Repotting is an essential component in the ongoing process of tree miniaturization. By strategically trimming the foliage and pruning the roots, we can continue the art of illusion that is bonsai. When we repot we want to make sure we get our timing, percentage of roots, and methods correct so the tree doesn’t skip a beat in its growth and development and is on a better path than before you repotted it.
TO REPOT, OR NOT TO REPOT...?
Repotting can be performed for the health of the tree, or because you want to upgrade the pot and give your tree a ‘new pair of shoes.’
Regular repotting based on pot size, growing conditions, and fertilization, is usually needed every 2-3 years. Shorter durations between repotting can be experienced due to smaller pot sizes, fervently growing species, and heavy fertilization.
For most species, a bonsai wants room to grow a dense root ball (a ball or hockey puck shaped collection of roots) of fine roots that have room to ‘breathe’ and absorb all the nutrients and water necessary to allow the plant to thrive.
One instance when we need to immediately repot a tree is when the health of the tree is declining because it has not been repotted for many years and the roots have started to cross and compact to such a level that the roots can no longer efficiently do their job. This level of distress can occur more quickly if the pot was very snug for the tree originally, or as the result of abundant healthy root growth. Conversely, the tree may need to be quickly repotted if the pot it is planted in is too large, the soil it is planted in is retaining too much water, and the roots are starting to rot, smell, or generally become ineffective and unhealthy.
Whether you want to repot to help your bonsai gain vigor, or you simply want to see your tree paired with a new vessel, the repotting process is going to consist of the same components: root selection, trimming, ‘tying’ or ‘wiring’ into the pot, filling with rocky bonsai soil and using a chopstick to make sure all the holes between the roots have been filled in.
You want to enjoy your tree and ideally enhance the features of your tree that you think make it special. When selecting a pot, consider shape, color and size. The rounded or straight edges, design of the feet, glazing or lack of glazing and overall design of your pot will have an effect on the story your tree tells.
It is easy to feel lost when selecting an appropriately sized pot. Some basic recommendations to keep in mind are: the length should be approximately ⅔ the height of the tree and the depth of the pot should be similar to the width of the trunk.
Two types of roots we will encounter when repotting a bonsai are tubular thicker roots and much finer ‘hair-like’ roots. The thicker roots can generally survive outside of the soil (think thicker roots that make up the nebari). They function as transportation tubes for nutrients and water to move into the trunk and out to the branches and foliage.
The finer ‘hair-like’ roots absorb the water and nutrients from the soil mass and should make up a large majority of your roots. When selecting roots, we want to keep enough so the tree can continue to absorb water and nutrients from the soil. We also want to trim enough so that the tree can fit in the bonsai pot, (the same pot it was in, or a new pot). When trimming, be sure to leave just enough room around and underneath the roots for a layer of your bonsai soil.
The reduction of large roots and very large root masses may need to be conducted over time and refined slowly after several repottings.
Once the wires and ties are cut from underneath the pot and the tree is removed; the true assessment of the roots can begin.
Carefully take a chopstick and start to drag the old soil from the top, bottom, and sides of the root mass. As you remove soil, observe the root growth of your tree and frequently check your roots against the shape and size of your pot until it fits.
Trim roots growing in matted spirals at the base and growing upwards. Trim or re-position roots starting to coil around the root ball.
If a thick root has grown particularly long (a benefit for trees in the ground, but not for a bonsai), trim it back to fine roots higher up the thick root, closer to your desired root ball. Once enough roots are removed so the root ball fits the pot; the tree is now ready to complete the next stage of repotting.
Choosing The Potting Orientation
Depending on the size of your tree and pot, you may be able to make some artistic decisions with the angle and orientation as the tree is tied into the pot. Explore what the tree looks like sitting to the left, right, middle, front, and back of the pot. Also, check what the tree looks like when it is angled or straight up and down.
Any of these decisions can be artistic and affect the story the tree tells. If potting the tree at an angle and near the side of the pot, make sure the tree and pot are balanced and will not tip over, especially if the tree has a lot of foliage and weight up top.
The pot should be prepared with mesh to cover the bottom drainage holes and wire or zip ties, which will be used to secure the tree to the pot.
Pour the rocky bonsai soil evenly across the entire bottom of the bonsai pot. Bonsai soil typically consists of inorganic matter like lava rock and pumice along with a small amount of organic material like small shards of pine bark.
Place the tree on top of the soil. If the tree sits at or slightly above the rim of the pot, it is ready for the next step. If the tree sits too low in the pot, or too high; remove or add soil as needed.
Next, twist or tighten the wire or zip ties over the roots as close to the trunk as you can without crushing or covering the nebari. Tighten all the way to the root mass, as tightly as possible, without breaking or tearing them. The tree is successfully potted when the tree sits in the pot sturdily without any assistance.
FILL WITH BONSAI SOIL
Bonsai soil can consist of many different particles depending on the tree species being potted. Lava rock has nooks and crannies that store water and nutrients for roots to access. Pumice aerates the soil. Pine bark provides small amounts of healthy organic nutrients and additional water retention.
Start to fill the pot with bonsai soil around the trunk like a moat around a castle. Move the chopstick up and down along the inside edge of the pot only to funnel the soil in between, around and under the roots.
It may help to gently press the soil down with a finger as the chopstick up wiggles up and down. This step is completed when no more soil can funnel in and the soil is at or just below the rim of the pot. When you tree is good and solid, the ‘tails’ of the zip ties may be trimmed to the soil.
After repotting, water the tree immediately after or the next day. If many roots were removed, some leaf drop may occur. If leaf drop happens, water the plant each time the soil dries out. Check frequently.
Check in with your tree to make sure it is responding well and feel free to reach out to us with questions.