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Marginal plants are plants which grow naturally or thrive in wetlands, bogs or shallow water. The term encompasses a wide range of plants, including water iris, cattails, water lilies and pitcher plants. They provide both shelter and food for aquatic animals, and even cleanse the surrounding water, making them invaluable to the ecosystem. There are two basic subsets of marginal plants - bog plants and emergent plants.
Bog plants are species that have adapted to growing in decomposed peat moss, as opposed to the waterlogged soil in which most marginal plants grow. The peat is both highly acidic and low in nutrients, which has resulted in many unique species. Most carnivorous plants are bog plants, and ingest insects to make up for the lack of nutrients in the soil. Bog plants also require a large amount of sunlight to increase the rate of photosynthesis, and produce more food.
Most marginal plants are in the emergent subset. These plants are rooted under the surface of the water, and do not flower or produce foliage until they emerge. Emergents grow this way to increase the amount of sunlight absorbed, and for reproduction. The stem of emergent plants is usually firm to keep the top of the plant from drifting. Common species of emergents include willows, reeds, lilies and cattails.
Marginal plants are commonly grown in water gardens, as they are attractive and easy to maintain. They can be grown directly from the soil or in sunken pots that rest below the water. Most marginal plants are extremely invasive, and will only behave if planted in a pot. This results in competition between different species, so providing plenty of space between plants is recommended.
All marginal plants share the need for consistently moist soil. However, marginals do not thrive in soil that is easily compacted. Constant moisture will often break down most potting soil, making it unsuitable for growing marginals. A commonly used soil combination is two parts peat moss, two parts washed sand, one part perlite, and one part bark chips. This will result in a soil that is high in nutrients, which will retain its basic composition when submerged.
Fertilizer is another common need of marginals. Soil in fens, wetlands, and around bodies of water is typically rich with nutrients from decomposing leaves and organic matter. Adding an aquatic plant fertilizer tablet to the soil mixture, following the manufacturer's directions, will substantially increase the health and growth of marginal plants.
There are often a lot of water loving marginal pond plants in public greenhouses and conservatories. I know the one near my house has a lot of different kinds that you wouldn't be able to find in the wild, simply because it is too cold.
They have massive lily pads, at least three feet across, as well as lotus flowers and cattails (which are found in the wild around here, actually). They also have pitcher plants, which are my favorite.
They usually hang them from the ceiling but if you stand on tip toe you can see into the bottom of the plant where the fly is captured.
It's pretty interesting and a good place to hang out in cold weather.
Wellington, the capital city of New Zealand, has a wonderful system that uses marginal plants to clean the runoff from city streets before it reaches the ocean.
It's really pretty as well. They have a series of terraces with short walkways through them, filled with marginal water plants like reeds and flax, and these help to purify any water that flows from the concrete after it rains.
I think it is the kind of approach that most cities should take. The Wellington harbor is clean enough that kids can swim in it, right in the middle of the city, which is not something that can be said of many places.