If a planting is designed according to the site’s trophic level, fertilising is not necessary. However, in artificially sealed ponds, it is recommended to reduce nutrients as efficiently as possible to achieve clear water and reduce Lemna minor carpets and other weeds. On the other hand, in such an enclosed system, species with a high nutrient absorption can suffer from deficiencies, especially in nitrogen. Symptoms show up as small, yellowed leaves, shorter branches and a low and halting growth. Butomus umbellatus even reduces the production of inflorescences. Other nutrient-demanding plants are species of the genera Pontederia, Iris, Hibiscus, Sagittaria, Alisma, Orontium, Trapa, Lysichiton and Calla. To increase nutrient availability, nitrogen should be applied directly to the needy plants without fertilising the water: either hoof and horn chips can be injected into the substrate surrounding the root-system (20-50 g per plant) or ureasolution (1%) is sprayed upon the emerged surface of the plants. Floating plants can only be treated with the spray method because they are not fixed in the substrate with roots. Fertilising should be carried out only when symptoms of deficiency are to be seen. Hoof and horn chips should be used only in spring (May to June). According to the author’s experience, even in bog – and fen-plantings nitrogen deficiencies can appear when they are fed with rainwater and there is no water movement through their substrate. In this case one to two urea treatments in spring are recommended as explained above.
Planted in the indicated water depth, nearly all species mentioned in this chapter are hardy. Protection in very cold climates, such as in Central Europe, particularly when temperatures fall under -15°C for several days, is recommended for:
– Darlingtonia californica and Sarracenia flava (brushwood layer)
– Pontederia lanceolatum (hardy when planted deeper than 50 cm under the water level)
– Nymphaea tetragona (overwinter under frostfree conditions)
– Salvinia natans (but light is also required).
Ponds can be protected from blown or fallen foliage by stretched-out nets. If this is not suitable, thick layers of leaves accumulated above the bottom or the pond can be removed with a small net mounted on a long handle or with a pump.