Fertilising guide
How to fertilise Greater Duckweed (Spirodela polyrhiza)— schedule & NPK
Also called Greater Duckweed, Common Duckmeat, Giant Duckweed.
More about greater duckweed
About Greater Duckweed
Spirodela polyrhiza · also called Greater Duckweed, Common Duckmeat · flowering
Greater Duckweed is the largest of the common duckweeds, with flat, rounded fronds 3–10 mm across bearing multiple rootlets on the underside. Native to every continent except Antarctica, it rapidly covers still water surfaces, providing shade to limit algae and shelter for invertebrates. An important waterfowl food and natural water-quality indicator.
Growth habit: Free-floating aquatic perennial; flat circular to oval fronds 3–10 mm with a reddish-purple underside and 7–21 thin rootlets hanging beneath.
Watch for — Rapid invasive spread: Spirodela can double biomass every 2–4 days in warm, nutrient-rich water. Control via regular skimming, shading part of the pond with marginal plants, or adding goldfish and koi that graze the fronds.
What fertiliser greater duckweed actually wants — and why
Greater Duckweed is an easy, light foliage feeder — a half-strength balanced liquid feed through the growing months keeps it green without forcing weak, sappy growth.
A balanced general houseplant feed (roughly even N-P-K) is exactly right — it is grown for foliage, so steady, moderate nitrogen for healthy leaves is the goal, not a bloom or root formula.
For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for greater duckweed: match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.
How often to feed greater duckweed, and which months
Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For greater duckweed:
Self-sustaining in a natural pond; absorbs nutrients from the water column. No feeding required. In ultra-oligotrophic (very clean) water or aquaria without fish, a minimal dilute aquatic fertiliser can be applied monthly. Treat that as monthly between spring through early autumn (roughly March to September); ease off in autumn and stop entirely in the low light of winter.
The dormant-season rule matters more than the exact interval: skip feeding entirely when greater duckweed is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.
What strength to mix for greater duckweed
Half strength is the safe default for greater duckweed — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.
Feeding always goes onto already-damp soil, never dry roots — water greater duckweed first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the greater duckweed watering schedule.
Signs you are over-feeding greater duckweed
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for greater duckweed:
- Brown, crispy leaf tips and edges with no sign of underwatering.
- A white, crusty salt deposit on the soil surface or pot rim.
- Weak, pale, stretched new growth that flops.
- Lower leaves yellow and drop while the soil is correctly watered.
Signs you are under-feeding greater duckweed
- Uniformly pale or yellow-green leaves, oldest first.
- Noticeably small new leaves and stalled growth in good light and season.
- A generally tired, lacklustre look despite correct watering and light.
If the symptoms point at watering, light or roots rather than nutrition, the full greater duckweed care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Flush the pot of greater duckweed with plain water until it runs freely from the base every couple of months in the feeding season — it washes out the fertiliser salts that cause brown tips.
Organic vs synthetic feeds for greater duckweed
Organic options
A diluted seaweed or worm-casting feed, or fish emulsion if you can tolerate the smell indoors. UK: Westland or Baby Bio Organic, dilute seaweed; US: Espoma Indoor! or Neptune's Harvest fish & seaweed. Slow, gentle and hard to overdo.
Synthetic / liquid feeds
A general-purpose houseplant liquid at half strength — UK: Baby Bio, Westland Houseplant Feed or Phostrogen; US: Miracle-Gro Indoor Plant Food or Schultz. Convenient and fast-acting; the only risk is overdoing it.
Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.
Fertilising greater duckweed — frequently asked questions
What fertiliser does greater duckweed need?
A balanced general houseplant feed (roughly even N-P-K) is exactly right — it is grown for foliage, so steady, moderate nitrogen for healthy leaves is the goal, not a bloom or root formula. Greater Duckweed is an easy, light foliage feeder — a half-strength balanced liquid feed through the growing months keeps it green without forcing weak, sappy growth.
How often should I feed greater duckweed?
Self-sustaining in a natural pond; absorbs nutrients from the water column. No feeding required. In ultra-oligotrophic (very clean) water or aquaria without fish, a minimal dilute aquatic fertiliser can be applied monthly. Self-sustaining in a natural pond; absorbs nutrients from the water column. No feeding required. In ultra-oligotrophic (very clean) water or aquaria without fish, a minimal dilute aquatic fertiliser can be applied monthly. Treat that as monthly between spring through early autumn (roughly March to September); ease off in autumn and stop entirely in the low light of winter.
What strength of feed for greater duckweed?
Half strength is the safe default for greater duckweed — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.
What does over-feeding greater duckweed look like?
Brown, crispy leaf tips and edges with no sign of underwatering. A white, crusty salt deposit on the soil surface or pot rim. Weak, pale, stretched new growth that flops. Lower leaves yellow and drop while the soil is correctly watered. Feeding greater duckweed year-round on a fixed schedule, including dark winter months, is the most common mistake — it cannot use the nutrients in low light and the surplus simply burns the roots and crusts the soil.
Should I flush the soil of greater duckweed?
Flush the pot of greater duckweed with plain water until it runs freely from the base every couple of months in the feeding season — it washes out the fertiliser salts that cause brown tips.
Keep reading
- Greater Duckweed care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water greater duckweed — the watering schedule
- The houseplant fertiliser schedule — feeding through the year
- NPK ratio explained — what the three numbers on the bottle mean
- How to fertilise stanhopea tigrina
- How to fertilise rodriguezia secunda
- How to fertilise aerangis luteoalba
- All 8452 fertilising guides in the Growli library