Like their landlubber counterparts, garden ponds require maintenance and care including proper fertilization. If you decide to buy fertilizer for water plants, be sure it is made for your particular pond environment. It should be formulated for water gardens--standard garden fertilizers can harm fish and other pond life and cause severe algae blooms.
Water plant fertilizers come in granular, tablet and liquid form. According to the University of Illinois Extension, slow release tablets or granular fertilizers with an analysis of 20-10-5, 5-10-5, 10-6-4, or 12-8-8 are all acceptable. Tablets are recommended because they are easy to push into the soil to fertilize individual plants. Granular formulas can be wrapped in a coffee filter or paper towel and pushed into the soil.
Clear pond water is visually appealing and it is important to plant health. Cloudy, algae-filled water is depleted of oxygen and light cannot penetrate to submerged plants. Avoid over fertilizing to keep water clear by testing your pond water regularly. Test kits are available at nurseries and garden centers that will test for pH (acid/alkaline balance) as well as nitrite and ammonia levels. The pH levels should be neutral (around 7) and high nitrite levels indicate too much fertilizer. High ammonia levels are dangerous if you have fish; products are available in pet stores and pond supply stores to lower these levels.
Water lilies and lotus should be fertilized at the time of planting and once a month until mid-August. For each cubic foot of soil, use 4 oz. of fertilizer. Emergent plants--those that grow along the perimeter of the pond--should be fertilized in spring and once again at mid-summer at a rate of 2 oz. per cubic foot of soil. Liquid fertilizers are available that add beneficial pond bacteria to the water and stimulate leaf and root growth. Water lilies and lotus plants are heavy feeders and may need additional concentrated feeding in addition to this general feeding.
If your pond plants appear stunted or are flowering poorly, they could need fertilizer. Yellow leaves are also a sign of nutrient deficiency. Be aware, however, that koi can damage water lilies and lotus plants. These valuable ornamentals should be caged to protect them from hungry fish. Plant lilies and lotus on plastic pots to protect the roots and use a plastic mesh fence to protect the stems and leaves.
End of Season Pond Care
Do not fertilize pond plants after mid-August. This could cause late season growth and harm the plant if temperatures drop suddenly. If your pond is shallow or you are growing tropical plants in a northern location, you will need to remove the plants from the pond. Wrap the pots in plastic and store them in a cool (50 degrees F) location. Keep them moist but do not fertilize them during this time.