Azolla rubra
red azolla
Overview
Azolla rubra is a minute free-floating aquatic fern that forms dense mats across the surface of still and slow-moving fresh water. Individual plants are 0.4-1 inch (1-2.5 cm) across, with branching stems clothed in tiny overlapping scale-like leaves and thread-like roots that hang into the water. The foliage is green in shade and shifts to red or reddish brown in full sun, cold, or nutrient stress, often colouring whole ponds. Each leaf encloses a cavity housing a nitrogen-fixing cyanobacterium, so the fern draws nitrogen from the air and can grow rapidly, doubling its area within days in warm, still conditions. It reproduces mainly by fragmentation, with spores formed less often. Azolla rubra is native to Australia and New Zealand and grows on dams, ponds, wetlands, and ditches. Dense mats can shade out submerged plants and lower oxygen levels in the water below, and the fern is killed by hard frost and by drying of its waterbody. In cultivation it is used on ornamental ponds and in aquaculture, though it spreads quickly and may need regular thinning.
Native Range
Azolla rubra is native to Australia and New Zealand, where it floats on dams, ponds, wetlands, lagoons, and slow streams. It favours still, nutrient-rich fresh water in open, sunny positions.Suggested Uses
A. rubra is used as a floating cover on ornamental ponds, as a green manure and fodder in aquaculture, and to suppress algae by shading the water. Its nitrogen fixing also enriches pond systems. Rapid spread means it can overrun open water and needs containment.How to Identify
Appearance
Size & Dimensions
Height1" - 1"
Width/Spread1' - 10'
Reaches mature size in approximately 1 years
Bloom Information
As a fern, A. rubra produces no flowers and spreads mainly by breaking apart into new plants. Spore-bearing structures form occasionally, mostly in summer and autumn. Growth and red colouring are strongest in the warm, bright months.
Detailed Descriptions
Foliage Description
Green turning redGrowing Conditions
Sun Requirements
Requires 4-10 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight
Care & Maintenance
Care Guide
Grow A. rubra as a floating layer on still or slow fresh water in full sun to part shade. It needs no soil and draws nitrogen from the air through its cyanobacterial partner, though it grows fastest on nutrient-rich water. The fern suits mild climates, roughly USDA zones 9-11, and is killed by hard frost and by the drying of its pond. Thin the mat regularly, as unchecked growth shades and deoxygenates the water beneath. Remove excess plants by netting to keep part of the surface open.Pruning
A. rubra is not pruned in the usual sense. Skim or net off surplus mat through the growing season to stop it covering the whole surface. Removing build-up also reduces the risk of oxygen depletion in the water.Container Growing
✓ Suitable for container growing
Minimum container size: 5 gallons