Chapter 59 Mosses For Bonsai

I admit to being a little infatuated with mosses. On nearly every walk or hike, I find myself admiring patches of verdant green, silvery gray, and golden amber mosses sporting a variety of textures. Wherever they grow, mosses create a sense of age and stability, that venerable quality the Japanese call shibusa.

59.1 What exactly IS moss?

Mosses are one of several kinds of primitive nonflowering plants. They tend to thrive anywhere there is relatively consistent moisture and moderate to cool temperatures for most of the year. Mosses predate the conifers, deciduous trees, and other flowering plants by more than 100 million years.

Mosses can be categorized by their preferred substrate: rock mosses, tree & wood mosses, lawn & open soil mosses, and woodland mosses. They can be subdivided further into shade and sun–loving mosses. Rock mosses can even be divided by the type of rock substrate they prefer (granite versus limestone, etc.)

A patch of moss is a colony of hundreds or thousands of individual plants, each less than 1/4 inch in diameter. Each moss plant consists of a single central stalk with tiny leaves arranged like scales. At the base, root–like hairs called rhizoids or holdfasts anchor it to the substrate. The rhizoids do not take up water or nutrients, because unlike flowering plants, mosses do not have a vascular system. Water and nutrients enter the leaves directly (like trees during foliar feeding), then are transported between the cells.

Anatomy of mosses. Link to original image

Mosses reproduce sexually. Spores are made and carried in capsules on female plants, that are fertilized by sperm from nearby male plants. Unlike pollen, the sperm are not carried by insects, but swim or are splashed from male to female plants in raindrops. Mature spores are extremely small, and can travel great distances on the breeze.

59.2 Misconceptions About Mosses

Which of these bits of common wisdom about moss true, partly true, or false?

Moss cannot be cultivated.
Mosses will die if they are fertilized.
Moss damages trees by feeding on and destroying bare wood.
Mosses in a bonsai pot prevent the tree from getting enough water.
Excess moss in a bonsai pot robs a tree of nutrients.
Mosses secrete acids that erode pots (and damage roof shingles).

Ready? Items #2 and #4 are partly true, and the others are all false.

Mosses are extremely durable plants. In one experiment, 10 year old fragments from dried lawn moss were put in a drop of water under a microscope. Within ten minutes, the leaves greened up and started photosynthesis! Spores remain viable even longer.

While durable, many moss substrates are extremely hard to recreate. Even commercial moss suppliers do not try to cultivate rock or bark mosses, selling mostly collected materials instead. Lawn and soil mosses are easier to grow, but still are very fastidious about their growing conditions. Each species has a particular range of conditions that it will tolerate; if those are not met, it will not grow. Period. This is how moss got its reputation for not being easy to cultivate. It also helps explain why moss dies when exposed to excessive fertilizer; excess salts alter the conditions so they are no longer favorable.

Moss depends on consistently moist conditions, and is specialized to absorb and retain water. Scattered pieces do not impede water flow significantly. However a thick layer of moss that is allowed to dry out will absorb quite a bit of water before allowing any to penetrate into the underlying soil. This is why it is a good practice not to cover more than about half of the soil surface with moss. It also is a good idea not to let the moss get overly dry.

Broken moss coverage in a large bonsai. As long as there are breaks where water can reach the soil directly, moss does not create a significant barrier to water penetration. Link to original image.

Rhizoids do not absorb nutrients from the substrate, so it is impossible for moss to rob nutrients from trees. Rhizoids do produce a small amount of acid, but not enough to damage the substrate significantly. A thick carpet of moss will need more than 25 years to erode an asphalt shingle roof. Air pollutions and natural weathering does much more damage than moss to a tile roof, clay pot, or stone sculpture. Mosses do not produce the enzymes needed to break wood down either. What the moss does is trap moisture, which encourages growth of wood-destroying fungi.

59.3 Cultivating Moss For Accents

Can we encourage mosses to grow when & where we want them? To answer this question I consulted Moss Gardening by George Schenk. He is a retired landscaper who has created moss gardens of various sizes on four continents.

George dispelled the myth that mosses can be propagated by grinding up fresh moss with milk, beer, compost tea, etc. This does not work because most species cannot regenerate from fragments. When moss is ground up and smeared on soil or a rock, any new moss comes from spores on the parent plants. In a head to head test, a stone coated with moss “milkshake” took 2 years to develop a covering of moss; a similar but unpainted stone next to it developed a nice covering of moss at about the same rate, just from spores floating in the air.

The easiest way to get moss in a bonsai pot is to find what you like, push it down on the soil, enjoy it for a while, then remove the moss when it dies. Moss can be collected and placed immediately, or maintained for several weeks after collection on a high-moisture fine soil mix.

59.3.1 Which Mosses to Collect, and How

Skip the rock or wood mosses. They are so finicky that your best bet is to simply add collected pieces of wood or rock that are already colonized to your bonsai arrangement. The odds of success are much better for mosses that grow in lawns, open soil, or woodland areas. A good type to start with are “crack mosses,” those small, compact colonies that form in the cracks of walkways and pavement.

Rock mosses specialized to grow on rocks. Link to original image.

Tree mosses are specialized to grow on bark. Link to original image.

Image 1. Ground or soil mosses like sphagnum can form large deep mats. Image 2. Soil mosses can fill spaces between rocks, but still require soil in which to anchor. Link to original image 1;image 2.

When collecting mosses, pick species that live in the same light conditions as the trees you intend to decorate with them. Use a knife, spatula, or some other flat utensil to pry up as much of the subsurface material as possible. Try to keep the colony in one piece. Place the moss in a single layer on a flat pan or tray lined with a damp layer of paper towels or cardboard. Before leaving the site, take specific notes about the local lighting (full sun, bright shade, deep shade; direction; time.) Keep the tray of moss moist, but not dripping wet, until you are ready to put it into a propagation tray.

Basic Moss Growing Mix

1 qt. fine sand
1 qt. sieved black topsoil or bark fines
1 qt. sieved compost
1/2 Tbsp. finely crushed water absorbent crystals

Unlike bonsai soil, this mix should be very fine. Use the material that passes through a standard window screen. The water absorbent crystals expand 300–fold when wet, then release water slowly. Combine the ingredients, add water, and let the mix hydrate thoroughly. Spread it to a depth of 1–2 inches in a nursery flat that has 1–2 drainage holes.

Most mosses prefer acidic conditions around pH 5.5. To mimic them, water the mix thoroughly with 1/2–strength rhododendron fertilizer; alternatively, dust the surface with 1 tablespoon of aluminum sulfate or other acidifying agent. Use your hand or a fine brush to rake the surface smooth.

Transfer the mosses you collected to the surface of the prepared mix. Press them down firmly, but do not bury them. DO NOT remove the excess material from underneath the moss; it will protect the rhizoids until they extend into the new container.

Now comes the big secret to cultivating moss: place the flat in light conditions that are as close as possible to the original collection site. Try to give the moss the same amount of light, direction of light, and level of light.

Mosses are heavy drinkers, so mist the flat daily, and water it regularly. Do not worry about using tap water; it is usually not a major factor. As the moss begins to grow, feed it a very dilute solution (1/5 or 1/10–strength) of rhododendron food. Avoid anything that contains calcium, or raises soil pH; either one will kill the moss (hence the myth about not using fertilizer.)

To add moss to a potted tree, cut a piece to size and lift the entire mass. Gently brush away loose cultivation mix, then press the moss onto the surface of the soil. To make watering easier, cover less than 50% of the surface with moss. Do not worry about the fine soil washing into the pot; the rhizoids will keep most of it on the surface. Keep the moss happy by misting it regularly and feeding it occasionally with dilute rhododendron food. Don’t fret about the moss competing with the tree for nutrients; remember, the rhizoids do not take up nutrients.

59.4 Commercial Sources

Moss Acres is a specialty supplier of live moss for landscaping projects. They sell a Moss Sampler Set containing 10 square feet of fern moss, and clumps of haircap moss, rock cap moss, and cushion moss (approximately 1 sq. foot of each type). This is enough to dress a couple dozen pots for show.