Growing trees in the ground or larger pots is a way to increase the trunk size quickly but as the trunk is growing, so are the roots underneath. At some stage the roots will need to fit into a bonsai pot so how can you tame those wayward roots?
Nebari is the term for the junction of trunk and roots, including the visible roots that radiate out away from the trunk before they disappear into the soil. A strong nebari is highly valued, particularly in maples. Good nebari is more than just a few strong roots sticking out from the trunk. Like the branches above, roots should radiate out from the trunk and divide and ramify before they disappear into the soil so that the bonsai looks old and stable. In order to produce such well ramified surface roots you need to root prune regularly and fearlessly during the development phase.
Through progressive trials I have found that trees actually need very few roots to survive transplant as shown in these photos of field grown trident maples as they are dug from the Shibui Bonsai grow beds.
trident after root pruning.
Cut vertical roots off completely.
I have also noted that, after root pruning, the vast majority of new roots form from the cut ends of the roots.
Roots grow from the end of previous pruning sites.
That means that there is very little point leaving roots long during the initial transplant. At some stage in the future they will need to be cut short so you can fit your bonsai into a suitable bonsai pot. This radical root shortening is better done earlier than later.
Cut roots short so they will ramify even more.
Most other deciduous species that I grow as bonsai are treated in a similar manner.
Chinese quince as dug
Chinese quince after initial pruning.
Japanese maple after initial pruning.
The Japanese maple shown above had a trunk diameter of 12 cm and recovered well after being dug in 2011.
Not all species can tolerate such radical reduction of roots. I generally leave more roots on evergreen species such as pines, junipers and most Australian natives. Ficus rubiginosa is one native species that can tolerate extreme root reduction provided it is done during the warmer months.
I have a set of tree and root questions. I have American Elm trees growing in the ground. The trees have been reduced significantly in height and pruned to basically bare trunks.
1. Is this a good time to also dig them up and cut the roots back significantly?
2. If yes to digging and root pruning….How do I know when to dig? Do I wait for buds to develop on the trunks?
3. Do I wait and dig them up next year when it is not a top pruning year?
Hi again John,
I think you are in the USA John so we need to speak in terms of seasons rather than months. I dig deciduous trees from mid winter through to when they start to grow the first leaves in spring. Timing does not seem to be important as long as the trees are dormant. I am not sure how advanced your spring is at this time but if the trees are not growing yet it should be a great time to dig them and prune the roots. If they have already started to grow new leaves I would leave them until next winter.
I prune the top and cut the roots in the same season. When we dig the field grown trees I prune the roots and also cut the top back very hard at the same time before replanting in the ground or transferring to pots ready for further development.
Hope that helps,
Neil