cushion plants

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Rock Gardens: An Introduction

Recently I helped build and plant a rock garden. It was a first for me, but something I had been wanting to do for a while. Rock gardens consist of plants that grow in rocky environments, such as rock outcrops on mountains or accumulations of rocks at the bases of cliffs or steep slopes. Rock garden plants are commonly called alpine plants – alpine refers to an environment that is very high in elevation or, in other words, in mountains above the tree line. Not all rock garden plants are native to alpine environments; however, in the rock garden community, the term “alpine” often refers to small, hardy plants that are ideal for rock gardens.

A rock garden mimics the environments of alpine plants by incorporating a mixture of large and small rocks placed in an aesthetically pleasing manner. Well-draining soil is brought in to fill the spaces between the rocks, and the plants are planted in these spaces. Rock garden plants are typically small and compact. Cushion plants (Silene acaulis, Saxifraga spp., etc.) are one example of a type of rock garden plant. Other popular rock garden plants include the following genera: Pulsatilla, Viola, Sedum, Daphne, Delosperma, Dianthus, Thymus, Primula, and Scutellaria. The list goes on. Many rock garden plants can be found at local garden centers, while others will require some searching, but there should be enough of them available to at least get you started.

A rock garden doesn’t have to mean a scattering of rocks laid out on the ground. They can also be built in raised beds or they can consist of a series of troughs or planters. Rock garden troughs are typically made of tufa or hypertufa. Tufa is a naturally occurring variety of limestone. Hypertufa is a human-made version of tufa that is composed of various aggregates cemented together.

To learn more about rock gardening and to join a community of rock gardeners, check out the North American Rock Garden Society, and stay tuned to Awkward Botany for future posts on rock gardens.

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Here is an example of a rock garden in a hypertufa trough. You can see this and more like it at Idaho Botanical Garden.

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Cushion Plants and Species Richness

Cushion plants are in the news. A study published in the journal, Ecology Letters, has demonstrated that cushion plants can help increase species richness (the number of unique species in an ecological community) by modifying their micro-environment, which in turn allows certain species to exist in the community that would otherwise be unable to survive the harsh conditions. Other studies have had similar conclusions, but what is unique about this study is how extensive it was, involving 77 alpine plant communities on 5 continents.

The term “cushion plant” refers to a specific growth form. It describes a plant that grows low to the ground, has numerous small leaves and a closed, tightly-packed canopy with dense non-photosynthetic living and dead plant tissues below the canopy. Above ground it appears as a lush, thick, spreading, green mat; below ground it has a long taproot and an extensive root system. There are around 338 species of cushion plants, spanning 78 genera and 34 plant families, which can be found around the world mainly in alpine (high-altitude, tree-less) environments. Around half of the cushion plant species are native to the Andes in South America.

So, how are cushion plants able to increase species richness in their communities? There are a few unique characteristics of cushion plants that lead to this result:

– The tightly-packed, low to the ground growth form of cushion plants helps to modify the temperature of the underlying soil, working as a living mulch to keep the ground warmer in the winter and cooler in the summer. Plants that otherwise could not abide in extremely cold soil conditions, can thrive inside of a cushion plant due to this modification.

– The shading and covering of the ground also helps to maintain a higher level of soil moisture below cushion plants, resulting in more available water throughout the growing season, which is especially important during warm months of the year when water becomes scarce elsewhere.

– Cushion plants may also increase nutrient availability in the surrounding soil. This could be due to their long taproots and extensive root systems allowing them to “mine” the soil and pull up nutrients (and water) that would otherwise be unavailable to shallow-rooted plants. It could also be due to the high degree of dead plant material found within cushion plants that leads to an increase in the amount of organic material in the soil below. The warm, moist conditions of a cushion plant’s underbelly could speed up the rate of decomposition and nutrient cycling, making essential nutrients available to plants growing within them.

Because of these features, cushion plants act as “nurse plants” to species that grow within their mats, providing them with more accommodating soil temperatures, greater access to water, and a higher level of nutrients compared to the surrounding open ground. Some of these plant species would have little or no chance of survival in the harsh environment outside of the cushion plant. Cushion plants are also considered foundation species or keystone species because they play such a strong role in structuring their ecological community, affecting the diversity of species found in the landscape and the abundances of those species.

Silene acualis

A common and popular cushion plant: Silene acaulis. Common name: moss campion. Plant family: Caryophyllaceae. Occurs in high mountains of North America and Eurasia. Photo credit: wikimedia commons.

cushion plant as nurse plant

An example of a cushion plant with another plant species growing within it. Photo credit: wikimedia commons.