How to Refresh Your Bonsai's Topsoil
Tools You’ll Need
Bonsai scissors or root-cutters
An appropriate quantity of fresh, species-specific bonsai soil
When should I refresh my bonsai's topsoil?
Anytime you notice a change in downward water flow into the soil and through the pot. Water might appear to stand or puddle on the surface for too long or doesn’t seem to flow out the bottom of the pot as quickly or forcefully anymore.
As long as you don’t greatly disturb or damage the roots, this topsoil refresh can be done any time of year. For me, I often do this in Autumn at the conclusion of the foliar growth period, and again in late winter or early spring, if I’m not doing a repot on that tree that year. Any season is fine if the bonsai fits the above criteria for changes in drainage.
Why should I care about the way water drains from the bonsai pot?
Poor drainage can result from a variety of factors, including over-watering, too much heavy rain, a bonsai being root-bound within the pot, and the natural breakdown of soil particles.
When water takes too long to flow through the root system and out the bottom of the pot, it can stagnate and damage the roots.
Root damage and rot will occur if left alone for too long. Root-rot can be trimmed away with sharp and clean bonsai shears, and this will promote new root growth and allow the tree to recover.
More dangerous than simple root-rot is a family of fungal infections called Phomopsis. Usually, only a lab test can determine the actual presence of Phomopsis, and once it takes hold of a tree’s vascular system, it is often fatal.
Unfortunately, the signs of root-rot and Phomopsis are nearly identical, and both share their origin in a too-damp environment. The spores of Phomopsis are found in rainwater and other natural sources. The spores grow when water stagnates.
The best way to protect your bonsai is always a preventative maintenance and care routine. Staying on top of any noticeable changes, and anticipating problems before they occur will keep your garden healthy. In this case, root problems can be avoided and possibly corrected by achieving adequate drainage of water from the bonsai vessel.
Replacing the upper layer of the soil gives you an opportunity to see the conditions of the soil underneath, and check for root rot and other signs of improper drainage, such as soil compaction and soil particle breakdown. Cosmetically, fresh topsoil can also improve the image of the tree for viewing.
How do I remove the old bonsai soil?
Using a simple and clean wooden chopstick, or clean pair of bonsai tweezers, gently and softly scrape away the top layer of soil. Start at the trunk (careful not to scratch the bark) and root base, and scrape away from the trunk directly toward the edge of the pot in a straight line. Be careful of larger roots, and sort of comb the smaller roots outward while scraping away the topsoil.
Proceed around the tree in a clockwise or counterclockwise direction, with each stroke reaching the edge of the pot before you start again at the base of the trunk. Always move slowly and lightly. It’s better to go over the area a couple of times than to damage any roots by digging too deeply or forcefully. You are “training” the surface roots to grow outward like the spokes of a wheel, while also removing any broken-down soil particles from the surface.
After the topsoil is gone, you should be able to see all the surface roots. Now you can check along the edge of the pot to see if the soil is too compacted. Choose a place to excavate some soil from between the rootball and the pot. Go down a bit, toward the bottom of the pot. If the soil particles are intact, or mostly intact, and not breaking down and clogging the water flow, you can simply pour some fresh bonsai soil back onto the surface roots, and maybe apply some top-dressing and moss spores to complete your bonsai image. (See the Bonsai Tip below) Carefully work the particles into any crevices with a clean chopstick, filling in the excavated areas, and gently pack the bonsai soil in by poking the chopstick downward. This will get rid of any air gaps left behind.
If you have noticed a substantial reduction in water flow from your bonsai pot over time, it’s likely you will see broken-down and compacted soil along the sides of the pot. In this case, you will also have to remove and replace this compacted soil to improve water flow and increase or regain the health of your bonsai.
Remove the old bonsai soil from between the rootball and wall of the pot as best you can, while not breaking the roots. Look for signs of root rot, and, using clean and sharp bonsai shears or scissors, cut away any expired or rotted roots to a point where there is no more rot. Healthy roots appear light in color and have fresh growth on the tips. Root rot causes the tip of the root to turn black. It then advances along the root toward the base and turns the whole root slimy and black. If it easily breaks, and the inside of the root is slimy and separates readily from the cambium sheath, it is rotted. If the whole root is rotted, the whole root comes off. When the dead or dying roots are properly cut away, new root growth can commence. Once you feel as though you have addressed any dead roots, and it seems like proper drainage has been restored, you can sprinkle some rooting hormone powder around the edges of the rootball. You can also mix some root powder with a few drops of water to make a paste, and spread this paste on the roots with a paintbrush.
Using fresh bonsai soil and a clean chopstick, work the soil into any spaces along the rootball between the tree and the pot. Gently push the chopstick into the bonsai soil, and pour more soil as needed, and continue to carefully work it into the pot around the tree, and among the roots. Sprinkle some bonsai soil onto the surface roots, and spread the particles among them.
Moss growing on the surface of the soil can help complete the image of your bonsai. It also helps retain moisture in hot weather. A Bonsai Top-Dressing, made with shredded sphagnum moss dyed with black ink and sufficiently dried out, then mixed with ground-up green moss from your yard, will add a new level of realism to your bonsai planting when sprinkled across the fresh topsoil. The shredded green moss will immediately look beautiful as it dots the soil surface among the dyed sphagnum top-dressing, and the green moss will take hold and grow naturally within a month or two. Different types of moss can be used together to present a variety of textures and colors among the surface roots and around the tree. The results with this easy method will be very pleasing to the viewer and will look much more realistic and natural than some randomly-placed clumps of moss.
The final step when replacing bonsai topsoil is to “water-in” this new soil. Water the tree until the drainage water runs completely clear. You should immediately notice improved drainage. Again, water until it runs clear.
If you had significant root-rot, and are worried about more rot, it can be good to allow the roots to dry out for a while before watering in the new soil. This slight period of drying can help the tree recover faster and spur the development of new roots. This makes sense if you consider the fact that the problem with the roots themselves was likely caused by water stagnating around the old roots. That being said, I still like to thoroughly water in the new soil anyway, and then allow it to dry for 1-3 days depending on the weather. To alleviate any confusion: When in doubt, water the tree!
If your bonsai was healthy, and you didn’t see any signs of root problems, you can simply put it back on the bonsai bench. Just keep a close eye on it, and monitor the water drainage for a while. As always, clean your tools and workspace after any bonsai work
If your tree needed some root-work because of rot, treat it as if it was just repotted, and keep it out of the direct sun for two weeks or so. If the roots were severely damaged, water it a bit less while it recovers. More oxygen is needed to jumpstart the root growth. You can also tilt the pot after your regular waterings to make sure all the excess water has flowed out. If the problem was only root-rot and was caught early enough, the tree should recover in time. If you notice that your tree continues to deteriorate, and begins to show signs of serious distress, or dies, it is quite possible that Phomopsis was present in the roots. Cleaning your tools and workspace thoroughly is always important, but it’s critical if you suspect a fungal or bacterial infection.
Thanks for reading. I hope you can use these methods and tips to promote thorough drainage, healthy root growth, and achieve a more pleasing and natural-looking topsoil around the base of your bonsai.