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.
Showing all 30 plants
| Plant | Zone and depth | Height | Flowers | Native status | Source |
|---|---|---|---|---|---|
| Purple loosestrife Lythrum salicariaNative to GB | Bog / moist soil Any reliably moist soil | to 1.2 m | summer, purplish pinkJFMAMJJASOND | GB: Native N. America: Introduced Regulated as invasive: Minnesota (prohibited); Washington State (Class B, quarantined) | RHS |
| Meadowsweet Filipendula ulmariaNative to GB | Bog / moist soil Moist or poorly drained soil | to 90 cm | summer, creamy whiteJFMAMJJASOND | GB: Native N. America: Introduced | RHS |
| Hemp agrimony Eupatorium cannabinumNative to GBVigorous | Bog / moist soil Any moist soil | about 1.5 m | late summer and early autumn, light pink or purpleJFMAMJJASOND | GB: Native N. America: Introduced Can spread quickly; very attractive to butterflies. | RHS |
| Ragged robin Silene flos-cuculiNative to GB | Bog / moist soil Moist, moderately fertile soil | about 75 cm | late spring and early summer, rose pinkJFMAMJJASOND | GB: Native N. America: Introduced | RHS |
| Greater pond sedge Carex ripariaNative to GBVigorous | Bog / moist soil Fertile, moist or wet soil | 1–1.5 m | early summer, dark brown spikesJFMAMJJASOND | GB: Native N. America: Not listed Spreads vigorously; best for a large pond. | RHS |
| Swamp milkweed Asclepias incarnataNative to N. America | Bog / moist soil Moist, fertile, loamy soil; thrives near a pond | 1–1.5 m | mid-summer to early autumn, purplish pinkJFMAMJJASOND | GB: Not native N. America: Native | RHS |
| Marsh marigold Caltha palustrisNative to GBNative to N. America | Marginal Boggy soil at the water's edge; RHS zone under 5 cm and mud | to 50 cm | spring, deep yellowJFMAMJJASOND | GB: Native N. America: Native One of the first pond flowers of the year. | RHS |
| Brooklime Veronica beccabungaNative to GB | Marginal Permanently wet soil, or water up to 10 cm deep | up to 10 cm | late spring to late summer, bright blueJFMAMJJASOND | GB: Native N. America: Introduced | RHS |
| Water forget-me-not Myosotis scorpioidesNative to GB | Marginal Wet soil at pool margins, in water no deeper than 10 cm | to 30 cm | early summer, bright blueJFMAMJJASOND | GB: Native N. America: Introduced | RHS |
| Lizard's tail Saururus cernuusNative to N. America | Marginal Boggy ground or pond margins, in water no deeper than 10 cm | 1–1.5 m | summer, whiteJFMAMJJASOND | GB: Not native N. America: Native | RHS |
| Blue flag iris Iris versicolorNative to N. America | Marginal Moist to wet soil; RHS zone under 5 cm and mud | to 80 cm | summer, bluish purpleJFMAMJJASOND | GB: Not native N. America: Native | RHS |
| Water mint Mentha aquaticaNative to GBVigorous | Marginal Muddy margins, or baskets in water up to 25 cm deep in a large pond | to 90 cm | summer, pinkish purpleJFMAMJJASOND | GB: Native N. America: Introduced Very aromatic; spreads by rhizomes. | RHS |
| Yellow flag iris Iris pseudacorusNative to GBVigorous | Marginal Wet soil at the margins of large ponds; RHS zone 5–15 cm | to 1.5 m | mid and late summer, bright yellowJFMAMJJASOND | GB: Native N. America: Introduced Regulated as invasive: Minnesota (regulated); Washington State (Class C, quarantined) Forms extensive colonies; best for large ponds. | RHS |
| Cardinal flower Lobelia cardinalisNative to N. America | Marginal Reliably moist soil; RHS zone 5–15 cm | to 90 cm | summer and autumn, scarletJFMAMJJASOND | GB: Not native N. America: Native | RHS |
| Pickerelweed Pontederia cordataNative to N. America | Marginal Baskets of loam in shallow water, 10–12 cm deep | to 1.2 m | late summer, deep blueJFMAMJJASOND | GB: Not native N. America: Native | RHS |
| Arrowhead Sagittaria sagittifoliaNative to GB | Marginal Deep mud and up to 20 cm of still or slow water | flower stems to 80 cm | July and August, whiteJFMAMJJASOND | GB: Native N. America: Not listed Regulated as invasive: United States (Federal Noxious Weed) | RHS |
| Flowering rush Butomus umbellatusNative to GB | Marginal Fertile mud at pond margins, or water up to 25 cm deep | to 1.5 m | late summer, rosy pinkJFMAMJJASOND | GB: Native N. America: Introduced Regulated as invasive: Minnesota (prohibited); Washington State (Class A) | RHS |
| Bogbean Menyanthes trifoliataNative to GBNative to N. America | Marginal Muddy margins, or baskets in water up to 25 cm deep | flower spikes to 25 cm | summer, pale pinkJFMAMJJASOND | GB: Native N. America: Native Forms wide mats with leaves just above the water. | RHS |
| Lesser spearwort Ranunculus flammulaNative to GBNative to N. America | Marginal Still or slow-moving water 15–22 cm deep | 0.5–1 m | early summer, bright yellowJFMAMJJASOND | GB: Native N. America: Native | RHS |
| Greater spearwort Ranunculus linguaNative to GB | Marginal Muddy pool margins in water 15–25 cm deep | 1–1.5 m | early summer, yellowJFMAMJJASOND | GB: Native N. America: Not listed | RHS |
| Water plantain Alisma plantago-aquaticaNative to GB | Marginal Pond margins in water up to 30 cm deep (the RHS pond list puts it with plants for 30 cm or more) | 0.5–1 m | late spring, whiteJFMAMJJASOND | GB: Native N. America: Introduced Deadhead to stop it self-seeding. | RHS |
| Branched bur-reed Sparganium erectumNative to GBVigorous | Marginal A shallow pool margin, to 45 cm deep | 1–1.5 m | summer, greenish whiteJFMAMJJASOND | GB: Native N. America: Introduced Regulated as invasive: United States (Federal Noxious Weed) Can spread indefinitely; only for large ponds. | RHS |
| Amphibious bistort Persicaria amphibiaNative to GBNative to N. AmericaVigorous | Deep water Moist soil, or as an aquatic in up to 80 cm; lower young plants in stages | floating leaves; 10–50 cm out of water | mid-summer to early autumn, pinkJFMAMJJASOND | GB: Native N. America: Native Small leaves suit small ponds (FHT); divide regularly. | RHS |
| American white waterlily Nymphaea odorataNative to N. America | Deep water Basket first under 15–25 cm of water, then lowered gradually to 50–60 cm | floating leaves | June to September, white, fragrantJFMAMJJASOND | GB: Not native N. America: Native | RHS |
| White water-lily Nymphaea albaNative to GBVigorous | Deep water Basket first under 15–25 cm of water, then lowered; RHS zone 75 cm–1.2 m | floating leaves up to 30 cm across | summer, whiteJFMAMJJASOND | GB: Native N. America: Introduced Regulated as invasive: Minnesota (non-native waterlilies are regulated) Vigorous; a lily for a large, deep pond. | RHS |
| Frogbit Hydrocharis morsus-ranaeNative to GB | Free-floating Floats; water deep enough not to freeze to the bottom, where its buds overwinter | floats | summer, whiteJFMAMJJASOND | GB: Native N. America: Introduced Regulated as invasive: Minnesota (prohibited); Ontario (restricted) Sinks to the bottom in winter. | RHS |
| Water soldier Stratiotes aloidesNative to GBVigorous | Free-floating Floats just below the surface; sinks in winter | leaves to 45 cm | summer, whiteJFMAMJJASOND | GB: Native N. America: Not listed Regulated as invasive: Northern Ireland (Schedule 9); Minnesota (prohibited); Ontario (prohibited) May become a nuisance in small pools. | RHS |
| Rigid hornwort Ceratophyllum demersumNative to GBNative to N. America | Submerged Floats freely in water up to 150 cm deep | submerged | May to July (inconspicuous), pale greenJFMAMJJASOND | GB: Native N. America: Native Overwinters as buds in the bottom mud. | RHS |
| Autumnal water-starwort Callitriche hermaphroditicaNative to GBNative to N. America | Submerged Baskets submerged in water 45–60 cm deep | submerged | summer (inconspicuous), greenJFMAMJJASOND | GB: Native N. America: Native Grow in baskets to restrict its spread. | RHS |
| Spiked water-milfoil Myriophyllum spicatumNative to GB | Submerged Baskets of loamy soil at a depth of 1 m | submerged; flower spikes above water | summer, dull redJFMAMJJASOND | GB: Native N. America: Introduced Regulated as invasive: Minnesota (prohibited); Washington State (Class B, quarantined) | RHS |
No plant matches all of those filters: try fewer.
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
- RHS, Choosing pond plants (depth zones for marginals and waterlilies, floating plants and oxygenators), How to grow waterlilies and each plant's RHS page, linked in the table (depth, height, flowering season, sun, native to GB/Ireland), checked 2 November 2026.
- USDA NRCS, PLANTS Database: native or introduced status in the lower 48 states and Canada, checked 2 November 2026.
- Freshwater Habitats Trust and partners, Creating Garden Ponds for Wildlife (2023): native plants for wildlife ponds and plants to avoid.
- Defra, Be Plant Wise (12 October 2010); GOV.UK, Invasive non-native plant species: rules in England and Wales (updated 18 November 2024); Wildlife and Countryside Act 1981, Schedule 9 (checked 2 November 2026); S.I. 2014/538, the 2014 sale ban, as amended.
- European Commission, Invasive alien species: 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.
- USDA APHIS, Noxious weeds program and 7 CFR 360.200(a), aquatic and wetland weeds; Minnesota DNR, invasive species laws; Washington State Noxious Weed Control Board (flowering rush, purple loosestrife, yellow flag iris); Ontario's Invading Species Awareness Program.
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.