Root pruning when repotting black pines

I’m a bit at a loss when it comes to repotting a black pine. Especially how much of the roots you can cut back without harming the tree. To my understanding, the general rule is to remove no more that 50% of the rootball and if you need to bare root, only half of the rootball to be safe.

I’ve been checking out some Japanese blogs which show more “agressive” methods of repotting and root pruning similar to this video I found. It looks like he is removing 80-90% of the roots and removing almost all of the old soil. His channel shows more repotting videos applying the same method. How come this is considered “safe” in Japan?

First, as I understand it, you never bare root a black pine. The soil as beneficial mycorrhizae (bacteria) in it and some soil should be preserved. You remove original soil slowly over several repots. Next the strength of black pines are in the roots, if the roots are cut back severely, it will harm or kill the tree. You cut back about 20% of the roots and make sure to leave small feeder roots. I find it takes several reports over multiple years to get all the original soil out of the pot and the roots developed to maximize the health of the tree.

Great question. A number of the Bonsai Shinshi videos are filmed at a friend’s workshop in Japan (Daisaku Nomoto). We talked about the difference in approach to repotting pines and other conifers. Some key takeaways:

  • When trees are well-suited to the local weather, it’s easier for them to recover after repotting.
  • Removing lots of roots and/or soil is possible but not always advisable depending on the care it gets afterwards. If the weather and watering is perfect, it’s much easier for a tree to regain its strength after a growing season or two.
  • Little to no work is done after extensive repottings. This lets trees recover their strength before they are worked on again.
  • The soil mix is also different. In the US, mixes with up to 1/3 akadama are popular where mixes up to 60-80% akadama are popular in Japan.

I occasionally repot conifers as they do in this video but I don’t typically recommend it as it requires that the trees are healthy to start with and receive good care - and rest - afterwards.

I also repot trees out of season. We can repot most trees most days of the year, but the extent of the rootwork will determine how well the tree recovers afterwards. I’ve had good results with repotting pines at decandling time, but I was sure to water lots in the following months - the time of year when Japan experiences heavy rain and high humidity.

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Haha, what a coincidence. Nice having a friend from Japan with such knowledge. Thanks for clarifying!

You should never bare root any pine. I tend to be very careful about how much to remove, usually no more that 25% if I can, depending on species.

Unless the trees are young - it’s no problem to bare-root 1-2 year old pines and sometimes even pines that are a few years older.