09 · Aquatic Plants
Plants are not decoration in a natural pond — they are the filter, the oxygenator, the shade, and the habitat. Plant generously.
The four functional groups
| Group | Role | Examples | Planting depth |
|---|---|---|---|
| Oxygenators (submerged) | Absorb nitrate, release O₂ | Hornwort, Anacharis, Myriophyllum | 30–80 cm |
| Floaters | Shade surface, cut algae light | Waterlily, Frogbit, Duckweed* | Surface |
| Marginals | Edge filtration, habitat | Iris, Rush, Cattail, Pickerelweed | 0–20 cm |
| Deep-water | Cover & cooling | Hardy & tropical lilies | 40–90 cm |
*Duckweed spreads explosively — use only in contained, fishless ponds.
How much to plant
- Aim to cover 40–60% of the surface with foliage at peak summer.
- Stock ~1 bunch of oxygenator per 1–2 m² of water.
- Marginals around the entire perimeter where shelves allow.
Planting technique
- Use aquatic planting baskets with mesh sides — they contain soil but let roots and water interact.
- Fill with heavy clay-loam (not potting mix, which floats and fouls water); top with a layer of gravel to stop fish digging.
- Lower baskets onto shelves; never plant directly in the liner substrate unless you have a gravel bio-layer.
Balance trick
If algae appears, add more plants rather than reaching for algaecide. More living biomass = fewer nutrients for algae.
Seasonal plant care
- Spring: divide crowded lilies, replant baskets, remove dead winter foliage.
- Summer: thin aggressive growers; deadhead lilies.
- Autumn: cut back dying marginal stems to ~5 cm; net the pond against leaves.
- Winter: sink tropicals or compost them; hardy lilies die back and regrow.
Planting procedure for marginals
- Use aquatic baskets (mesh, not solid pots) so roots can access water.
- Line baskets with hessian or newspaper to prevent soil washing out.
- Fill with heavy garden soil, not potting compost (it floats).
- Plant at the correct depth: crown just above water level.
- Top with a layer of gravel to prevent fish digging and to suppress algae.
- Lower gently onto the shelf. Do not bury the crown.
Top 5 oxygenating plants
- Hornwort (Ceratophyllum) — no roots, just drop in
- Canadian waterweed (Elodea canadensis)
- Spiked water-milfoil (Myriophyllum spicatum)
- Bladderwort (Utricularia) — also traps mosquito larvae
- Water-starwort (Callitriche)
Expect die-back
Most aquatic plants look terrible for the first 2–3 weeks after planting. Do not panic and do not add fertiliser. They are adjusting to new water conditions. New growth appears within 4–6 weeks.