Water gardens are a great source of beauty and interest for the owners of small ponds. There are four types of water-garden plants: deepwater plants, submerged plants, marginal plants and floating plants. Unfiltered ponds are likely to have cleaner water by having a mix of floaters, submerged and deepwater aquatic plants. All water plants, except for floaters, should be potted up. Since they are beneath the water, black plastic nursery pots work well. Use a good quality potting soil without fertiliser, peat or perlite, which can encourage algae and be harmful to fish and other pond creatures. There are aquatic fertilisers available, which benefit water plants after they are established. After potting up the plant, spread a layer of gravel or small rocks over the top to keep the soil from spilling out into the water.
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Water lilies and lotuses are deepwater plants that sit in pots on the bottom of the pond. When they are small, use bricks or other supports to raise their heights so that their leaves and flowers float on the surface. As the plant grows, remove some of the bricks to lower the pot in the water, so that only their leaves and flowers float above the surface of the water. There are both hardy and tropical water lilies and lotuses. When purchasing pond plants, use the guidelines written on the plant's tag to determine its hardy zone and needs.
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Submerged plants grow in pots placed at the bottom of the pond, with their foliage remaining underwater. They are some of the most useful water plants. They can add small amounts of oxygen to the water and help keep algae growth down by absorbing carbon dioxide and minerals. They provide good cover for fish to spawn, and some varieties also provide food for fish. They do not require aquatic fertiliser because they get their nutrients from minerals that have dissolved in the pond. Often called oxygenators, examples of submerged plants include fanwort, Canadian pondweed and wild celery. Mix the varieties of submerged plants for the best results.
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Floaters are the easiest of all water plant types to grow. They float above the water with their roots dangling beneath them, so they do not require pots. Some floaters act as water filters, and some provide food for fish and other water creatures. They also work with the deepwater plants to provide shade for the pond. For the health of the pond, about one-half of its surface should be covered with plant material to shade the surface. But, be aware that if two-thirds or more of the surface is covered, carbon dioxide and other gases can be trapped in the water, which may be harmful to plants, fish and other water inhabitants. If this happens, simply pull out some of the floaters. Be careful in selecting floating plants; some are invasive and are prohibited in some (but not all) states. These include water lettuce, water hyacinth and duckweed. There are a number of other varies of floating plants from which to choose.
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When a pond is dug, shelves are often formed into its sides. These shelves are for the primary purpose of supporting shallow-water marginal plants; many are also called bog plants. These plants are valuable for their decorative accents around the edge of the pond. They are grown in containers that sit beneath the water's surface on the shelves and can be easily lifted out to groom or divide. Except for the deepwater lilies and lotuses, these marginal plants are usually the most colourful. Some examples include iris, marsh marigold, arrowhead and cattail.