A few notes on what seems like a simple operation.
At some point, we have all experienced this scenario...when your new plants arrive and you are eager to re-pot them. You may say to yourself, "I'll just transfer them to another container, it shouldn't cause any harm."
The truth is, you will only be creating more work for yourself next spring, and you may end up causing damage to your new plants, which will require both your time and affect their health.
Despite the various videos on YouTube making it out to be a simple non-harmful thing to do, the technique is not recommended by most bonsai professionals.
My advice is simply don't do it.
Disturbing the roots during the incorrect season should only be considered if absolutely essential. This includes situations of evident root rot or if a tree is toppled over and its container shatters.
The tips of the roots are covered with tiny, nearly microscopic hairs known as root hairs. These are simple structures and the only structures that allow the plant to take up water and nutrients from the soil. The are usually just one cell in width. Being so fine they are extremely fragile and easily damaged.
Even taking the plant out of the pot to have a look at the roots is going to do damage to the root hairs. You also run the risk of the soil falling off the roots completely which will only stress your tree further, especially with juniper and other conifer species.
You may not think you have done any harm but when the tree is stressed in hot weather and needs to take in water you will find out how much damage has been done when your previously healthy plant suddenly looks very unwell or even dies.
There is a good video here on YouTube where Eric Schrader explains some of the details of slip potting and why it may not be such a good idea.
I hope this has been helpful
Happy Bonsaiing
I am so glad you have written this Fred. I found out how bad a practice this is after purchasing a Japanese White Pine from a UK bonsai nursery that almost died due to this. When I purchased it it had a cluster of yellow needles and they assured me that this was normal and seasonal (in August) . Being a relative newbie I accepted the explanation and had done what I thought was my homework and was very mindful not to overdo water. It went from bad to worse as the yellow needles turned brown and dropped off and I kept it protected over winter not knowing what I was doing wrong. Come late Feb I decided I had…