Assistant Curator’s Blog: Collecting Moss and Creating Top Dressing

As bonsai practitioners, we are always thinking about the past, present, and future aspects of our trees and the environments in which they live and grow. We constantly plan and prepare for the next step in the wonderful journey of bonsai and penjing. One preparation that can be completed any time of the year and is extremely important for the repotting process is collecting moss and creating top dressing. 

Bonsai and penjing are commonly grown in small containers with inorganic substrate, or the layer of matter that helps trees grow and obtain nourishment. The substrate National Bonsai & Penjing Museum staff uses consists of the Japanese clay substrate called akadama, pumice, and lava rock, although substrates like tree bark and perlite could be used.

This well-drained substrate retains water to provide to the tree while allowing oxygen to flow through the soil, keeping the containerized tree healthy. To ensure the health of bonsai and penjing, a practitioner must constantly check and carefully manage watering to maintain the delicate balance of oxygen and water.

Moss is cultivated on the substrate surface by applying a layer of top dressing. This dressing is composed of local mosses typically collected from places like parking lots, driveways, or a nearby forest. The mosses may be mixed with high-quality sphagnum moss for larger tree collections. The live and sphagnum mosses are then ground to a smaller size, and soil and fine particles are removed.

All you need to collect moss is a container, a scraping tool and scissors, shown here. 

The ideal moss is tight and low growing. Species that spread and climb will create issues when the moss grows up the trunks. This is especially problematic on rough bark trees, which will need to be sprayed carefully with vinegar to kill and remove the moss.

Once collected, place the moss in trays to dry, allowing for the soil and other fines to easily be sifted when grinding.

After the moss has dried, use bonsai substrate-sifting screens that are one-quarter, one-eighth, and one-sixteenth size. Take the moss and rub against the screens, starting with the one-quarter size. It may not be necessary to grind past the one-eighth size screen, depending on the size of the bonsai substrate, and the one-eighth screen can be used to remove the smaller particles.

The top left image shows the sphagnum moss before and after grinding. The top right image shows the locally collected moss before and after grinding. The bottom image shows both the ground sphagnum moss and the locally collected moss separately, as well as after mixing both together with about a one-to-one ratio.  

Spread a thin layer of topdressing to cover the fresh substrate after repotting is complete. 

Applying top dressing to the surface of bonsai and penjing at the end of the repotting process provides multiple benefits. The moss establishes the aesthetic of the composition, increasing its value. The moss layer also holds moisture in the top portion of the media, which would dry out much faster than the lower portion if left uncovered. The moss layer also helps prevent breakdown and erosion of the substrate in the container. 

Once you collect moss, the empty trays can be filled with soil and a layer of top dressing. This allows you to propagate specific moss varieties and avoid having to collect moss each year from other sources.

As important as moss and top dressing is for bonsai and penjing, you also want to remember to follow and stay within specific guidelines. Although it may seem and sound harmless, it is not always legal to collect moss in every area you find, so be sure to always ask permission and obey trespassing signs if you are collecting!

Sincerely,

Andrew Bello
Assistant Curator

The National Bonsai and Penjing Museum
U.S. National Arboretum

ASSISTANT CURATOR'S BLOG: Fall at the National Bonsai & Penjing Museum

Figure 1Trident Maple (Acer buergerianum) donated by Ted C. Guyger in 1990, in training since 1975.

Practicing the art of bonsai and penjing is rewarding in many ways, and each season provides a multitude of beauty, work, and lessons. As fall continues, all the hard work we bonsai practitioners have put in throughout the growing season on our deciduous trees is honored by an array of yellows, oranges, and reds.

Now in mid-to-late fall, we have a moment to breathe the cool autumn air and enjoy the vibrant colors. But like many things in life, this moment of visual pleasure and relaxation feels all too short lived. The pigments slowly begin to fade, and leaves tumble to our display areas, signaling that the next phase of seasonal work has arrived. 

Leaves dropping from our deciduous trees conveys that the stored resources in the leaves have been reabsorbed into the twigs, branches, trunks, and roots. This reserved energy will power our trees when they awaken to grow again in the spring.

Now we can begin tertiary structural pruning of many of our deciduous trees, like the much-beloved trident maples and other non-flowering species. This pruning process includes spotting twigs that have thickened in the outer canopy, detracting from the delicate, finely ramified structure we strive for. We also look for areas where more than two twigs are growing from one location, known as a node. By reducing these areas to two twigs, we can prevent unwanted swelling and promote a smooth transition of taper from the trunk to the tip of the twig. 

Ginkgo biloba – donated by Kiku Shinkai in 1976, in training since 1926

Lastly, we look at areas where twigs have elongated too far and prune them back to scale with the design of the tree. This work may be completed after the leaves are done changing color until late winter but should finish before the buds begin to elongate and open. If we prune during the dormancy period, we reduce the chance that new growth will be stimulated due to the removal of growth inhibiting hormones. We prune judiciously to perpetuate health and the execution of the design we have been working so hard to create.

While pines, spruces, and junipers don’t lose all their foliage, these conifers also need some attention before the onset of winter dormancy. Pines and spruces can now be cleaned of old needles, and their designs can be refined with wiring. Weaker foliage on junipers can be removed, and adventitious growth can be eliminated from the crotches of branches.

Cleaning out old needles and growth allows more light to enter the canopy and stimulate interior buds, helping them more strongly develop. Cleaning trees this time of year can be tedious, especially on a large collection, but I think this intimate operation brings us closer to our trees and gives us an even better understanding of their health and growth habits. 

This is also an important time to take advantage of the last warm months to ensure all winter preparation and some spring preparation is completed, and those techniques vary greatly. The simplest is arranging the area where trees will be placed on the ground out of the cold drying winds. Raised beds can be constructed to heel trees into mulch to protect the delicate root systems. Cold frames can be built and prepared to provide protection from wind and frost damage.

Cleaning buildings and testing heating and ventilation systems in greenhouses should be done to ensure all mechanisms are functioning properly. No matter the method of protection, be on top of this task before the temperatures dip below 40° F. If a random cold snap of 28° F or below arrives before everything is prepared for winter, the best thing for your trees is to place them on the ground for the night.  

American Beech (Fagus grandifolia) donated by Fred H. Mies in 2003, in training since 1979

Many other tasks may be completed at this time of year, but the ones shared above are what I feel to be the most important. As the leaves on all of the trees in the landscape and in our bonsai and penjing gardens begin to fall, and winter approaches, we, like the trees, must try to store our energy, shed our stress, and prepare for the challenging cold months ahead.

Sincerely,

Andrew Bello
Assistant Curator

The National Bonsai and Penjing Museum
U.S. National Arboretum