Water plants in their natural habitats draw nutrients from the soil below the pond or stream and water as it carries them from one part of the watershed to another. Plants like water lilies that live in pots in garden ponds unfortunately do not have access to the same nutrients. They need a special type of fertilizer designed to work in aquatic environments.
Most aquatic plants are heavy feeders, but the heavy soil and gravel in which most are planted provide little nitrogen, phosphorus or potassium (N-P-K) to say nothing of the minerals and trace elements necessary for healthy plants. Although many emergent or marginal plants can be planted in soil along water feature banks, floating plants like duckweed and water hyacinth derive all of their nutrition from the water. The roots of submerged plants like hornwort and the surface plants like lotus and water lilies are confined to pots, much like houseplants. They require special feeding.
With the growing popularity of water gardens and ponds, fertilizer manufacturers have begun providing special fertilizers for use in aquatic environments. Liquid fertilizer goes directly into the water for floating plants. Granular fertilizer or tablets are added to potting soil when plants are potted up, and tablets can be sunk into potting soil for monthly feedings.
Fertilizers must be more water soluble than most other garden fertilizers to be effective in aquatic environments. They must release nitrogen slowly to avoid upsetting the chemical balance of the water for other inhabitants like fish. The University of Illinois recommends granular and tablet fertilizers with an N-P-K analysis of 20-10-5, 5-10-5, 10-6-4, or 12-8-8.
The University of Illinois recommends waiting until temperatures reach at least 50 degrees in spring before setting out and beginning to fertilize hardy lilies and until the air is 70 degrees for tropical plants. End fertilization in August so that water plants can begin to enter dormancy. Directions on different fertilizers vary, but most recommend feeding at least every four to six weeks.
Amount and frequency of application also depend on temperature, depth of water and the amount of sun plants will get. Emergent plants require less fertilization because they are planted in the ground rather than pots. Kit and Ben Knotts, breeders of Victoria water lilies, recommend applying fertilizer less frequently during cool weather and as frequently as every two weeks when the weather is hot. Because they plant in sand rather than soil, they favor tablets rather than granular feeding to avoid burning roots.