Plants

Marginal plants grow around the edges, or margins, of the pond where the water is shallow. They usually have their soil and their crown (their growing point) underwater, and sometimes their lower foliage as well. They are generally placed on shallow planting shelves within the pond, but if you don’t have shelves their pots can be stood on things such as house bricks to raise them up to the correct height. In order to be considered a true marginal pond plant, the variety must be able to tolerate fully waterlogged soil or water over its crown all year. A plant which will tolerate permanently moist soil but will not tolerate water over its crown or foliage, is considered a marsh plant. There is a huge range of marsh plants available, but a much smaller range of true marginal plants. Unfortunately, many marsh plants are sold as marginal plants, but they will not survive in a pond in the long term.

Marginal plants usually have recommended planting depths - these refer to the depth of water over the crown, or growing point, of the plant (which is about the same thing as the depth of water over the soil level). So a plant with a recommended planting depth of 0 - 4 inches, should be grown anywhere from wet or waterlogged soil (0 inches) up to 4 inches of water over its crown. You can actually grow most marginals in less water than this if you need to, such as in an ordinary flowerbed, provided that their soil is not allowed to ever completely dry out. However, you should never grow them in deeper water than the recommended maximum.

Good examples of marginal plants would be water irises, and marsh marigolds. Most people consider marginal plants essential to make the pond look natural and attractive, and they also provide cover and habitats for all kinds of wildlife. 

Oxygenating plants are plants that have all their foliage under the water. They may live on deeper shelves or on the bottom of the pond, or even float suspended in the water – they will grow at any depth where there is light. They are the plants that are sometimes called 'pond weed'; they usually have fine, delicate foliage. Good examples would be spiked water milfoil, or hornwort. They are generally fast-growing and can take up food through their leaves as well as their roots, which means that they are good at absorbing excess nutrients from the water. This can help 'starve out' algae and blanketweed and keep water from going green. These plants also provide important habitats for aquatic invertebrates, and spawning sites for amphibians and fish. In addition, because all plants give off oxygen, their submerged vegetation will increase the oxygen levels in the pond during daylight hours. This is where the name comes from, although because the oxygen is lost at night this is actually not an important function. They also give a very natural and pleasing look to the pond, with their luxuriant underwater foliage. Unlike marginal plants, oxygenators can't usually be grown outside of the pond in a flowerbed or similar place.

Floating plants, as the name suggests, are any plants that float freely in the water and do not have true roots. Some of these plants are also oxygenators (having fine foliage under the water, such as hornwort), while others are more like waterlilies (such as frogbit) and have all their foliage on the water surface. Floating plants are quick to put in to the pond because they don't need potting, and quick and easy to remove if they ever need thinning out. They also provide shade for the pond and, because they take up some of their nutrients directly from the water, they will compete with algae and blanketweed and help keep these in check. Floating plants, like oxygenating plants, can't be grown outside of the pond.

Deep-water aquatic plants are plants that grow on deep shelves or on the bottom of the pond, but unlike oxygenators, most of their foliage is on, or above, the water surface. This category would include waterlilies (although for convenience, waterlilies are usually listed separately in plant catalogues). Apart from waterlilies, good examples would be flowering rush or floating four-leaf clover. Aside from their attractive appearance, these plants can also provide shade for the pond (which can help with green water) and cover for fish and pond wildlife. Because they start fairly deep under the water and then emerge up into the air, they are also the best plants for dragonfly and damselfly larvae to crawl up. Like oxygenating plants and floating plants, deep-water aquatics also won't grow outside of the pond in, for example, a damp flowerbed or bog garden - they need water over their soil in order to grow.