arising from stem nodes
• Typically 2–5 cm long, used primarily for anchorage rather than nutrient uptake (most nutrient absorption occurs through the leaves and stems)

Egeria densa inhabits freshwater environments and is highly adaptable to a range of conditions, which contributes to both its popularity in aquaria and its invasiveness in natural waterways.

Habitat:
• Still or slow-moving freshwater: ponds, lakes, reservoirs, canals, ditches, and low-gradient streams
• Found at depths from shallow margins to approximately 4–6 meters in clear water
• Tolerates a wide temperature range (~10–30°C), with optimal growth at 15–25°C
• Prefers nutrient-rich (eutrophic) waters with moderate to high light availability

Ecological Role:
• Provides habitat and shelter for aquatic invertebrates, juvenile fish, and amphibians
• Produces oxygen through photosynthesis, improving dissolved oxygen levels in water
• Absorbs excess nutrients (nitrogen, phosphorus), helping to improve water quality in controlled settings

Invasive Behavior:
• Forms dense, monotypic stands that can block waterways, impede water flow, and displace native vegetation
• Fragments easily; even small stem fragments can regenerate into new plants, facilitating rapid spread via water currents, boats, and equipment
• Listed as a noxious weed or invasive species in numerous countries including the United States, United Kingdom, Australia, New Zealand, Japan, and parts of Europe
• Management methods include mechanical harvesting, herbicide application, and biological control (e.g., introduction of grass carp, Ctenopharyngodon idella)

Egeria densa is one of the easiest aquatic plants to cultivate, making it ideal for beginner aquarists and classroom demonstrations.

Light:
• Moderate to high light intensity promotes compact, dense growth
• Under low light, internodes elongate and growth becomes leggy and sparse
• Tolerates a range of lighting conditions, contributing to its reputation as a hardy aquarium plant

Water Parameters:
• Temperature: 15–25°C (tolerates 10–30°C)
• pH: 6.5–7.5 (tolerates slightly broader range)
• Water hardness: soft to moderately hard (3–15 dGH)

Substrate:
• Can be planted in aquarium gravel or sand, or left to float freely
• When planted, bury only the lower portion of the stem to avoid rotting
• Most nutrient uptake occurs through the water column rather than roots

Fertilization:
• Benefits from liquid fertilizers rich in nitrogen, potassium, and micronutrients
• CO₂ supplementation is not required but promotes faster, bushier growth

Propagation:
• Extremely easy — stem cuttings of 10–15 cm readily root and grow into new plants
• Simply cut a healthy stem segment and plant it in substrate or allow it to float
• A single plant can produce numerous cuttings per month under optimal conditions

Common Problems:
• Melting or leaf loss when first introduced to a new aquarium (transplant shock) — usually recovers within 1–2 weeks
• Algae growth on leaves under excessive light without adequate CO₂ or nutrient balance
• In outdoor ponds, may require trimming to prevent overgrowth and oxygen depletion at night