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A cushion plant is a plant which has adapted to a harsh environment by growing slowly, producing a dense, matted pile of plant material which grows close to the ground, insulating the plant from hostile conditions. Cushion plants can be found all over the world, and they are represented in a wide variety of plant families. Biologists often use the cushion plant to illustrate convergent evolution, showing how entirely different plants grow in much the same way because they have discovered that this particular way is the most efficient.
As you might gather from the name, a cushion plant looks rather like a cushion, although if you press down on it, you will discover that it is not very soft; some biologists jokingly refer to cushion plants as “bolsters” in reference to this. Cushion plants grow very slowly, and also extremely evenly, putting out rosettes of leaves all at once so that no one part of the plant is more exposed than others. When they flower, the flowers are small and often closely nestled in the leaves for protection.
Cushion plants are most commonly found in the arctic and alpine regions of the world, where the weather is cold, dry, and often windy. Normal plants could not thrive in these conditions, because they would be broken by heavy winds, frost-nipped, or unable to access water and nutrients. A cushion plant, on the other hand, can thrive because it insulates itself, and it has time to develop very deep, thick taproots which can penetrate the ground in search of water and nutrients.
The dense foliage of a cushion plant acts as a layer of insulation, protecting the roots and stems from inclement weather. The plants are typically further insulated with layers of dead plant material, and they sometimes host insects and small mammals which burrow into the plant for protection. Cushion plants also help their environments, reducing erosion and weathering while sequestering water.
These matted plants are very hardy, as you might imagine, which makes them a popular choice with some gardeners. However, gardeners need to think about the fact that cushion plants grow very slowly, and they will not be hurried. A cushion plant can make a great groundcover, when a lot is planted and the gardener has patience, but these plants are not candidates for rapid landscaping.