---
title: "Pond Plant Depth Guide: Marginals to Lilies | Pond Mosaic"
description: "How deep to plant pond plants: RHS depth zones from marginals to waterlilies, with height, flowering months, native status and invasive plants to avoid."
url: https://pondmosaic.me/pond-plant-depth-guide/
---

RHS planting-depth zones

# Pond plant depth guide

How deep should pond plants go? Depth means water over the top of the pot or crown. The RHS sorts marginals into **under 5 cm, 5–15 cm, 15–30 cm and 30 cm or more**, and waterlilies from 10–30 cm up to 75 cm–1.2 m. Filter the table by zone, height, flowering month or native status.

## Invasive pond plants to avoid

These non-native plants spread fast, smother ponds and escape into the wild. Never buy, plant or pass them on, and compost or bin any you pull out rather than tipping them anywhere near water.

### Great Britain

- Floating pennywort (*Hydrocotyle ranunculoides*)
- Parrot's feather (*Myriophyllum aquaticum*)
- New Zealand pigmyweed, or Australian swamp stonecrop (*Crassula helmsii*)
- Water primroses (*Ludwigia grandiflora*, *L. peploides*, *L. uruguayensis*)
- Water fern (*Azolla filiculoides*)
- Curly waterweed (*Lagarosiphon major*)
- Nuttall's waterweed (*Elodea nuttallii*) and the other *Elodea* waterweeds, such as Canadian waterweed
- Water lettuce (*Pistia stratiotes*) and water hyacinth (*Eichhornia crassipes*)
- Duck potato, or broadleaf arrowhead (*Sagittaria latifolia*)
- Fanwort (*Cabomba caroliniana*), broadleaf watermilfoil (*Myriophyllum heterophyllum*) and salvinia moss (*Salvinia molesta*)
- American skunk cabbage (*Lysichiton americanus*) and giant rhubarb (*Gunnera tinctoria*) at the pond edge

In England and Wales many of these are “species of special concern”, which it is illegal to keep, sell, grow or let reproduce; the others are on Schedule 9 of the Wildlife and Countryside Act 1981, which bans planting them in the wild. Selling water fern, New Zealand pigmyweed or *Ludwigia uruguayensis* has also been banned in England since 6 April 2014. Scotland and Northern Ireland have their own lists; in Northern Ireland they include water soldier and fringed water-lily.

### European Union

The EU's list of invasive alien species of Union concern includes, among its water and wetland plants, floating pennywort, parrot's feather, curly waterweed, Nuttall's waterweed, the two water primroses, water hyacinth, fanwort, broadleaf watermilfoil, salvinia moss, alligator weed (*Alternanthera philoxeroides*), Senegal tea plant (*Gymnocoronis spilanthoides*), American skunk cabbage and giant rhubarb. They may not be kept, sold, grown or released. The list was last updated with effect from 7 August 2025; check the current list before buying.

### United States and Canada

The federal noxious weed list includes hydrilla, giant salvinia and its relatives, curly waterweed, water spinach and, notably, two European natives: arrowhead (*Sagittaria sagittifolia*) and branched bur-reed (*Sparganium erectum*). States add their own: Minnesota, for example, prohibits purple loosestrife, flowering rush, European frogbit, water soldier, fringed water-lily (yellow floating heart) and Eurasian watermilfoil. State and provincial lists differ, so check yours.

## How to use the depth guide

Each plant shows the RHS's own wording for where it grows, and its height, flowering season and native status. Depth is measured from the water surface to the top of the compost in the basket, or to the crown. Stand baskets on bricks to get the depth right on a deep shelf.

- **Marginals** grow at the edge in shallow water; many also grow in wet mud.
- **Waterlilies** start shallow, with the basket under 15–25 cm of water so the young leaves reach the surface, then go down in stages to their full depth.
- **Floating plants and oxygenators** need no soil. Keep at least half the surface clear of floating leaves.

Flowering months are the RHS season translated into months (spring is March to May, summer June to August), so treat them as approximate: a cold spring or a northern garden flowers later. “Vigorous” marks plants the RHS or the Freshwater Habitats Trust warn can take over a small pond.

## Sources

1. RHS, [Choosing pond plants](https://www.rhs.org.uk/ponds/pond-plants) (depth zones for marginals and waterlilies, floating plants and oxygenators), [How to grow waterlilies](https://www.rhs.org.uk/plants/waterlilies/growing-guide) and each plant's RHS page, linked in the table (depth, height, flowering season, sun, native to GB/Ireland), checked 2 November 2026.
2. USDA NRCS, [PLANTS Database](https://plants.usda.gov/): native or introduced status in the lower 48 states and Canada, checked 2 November 2026.
3. Freshwater Habitats Trust and partners, [Creating Garden Ponds for Wildlife](https://freshwaterhabitats.b-cdn.net/app/uploads/2023/06/Creating-Garden-Ponds_2023.pdf) (2023): native plants for wildlife ponds and plants to avoid.
4. Defra, [Be Plant Wise](https://www.gov.uk/government/news/be-plant-wise-help-the-fight-against-the-weeds-that-are-taking-over-our-waterways) (12 October 2010); GOV.UK, [Invasive non-native plant species: rules in England and Wales](https://www.gov.uk/guidance/invasive-non-native-alien-plant-species-rules-in-england-and-wales) (updated 18 November 2024); [Wildlife and Countryside Act 1981, Schedule 9](https://www.legislation.gov.uk/ukpga/1981/69/schedule/9) (checked 2 November 2026); [S.I. 2014/538](https://www.legislation.gov.uk/uksi/2014/538/made), the 2014 sale ban, as amended.
5. European Commission, [Invasive alien species](https://environment.ec.europa.eu/topics/nature-and-biodiversity/invasive-alien-species_en): 49 plants on the Union list, fourth update in force from 7 August 2025 (Implementing Regulation (EU) 2025/1422). The plants named above were checked against the list as updated in 2019; later additions are not shown here.
6. USDA APHIS, [Noxious weeds program](https://www.aphis.usda.gov/plant-pests-diseases/noxious-weeds) and 7 CFR 360.200(a), aquatic and wetland weeds; [Minnesota DNR, invasive species laws](https://www.dnr.state.mn.us/invasives/laws.html); Washington State Noxious Weed Control Board ([flowering rush](https://www.nwcb.wa.gov/weeds/flowering-rush), [purple loosestrife](https://www.nwcb.wa.gov/weeds/purple-loosestrife), [yellow flag iris](https://www.nwcb.wa.gov/weeds/yellow-flag-iris)); Ontario's [Invading Species Awareness Program](https://www.invadingspecies.com/invaders/plants/water-soldier/).

## Pond plant questions

**How deep should marginal pond plants be?**

It depends on the plant. The RHS groups marginals into under 5 cm (2 in) and mud, 5–15 cm, 15–30 cm, and 30 cm (1 ft) or more, measured from the water surface to the top of the pot.

**How deep do you plant a waterlily?**

Start with the basket under 15–25 cm of water, then lower it in stages. Dwarf lilies end up in about 30–45 cm, medium ones 45–75 cm and large ones 75 cm–1.2 m (RHS).

**What pond plants are native to the UK?**

Marsh marigold, water forget-me-not, brooklime, lesser spearwort, bogbean, flowering rush, water plantain and hornwort are among them. Filter the table to “Great Britain” for the full list.

**Which pond plants are banned in the UK?**

Floating pennywort, parrot's feather, water primroses, curly waterweed and water hyacinth may not be sold or kept in England and Wales; water fern and New Zealand pigmyweed are also banned from sale in England.