Are Lily Pads Good for a Pond? Cover, Shade and the Limit

Lilies are among the most useful things you can put in a pond — up to about half the surface. Past that the same plant starts working against the water.

What they do for the pond

• Shade. Algae need light and warmth; surface cover removes both. One of the better non-chemical algae controls there is
• Temperature. Meaningful in a shallow pond in July
• Nutrient uptake, competing directly with algae
• Cover for fish from herons, cats and sun — the most practical protection a small pond has
• Spawning habitat, among the stems
• Invertebrate habitat on the undersides, which the fish graze
• Somewhere for frogs to sit, which is not nothing

Where the line is

Judge coverage in midsummer at full growth, not in May. Lilies expand a great deal through the season, and a pond that looks sparse in spring is often over the line by August.

Planting them properly

• Aquatic baskets, always — never planted into pond soil, which is how they take over
• Heavy aquatic compost, not ordinary potting compost, which floats and leaches nutrients straight into the water
• Gravel topping, an inch of it, to hold the compost down and keep fish out of it
• Depth by variety: dwarf lilies 8–18 in, medium 18–30 in, vigorous 24–48 in. Match the variety to the pond, which is where most overgrowth starts
• Lower them in stages as the leaves grow, rather than dropping a new plant straight to full depth
• Full sun. Lilies flower poorly with less than about six hours
• Still water. They dislike being near a fountain or a waterfall outfall

Thinning what has taken over

1. Lift the baskets in spring and divide the rhizomes, cutting each back to a healthy growing point with a few leaves
2. Repot into fresh aquatic compost, gravel-topped
3. Through the season, cut excess pads at the stem well below the water. Do not pull — tearing the rhizome invites rot
4. Remove no more than about a third of the cover at once. A hard cut removes the shading before anything else takes up the nutrients, and you get an algae bloom within days
5. Remove yellowing pads and spent flowers continuously — they decay in the pond otherwise
6. Rake out any that have escaped into the pond bottom

Lilies and koi

The awkward combination. Koi root in baskets and tear at leaves and stems, and a koi pond with unprotected lilies usually ends with neither.

• Heavy gravel or cobble topping over the compost
• Mesh over the basket, so the plant grows through and the fish cannot dig
• Raise the baskets on blocks out of the main swimming area
• Or accept it and use marginal planting for cover instead — irises and rushes are less interesting to koi

Alternatives and companions

Lilies are surface cover. A pond needs more than surface cover:

• Submerged oxygenators — hornwort, elodea. These do the actual oxygen work and compete with algae in the water column
• Floating plants — water lettuce, frogbit. Easy cover, easy to remove, though some are restricted or invasive in parts of the US. Check your state list
• Marginals — iris, rushes, pickerel weed. Nutrient uptake at the edges, and they hide the liner
• Water hawthorn for a shallow pond, flowering when lilies are dormant

The combination that works is lilies for shade, oxygenators for oxygen, marginals for nutrients and edges — and, underneath all of it, feeding the fish less, which is the usual cause of the nutrient load in the first place.