Pond Plants: Water Lilies, Marginals and Oxygenators

Planting zones of a garden pond, water lilies and marginals, submerged oxygenators, baskets and soil, surface coverage for algae control, and the legal status of invasive pond plants.

Plant zones in a pond

Botanists group aquatic macrophytes by where they grow relative to the water surface (Wikipedia): emergent plants are rooted under water but hold most of their foliage in the air (reeds, cattails); submerged plants grow entirely under water, either rooted like Myriophyllum spicatum or unrooted like hornwort, Ceratophyllum demersum; floating-leaved plants are anchored in the bottom with leaves at the surface, the water lilies (Nymphaeaceae) being the classic example; and free-floating plants such as Lemna or water lettuce drift with no attachment to the substrate.

Water-garden practice translates these groups into planting depths. Penn State Extension distinguishes shallow marginals that stand in about 3 inches (7–8 cm) of water, deep-water plants such as water lilies that grow in 4 to 10 or more inches (10–25+ cm), submerged "oxygenators" that are usually sold as rooted cuttings, and floating plants that need almost no soil. Wikipedia places submerged plants typically 1–2 ft (30–60 cm) below the surface and marginals with the top of the pot at or barely below the water level.

Water lilies

Nymphaea are rhizomatous or tuberous perennial herbs with mostly floating leaves and contractile roots that anchor them in the pond bottom (Wikipedia). Leaf blades range from about 2.5–3 cm to 40–60 cm across, and petioles can reach several metres in deep water. Texas A&M AquaPlant describes oval leaves, green above and often purple or red beneath, on fleshy rhizomes in ponds, lakes and slow streams; the submerged parts shelter invertebrates that feed fish.

The genus contains hardy and tender species. Hardy lilies tolerate winter dormancy under ice, while tropical lilies need warm water year-round (Wikipedia); University of Georgia Extension notes that in USDA zones 7–8 hardy lilies overwinter in the pond while tropical lilies are treated as annuals or moved indoors. Penn State lists the native fragrant water lily, Nymphaea odorata, as hardy in zones 4–8, flowering from March to October in the Mid-Atlantic. Wikipedia adds that the shade cast by lily pads helps limit algae growth.

Marginal plants

Marginals occupy the shelf. Penn State names blue flag iris and pickerelweed (Pontederia cordata, hardy in zones 3–10, up to 4 ft tall) as shallow-water natives; Wikipedia adds iris, cattail, arrowhead and taro. University of Georgia Extension recommends growing vigorous spreaders such as cattails in pots so that they cannot colonise the whole pond.

Submerged oxygenators

Submerged plants release oxygen through photosynthesis and compete with phytoplankton for nitrogen and phosphorus, which is why they are sold as "oxygenators" for algae control (Wikipedia). Penn State notes that Elodea is one of the few pond plants that stays green through winter. Practical Fishkeeping points out that plants oxygenate water by day but consume oxygen at night, so heavily planted ponds without aeration can run short before dawn.

Planting baskets and soil

Extension sources agree on heavy, lean soil. Penn State recommends ordinary topsoil without organic matter in containers with drainage holes lined with burlap or landscape fabric; Clemson Cooperative Extension specifies sandy loam and warns against fertiliser, potting mix or compost, which feed algae blooms. Containers can be raised on blocks so that each plant sits at its preferred depth (Wikipedia). Penn State also advises rinsing new plants in a light-coloured bucket of tap water to remove hitch-hiking organisms, and disposing carefully of packing water.

Surface coverage and algae control

Shade from floating leaves limits the light available to algae, but complete cover suffocates the pond. Penn State Extension considers 20–30 % plant coverage ideal for an ecological balance between plants and aquatic life, and Clemson gives 20–50 % for wildlife ponds. Practical Fishkeeping frames the same rule from the fish side: keep about two-thirds of the surface clear. Penn State warns that a pond entirely covered by lilies or by free-floating plants receives too little light beneath the surface and loses gas exchange with the atmosphere, leaving it oxygen-poor. Free-floating plants give shade but multiply extremely fast and need regular thinning (Wikipedia).

Invasive species and legal notes

Several plants long sold for water gardens are now regulated. Wikipedia observes that many invasive aquatic plants were marketed as oxygenators, and Penn State reports that prohibited species often arrive unnoticed in plant orders or packing material.

Water hyacinth, Pontederia crassipes

Formerly Eichhornia crassipes, this free-floating plant from the Amazon basin can double in size in one to two weeks; dense mats shade out submerged plants and their decay depletes dissolved oxygen, killing fish (Wikipedia; Texas A&M AquaPlant). It is on the European Union list of invasive alien species of Union concern since 2016, which prohibits import, keeping, cultivation, sale and release. In the United States a 1956 federal ban on its sale and shipment was repealed in 2020, and regulation now sits with the states: Texas makes possession or transport illegal (AquaPlant) and South Carolina lists it as illegal (Clemson). The USDA APHIS Federal Noxious Weed List names the related anchored water hyacinth, Eichhornia azurea, not P. crassipes.

Parrot’s feather, Myriophyllum aquaticum

A South American perennial with feathery leaves in whorls of four to six and emergent stems that can exceed 1.5 m, it is grown in ponds and aquaria but has become a noxious weed on every continent except Antarctica (Wikipedia). Only female plants exist outside its native range, so it spreads entirely from fragments of rooted stems (Wikipedia; AquaPlant). It has been on the EU Union list since 2016 and banned from sale in the United Kingdom since April 2014; in the US it is declared a noxious weed in Alabama, Connecticut, Massachusetts, Maine, Vermont and Washington, while Texas does not prohibit possession but advises against introducing it to new water bodies.

Other regulated pond plants

• USDA APHIS Federal Noxious Weed List (aquatic section): Azolla pinnata, Eichhornia azurea, Hydrilla verticillata, Lagarosiphon major, Limnophila sessiliflora, Salvinia auriculata, S. biloba, S. herzogii, S. molesta, Ipomoea aquatica and others.
• Clemson lists 28 species illegal under South Carolina or federal law, including Pistia stratiotes, Egeria densa, Salvinia molesta, Myriophyllum spicatum and Hydrilla verticillata.

The legal instruments differ by region. In the EU, Regulation (EU) No 1143/2014 and Implementing Regulation (EU) 2016/1141 with its updates restrict keeping, importing, selling, breeding, growing and releasing listed species; the European Commission reports 114 species on the list after the August 2025 update, 49 of them plants. In the United States, the Plant Protection Act and the APHIS Federal Noxious Weed List regulate import and interstate movement, and state lists add further prohibitions. Verify current status before purchase; lists change.