Mosses have been an intentional part of landscape design in the form of moss carpets and gardens for nearly one thousand years, since Japan's feudal era began in the 1100's. Japan is famous for "moss gardens", and arguably the most famous moss garden of all is at the Saiho-ji Temple in Kyoto, Japan. It was created in the early 14th century by a Zen Buddhist monk and esteemed garden designer Muso Soseki. Today tourists from around the world plan months in advance to visit Saiho-ji (by reservation only). The experience of witnessing vibrant green moss carpets is certainly soothing; in fact, the guided tour of Saiho-ji includes a meditation service!

Bonsai enthusiasts share the monks' appreciation for mosses, and moss is incorporated into many bonsai compositions. A layer of thick green moss surrounding a tree in a bonsai container ignites the imagination, adding depth and texture. A bonsai tree nestled upon a moss bed can complete the look of a miniature landscape, called a Saikei in Japanese. Live moss is a wonderful addition to a bonsai tree as its prepared for showing, but it's use is far more than just aesthetic. Moss greatly benefits bonsai trees. It holds in heat during the winter, and helps keep the soil cool during summer, which helps protect the trees' roots throughout the year. It also prevents erosion or soil loss from watering when it grows on the surface of the soil, as it will adhere to the top layer of bonsai soil and keep it where it belongs during watering.

What is Moss and How do Mosses Grow?

Moss (division Bryophyta) is a rootless plant that grows in a mat of fine threads. Mosses are categorized as bryophytes, or non-vascular plants. They don't have roots, they have rhizoids; wispy root-like extensions of the stem. Since mosses don't have roots or a vascular system, they transport water and soil nutrients osmotically, from cell to cell. Some mosses prefer hanging out in damp, shady areas; others do well in partial shade or even full sun.

Moss spreads in different ways, including dispersal of moss spores through wind and insects; fragmentation; and shoot growth. Moss doesn't have flowers. Mosses reproduce by spores that travel on air or water currents. With fragmentation, a tiny part of moss develops into a larger plant and spreads over time to cover a large area. Moss can also spread by sending out shoots; moss stems can multiply and become a colony.

Types of Moss

There are both different species of moss and different types of moss; wherever you live, the mosses and plants you see growing naturally (e.g. natives) are the best use to use with your bonsai and in your garden. Here are a few common mosses:

• Sphagnum moss: There are more than 350 species of sphagnum mosses. It's long-stranded, and a thick layer of sphagnum moss is often used between the bonsai soil and the top dressing moss. Peat moss is actually from the same plant except it's dead and decayed, and usually dried and sterilized after harvest.
• Silver moss*: Bryum argenteum is a type of carpet moss so expect more horizontal growth than vertical. 
• Sheet moss*: Hypnum cupressiforme is typically seen on the ground in forests; if you're an avid hiker, you've definitely crossed paths with sheet moss!
• Common Fern moss: Thuidium delicatulum produces tall leaves up to 3.5 inches in height. Don't use if for bonsai unless you have a large bonsai (30 inches or taller) because it could cover over a third of a smaller tree trunk and make the composition look overgrown. 
• Atrichum Moss*: Two popular options are Big Star moss (Atrichum Undulatum) is taller, growing up to 3 inches; and the miniature Starburst moss (Atrichum Angustatum), that grows only to an inch in height. Both of these options are best for trees that prefer partial shade.

*these are the best options for creating a moss layer on the soil surface of a tree in a bonsai pot.

Where to Find Suitable Moss for Bonsai

Fortunately, it's not hard to find fresh moss at the appropriate times of the year, whether in urban areas or outside city limits. Fresh moss grows all around after a rain; in sidewalk cracks or along freeway or highway underpasses (be careful when collecting moss near moving vehicles!) Jonas Dupuich of Bonsai Tonight takes note of all the patches of attractive moss he finds growing on public property. In January (ideal collecting time in Northern California), Jonas knows exactly where to go moss collecting! Moss can also readily be found in a nearby forest or park.  Note that Irish and Scotch moss, sold as groundcover commonly, are not true mosses, they are vascular plants that will compete with your tree for nutrients. They are difficult to eradicate once they become established. (pictured below is Irish Moss) 

How to Collect Moss and Transplant Moss for Bonsai

The best time to collect moss is when there is the most rainfall. That could be spring or fall, depending on locale; in Northern California, January is often a great time to collect moss. Collect the moss by using a putty-knife, spatula, paring knife, or a similar flat, wide-bladed knife and try to collect relatively large patches. Keep the moss moist and transplant it quickly, laying a patch of moss in the location you would like for it to grow; ideally on top of a thin layer of nutrient-rich, water-retaining bonsai soil. To cover a large area you, use a plug method such as is used with grass. Place small pieces of moss at regular intervals over the area, and the moss will eventually grow together. After planting new moss, water it thoroughly. Keep the area damp with regular watering for the next year or so to help the moss establish well. Once established, transplanted moss should be watered at the same interval as the tree it's growing alongside. Watch this video "How to Put Moss on Your Bonsai for Show Preparation," to see this process in action! 

In his book "Moss Gardening," (Timber Press, 1997) author George Schenk introduced an example of moss container gardening tied to bonsai that he called mame mosses. A mame moss is "one small enough to fit comfortably into a mame pot" and can serve as a playful and imaginative accent plant in a bonsai display.

Caring for Moss: Sunlight, Water, Soil

“The forest is blanketed by the greenest ferns and moss and bonsai-like trees, a wild majesty that beckons hobbits and pixies and elves and dreamers.” - author Shannon M. Mullen

References:

"Moss Gardening, Including Lichens, Liverworts and Other Miniatures" by George Schenk, Timber Press, 1997.  ISBN: 9780881923704

https://www.mosskillers.co.uk/knowledge-hub/how-to-guides/how-does-moss-spread-and-reproduce

https://www.gardeningknowhow.com/ornamental/foliage/moss/growing-moss-gardens.htm

https://www.bonsaioutlet.com/bonsai-articles/few-things-accompany-a-bonsai-better-than-moss

https://www.bonsaiempire.com/blog/bonsai-moss

https://www.lovemybonsai.com/blogs/bonsai/how-important-is-moss-for-your-bonsai

https://bonsaitonight.com/2010/02/16/moss-technique/

https://plantophiles.com/plant-care/the-best-moss-for-bonsai/

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/sphagnum-moss-vs-sphagnum-peat-moss.htm