Aquatic plants purify pool water through three mechanisms: nutrient absorption by roots removing nitrogen and phosphorus that feed algae growth, oxygen production through photosynthesis that supports beneficial bacterial colonies, and physical filtration where plant stems and roots retain suspended particles. Emergent species (Typha, Phragmites, Juncus) rooted in gravel substrate absorb nutrients directly from the water column while providing surface for bacterial biofilms that perform nitrification converting ammonia into nitrate, submerged species (Ceratophyllum, Elodea) oxygenate the water through underwater photosynthesis, and floating species (Pistia, Eichhornia) shade the surface reducing light-driven algae blooms.
This guide covers plant nutrient absorption mechanisms, bacterial colonisation on root surfaces, plant selection by water depth zones (emergent 20–60 cm, submerged 60–200 cm, floating at surface), gravel substrate requirements, water circulation through planted regeneration zones, plant density calculations (5–10 plants per square metre), seasonal maintenance, and comparison with chemical and mechanical filtration systems.
Key Takeaways
- Plants absorb dissolved nutrients – nitrogen and phosphorus absorption deprives algae of food preventing green water
- Root biofilms harbour bacteria – beneficial bacteria on roots convert ammonia into harmless nitrogen gas
- Oxygen production is essential – submerged plants release oxygen supporting aerobic bacterial activity
- Gravel substrate is fundamental – a 30–40 cm layer provides bacterial colonisation surface and root anchorage
- Regeneration zone sizing: 40–60% of total pool area – adequate plant biomass for filtration capacity
- Plant density: 5–10 per m² – initial planting establishes coverage and plants spread naturally
- Circulation required – water pumped through planted zone every 6–24 hours for contact time
- Zero chemicals required – plants and bacteria replace chlorine, salt and ozone disinfectants
Nature Figured Out Filtration Long Before We Did
How Plants Purify Water
Nutrient Absorption
Nitrogen removal: Plants absorb nitrate (NO₃⁻) through roots as a primary nitrogen source. Emergent plants like Phragmites and Cyperus store substantial nitrogen in tissues, permanently removing it from the water when foliage is seasonally harvested.
Phosphorus removal: Roots absorb phosphate (PO₄³⁻) from the water and substrate. Phosphorus stored in plant biomass prevents algae from accessing this growth-limiting nutrient.
Starving algae of food: Algae require nitrogen and phosphorus to grow. Plants outcompete algae for these nutrients due to their extensive root systems, faster absorption rates and continuous uptake. Without adequate nutrients, algae cannot bloom causing green water.
Permanent removal: Seasonal trimming of plants physically removes from the pool system the nitrogen and phosphorus stored in plant tissues. Cut foliage is composted or discarded and not returned to the water.
Oxygen Production
Photosynthesis: Submerged aquatic plants produce oxygen through underwater photosynthesis. Oxygen is released directly into the water column from leaf surfaces.
Oxygen saturation: A healthy planted regeneration zone maintains dissolved oxygen of 6–10 mg/L (saturation). High oxygen levels support aerobic bacterial activity essential for ammonia conversion.
Night-time consideration: Photosynthesis stops at night and plants consume oxygen through respiration. Sufficient daytime oxygen production compensates for night-time consumption maintaining a positive oxygen balance.
Bacterial Colonisation on Roots
Root surface area: Plant roots provide enormous surface area for beneficial bacteria attachment. One square metre planted with bulrushes provides 50–100 m² of bacterial colonisation surface through the root mass.
Aerobic zones: Oxygen transported from leaves through stems to roots creates oxygen-rich zones around root tips that support nitrifying bacteria (Nitrosomonas, Nitrobacter).
Anaerobic zones: Deeper substrate layers with lower oxygen support denitrifying bacteria that convert nitrate into nitrogen gas (N₂) that escapes to the atmosphere.
Biofilm formation: Bacteria form biofilms (slime layers) on root surfaces and gravel particles. Biofilms trap particles, harbour diverse bacterial communities and accelerate organic matter decomposition.
Plant Selection by Depth Zones
Emergent Plants (20–60 cm Water Depth)
Bulrush (Typha latifolia, T. angustifolia): Aggressive spreading, rapid nutrient absorption and toleration of 10–80 cm depth. Native to Portugal, cold-hardy to -20°C. Require containment or regular division to prevent overcrowding.
Common reed (Phragmites australis): Tall (2–4 metres), extensive rhizome system and excellent phosphorus absorption. Can be invasive. Plant sterile cultivars or in contained areas. Native throughout Europe.
Rushes (Juncus effusus, J. inflexus): Clump-forming (no spreading), moderate nutrient absorption and attractive vertical form. Tolerates 5–40 cm depth and partial shade.
Water iris (Iris pseudacorus, I. versicolor): Yellow flowers in May–June, moderate growth rate and less aggressive than bulrushes. Native I. pseudacorus suitable for Portugal. Tolerates 10–30 cm depth.
Clubrush (Schoenoplectus lacustris): Cylindrical stems, tolerates warm water and is native to the Mediterranean. Excellent for Portugal’s climate. Depth range 20–80 cm.
Submerged Plants (60–200 cm Water Depth)
Hornwort (Ceratophyllum demersum): Free-floating (no roots), absorbs nutrients through the entire plant surface and is an excellent oxygenator. Can become prolific requiring thinning. Tolerates 30–300 cm depth.
Elodea (Elodea canadensis): Aggressive oxygenator, cold-tolerant and provides habitat for fish and amphibians. Plant in pots to contain spreading.
Water milfoil (Myriophyllum spicatum): Feathery foliage, good oxygenation and attractive underwater texture. Requires intense light and benefits from CO₂ supplementation in low-nutrient water.
Floating Plants (Surface)
Water lettuce (Pistia stratiotes): Tropical, not frost-hardy (remove before first frost in Portugal’s inland regions). Aggressive spreading and excellent nutrient absorption through suspended roots.
Water hyacinth (Eichhornia crassipes): Purple flowers, prolific nutrient absorber and tropical. Extremely invasive in frost-free climates so avoid permanent installation.
Duckweed (Lemna minor): Tiny floating plant, covers surface rapidly. Efficient nutrient absorber but difficult to control once established. Use sparingly.
Coverage limit: Floating plants should cover a maximum of 20–30% of the regeneration zone surface. Excessive coverage blocks light for submerged plants and reduces oxygen exchange at the water surface.
Gravel Substrate Requirements
Substrate Function
Bacterial surface area: Gravel provides surface for bacterial biofilm colonisation. Gravel with 10–30 mm diameter offers an optimal surface-to-volume ratio – greater surface area than coarse gravel and better flow than fine gravel.
Root anchorage: Plants root into the gravel gaining stability and accessing nutrients stored in substrate interstices.
Particle filtration: Water flowing through the gravel bed physically filters suspended particles retained between the stones.
Specifications
Gravel size: 10–30 mm diameter. Smaller (< 10 mm, sand) compacts restricting flow and oxygen. Larger (> 50 mm) provides insufficient surface area.
Depth: 30–40 cm layer. Provides adequate volume for root growth, bacterial colonisation, and particle storage.
Material: Washed river gravel, crushed rock, or volcanic rock (lava stone). Must be clean as unwashed gravel clouds water initially from sediment/dust.
pH-neutral: Avoid limestone, marble, and shell-based gravels that raise pH. Use granite, basalt, quartzite, and river stone (pH-neutral).
Flow Distribution
Uniform flow critical: Water must flow uniformly through the entire gravel bed and not shortcut through preferential pathways.
Design: A perforated distribution pipe buried in the gravel distributes incoming water uniformly. Multiple inlet points along the width of the zone. A slight slope (1–2%) toward the collection point ensures drainage.
Monitoring: Observe plant health. Yellowing in specific areas indicates poor circulation/nutrient deficiency signalling dead zones in flow patterns.
Regeneration Zone Design
Sizing
40–60% of total pool area: Regeneration zone surface area should equal or exceed the swimming zone area for adequate biological filtration capacity.
Example: A 50 m² swimming zone requires a minimum 50–60 m² regeneration zone. Total pool area of 100–110 m².
Undersizing consequences: Insufficient plant biomass and bacterial surface area cannot process swimmer pollutants. Results in cloudy water, algae blooms, and system failure.
Depth
30–80 cm water depth: Shallow water is optimal for emergent plant growth, bacterial activity, and oxygen penetration.
Depth variation: The zone can include varying depths. 20 cm shallow margins for iris, 40–60 cm main area for bulrushes/rushes, and 80 cm deeper pockets for transition to submerged plants.
Bottom substrate: The 30–40 cm gravel layer sits on the pool base. Water depth is measured from the gravel surface.
Swimming Zone Separation
Physical barrier: An underwater wall, shelf edge, or planted border prevents swimmers from entering the regeneration zone while allowing water flow between zones.
Typical barrier: Concrete or natural stone wall built 5–10 cm below the water surface. Creates visual separation, prevents accidental entry and maintains distinct zones.
Aesthetic integration: The barrier can be decorative with natural stone edging, gabion walls, or architectural elements that integrate the regeneration zone into the landscape.
Water Circulation System
Pump Capacity
Complete exchange every 6–24 hours: All pool water must pass through the regeneration zone for biological contact.
Flow rate calculation: A 60 m³ pool requiring an 8-hour turnover needs a pump of 7,500 litres/hour (60,000 L ÷ 8 hours).
Pump type: Submersible pump in the swimming zone or external pump in the equipment area. Energy-efficient variable speed pumps reduce electricity costs.
Flow Pattern
Intake: Swimming zone (draws pollutants for treatment).
Distribution: Evenly along the regeneration zone gravel bed through perforated pipe.
Collection: Collection point opposite the distribution side after water flows through the entire planted bed.
Return: Back to the swimming zone through underwater jets or surface cascade elements.
Retention Time
15–30 minutes contact time: Water should spend at least 15–30 minutes flowing through the regeneration zone for adequate bacterial processing and plant nutrient absorption.
Calculation: Zone volume ÷ pump flow rate = retention time. Example: 30 m³ zone volume, 2,000 L/hour flow rate = 15-hour retention (excessive so reduce flow rate or increase zone volume).
Plant Density and Establishment
Initial Planting Density
5–10 plants per m²: Spacing allows root establishment while achieving 60–80% coverage in the first growing season.
Example: A 50 m² regeneration zone requires 250–500 plants at initial installation, depending on species (aggressive spreading species need lower density; clump-forming species need higher density).
Species distribution: Mix of 60% emergent (bulrushes, rushes, iris), 30% submerged (hornwort, elodea), and 10% floating (water lettuce, minimal duckweed).
First Season Establishment
Growth timeline: Emergent plants establish roots in the first 4–6 weeks, visible growth begins at week 6–8, and full canopy development occurs at weeks 12–16.
Water quality: May remain slightly cloudy during the first 2–3 months as bacterial colonies establish and plant roots grow. Patience is essential as biological systems require time to mature.
Fertilisation: Not required. Swimmer pollutants (sweat, skin cells, urine) provide the nitrogen and phosphorus plants need. Additional fertilizer feeds algae.
Mature System Management
Coverage target: 60–70% plant coverage is ideal. Allows light penetration for submerged plants and prevents complete surface blocking.
Thinning: Aggressive species (bulrushes, reeds) require division every 2–3 years to prevent overcrowding. Divide in spring, replant divisions or discard excess.
Replacement: Some plants (annual species, frost-damaged tropicals) require annual replanting. Most hardy perennials persist indefinitely with minimal intervention.
Seasonal Maintenance
Spring (March–May)
Plant activation: Remove dead foliage from standing stems of the previous season.
Division: Divide overcrowded clumps, replant healthy divisions and discard excess.
New planting: Fill gaps from winter losses and add new species for diversity.
Pump start-up: Start circulation (if stopped during winter) and monitor flow patterns.
Time required: 2–4 hours for a typical 50 m² regeneration zone.
Summer (June–September)
Peak growth: Maximum nutrient absorption and oxygen production. Bacteria are most active at 20–25°C.
Monitoring: Thin overcrowded areas maintaining 60–70% coverage. Remove excess floating plants if coverage exceeds 30%.
Water clarity check: Should maintain 1–3 metre visibility. Cloudy water indicates undersized zone or pump failure.
Time required: 1–2 hours monthly inspection.
Autumn (October–November)
Dormancy: Deciduous plants (bulrushes, reeds) die back naturally. Cut stems to 15–20 cm above water removing nutrient-rich foliage, or leave standing for wildlife habitat and winter visual interest.
Leaf management: Remove fallen tree leaves from the pool surface before they sink. Leaf nets prevent accumulation.
Time required: 2–3 hours seasonal cutting if carried out.
Winter (December–February)
Dormancy: Most plants are dormant. Bacterial activity slows but continues at a reduced rate in Portugal’s mild climate (water temperature 8–14°C).
Minimal intervention: The system is self-sufficient. Check the pump monthly ensuring operation.
Time required: < 1 hour monthly.
Natural Pool vs Chemical Systems
Filtration Comparison
Chemical pools: Mechanical filter (sand, cartridge, DE) retains particles. Chemical disinfectant (chlorine, bromine, salt-generated chlorine, ozone) eliminates microorganisms. Requires daily testing, weekly chemical additions, and shock treatments.
Natural pools: Biological filter (plants and bacteria in gravel) absorbs nutrients, breaks down organic products and reduces pathogens. Requires only seasonal plant maintenance. Zero chemical testing and zero disinfectant additions.
Operating Costs
Chemical pools: €200–500 annually (chlorine/salt/chemicals, filter media replacement and equipment repairs).
Natural pools: €100–300 annually (pump electricity only). Zero chemicals, zero salt and zero disinfectant costs.
Water Quality
Chemical pools: Sterile, chemically disinfected, artificial blue colour and chlorine smell. Can irritate skin/eyes at higher chemical concentrations.
Natural pools: Biologically balanced, natural appearance (may have a slight tea tint from tannins and very slight algae green), no chemical smell and soft feel. Visibility typically 1–3 metres.
Maintenance Time
Chemical pools: 1–2 hours weekly (debris removal, vacuuming and chemical testing/adjusting).
Natural pools: 1–2 hours weekly (debris removal, vacuuming) + 4–8 hours seasonally (plant care).
Annual total: Similar time investment, different tasks. Chemical pools require chemistry knowledge; natural pools require plant knowledge.
Oásis Biosistema’s Natural Swimming Ponds
Oásis Biosistema designs natural swimming ponds where the entire pond is available for swimming. There are no separated zones, no areas off-limits. Aquatic plants and ornamental fish contribute to a living, balanced ecosystem, but the system does not rely on plants alone.
A custom biological filtration system combining mechanical filtration, UV sterilisation and ozone technology actively supports the natural processes, capturing debris before it decomposes and preventing algae proliferation in all conditions.
The base uses special-grade white quartz sand that keeps the bottom permanently visible, does not alter water parameters, and prevents sludge formation. The result is water that is always crystal clear, chemical-free, and stable year after year without full water changes.
Conclusion
Aquatic plants purify natural pool water through nutrient absorption that removes nitrogen and phosphorus preventing algae growth, oxygen production through photosynthesis that supports beneficial bacteria that convert ammonia into nitrogen gas, and physical filtration through root systems that retain suspended particles.
Regeneration zones sized at 40–60% of total pool area contain emergent plants (Typha, Phragmites, Juncus) in 30–40 cm gravel substrate that provides bacterial colonisation surface, submerged oxygenators (Ceratophyllum, Elodea), and controlled floating species coverage below 30%, with water circulation completing full exchange every 6–24 hours maintaining biological contact time.
Oásis Biosistema designs natural swimming pools integrating planted regeneration zones into Portuguese landscapes, creating chemical-free pools where plants and bacteria replace chlorine, salt and ozone systems through biological filtration that functions year-round in the Mediterranean climate.
FAQ
What is the relationship between plants and water?
Plants require water for photosynthesis, nutrient transport, structural support and cooling through transpiration. Water dissolves soil minerals, transports them through stems to leaves and maintains cellular turgor pressure that keeps plants upright. Approximately 90% of plant mass is water. Plants absorb water through roots and release it through leaf stomata, regulating internal processes.
Do plants reduce cortisol?
Yes, plants reduce cortisol levels. Studies show that indoor plants reduce stress hormones, decrease anxiety and improve mood through visual contact and care routines. Gardening activities particularly reduce cortisol. Plants improve air quality and create calming environments. Even viewing images of nature reduces stress. Regular interaction with plants promotes relaxation, lowering cortisol by 10–15% in some studies.
How do you keep plants watered for 2 weeks?
Keep plants watered using self-watering stakes, drip irrigation systems or wine bottle reservoirs. Water thoroughly before leaving, group plants to retain moisture, move them away from direct sunlight and use water-absorbing crystals in the soil. Capillary mat systems, automatic timers or asking neighbours to water work well. Consider drought-tolerant plants for extended absences.
What are the signs of overwatering plants?
Signs of overwatering include yellowing leaves (especially lower leaves), wilting despite wet soil, brown and mushy roots, mould/fungus on soil surface, leaf drop and soft stems. Soil smells sour or rotten. Leaves develop brown tips or water-soaked spots. Growth slows and roots appear black instead of white. Persistently wet soil attracts fungus gnats.
