Like many of their landlubber cousins, pond plants can be grown without dirt. Floating plants need no planting medium: their dangling roots obtain nutrients directly from the pond water. Submerged and marginal plants, however, need a place to put down roots. Regular garden dirt may contain pesticides, compost and other algae-causing components, so a neutral potting medium is a better choice when planting water garden plants.
Obtain a neutral pH, water garden potting medium from a pond specialty store. Commercial products made for this purpose are composed of kiln-fired clay pebbles. This material can be mixed with garden loam or used alone and is suitable for water lilies and lotus, as well as marginals such as bog lilies and cattails. Submerged, oxygenating plants like cabomba can be planted in gravel alone.
Choose the right container. Water lilies and lotus need a 10- to 12-inch basket. Baskets are preferable to solid pots because the perforated sides allow water to flow over the roots, mimicking their natural growing environment. Plant smaller submerged plants such as anarchis in 4- to 6-inch baskets.
Line the baskets with a 1/2- to 1-inch layer of sphagnum moss. Alternatively, you can use cut-to-fit filter pad liners available from water garden specialty stores. The moss or filter pad liners will keep the potting medium from leaking out the sides of the planting baskets.
Fill the pots halfway with potting medium and set your plants in the medium. Spread the roots out evenly over the medium. Add more potting medium, leaving 2 inches of space between the surface and the rim of the basket.
Add a 1/2- to 1-inch layer of builders’ sand or pea gravel to the surface of the potting medium. According to the University of Illinois Extension, pea gravel “will help prevent soil particles from clouding the water and will discourage fish from digging into the soil.” Water the containers thoroughly before setting them in the pond.
Fertilize lilies and lotus plants. Use compressed fertilizer tablets formulated for pond plants. Using regular garden fertilizers will promote algae blooms and other pond water clarity problems.