I picked up a blue atlas cedar pre-bonsai about a couple of months ago. It’s in a ~5 inch nursery pot nested inside another pot with rock rubble drainage. It’s in an organic soil mix. My issue is that I think it’s in desperate need of an up-pot; it’s growing vigorously to where the roots have split the inner pot and are entangled in the rock rubble drainage. I’m in Massachusetts, U.S., so my spring window for doing any type of root work with a repot is long gone.
Should I slip pot it into a slightly larger container, e.g., 8-10" nursery pot, oversized bonsai pot, or wood box now without touching any of the roots? I’m worried about the root ball getting too much bigger/deeper than it already is since these have sensitive roots so I don’t want to have to do even more root work when it comes time to move it into a bonsai container. If I do slip pot into a larger container, would it be best to stick with a similar organic soil mix, or could I fill in the new space with something like 1-1-1 akadama, pumice, lava?
I had a similar situation with a shore pine where the collected trees roots broke the clay pot it was in. However, I waited until the spring of the following year by taping the pot together with duct tape to hold the trees roots together best I could until the right season for repot. I highly recommend waiting until next Spring for a repot. That was you can properly repot and reduce the roots to fit into your ceramic. Blue Atlas Cedar can be finicky and I would venture that the tree might not thrive if you try a repot (slip pot) now. Do what you can to put it in another pot so the roots stay hydrated until you can repot. That is the best advice I can give.
How deep is the rock rubble below the inner pot? If it is rather deep then I understand why you are concerned about the root growth the rest of this year. I have found that nursery stock blue atlas cedar can take a lot more root work than those that have been growing in a bonsai pot. You should be able to slip pot now by carefully removing the broken inner pot and putting into a slightly larger pot. I would use 2-1-1 or a 1-1 mix of organic and pumice to better match the current soil and provide the higher level of moisture that blue atlas cedar prefer.
I removed most of the organic soil from a slightly nursery stock this spring when the buds were definitely pushing and potted in 1-1 organic-pumice. It lost most of the older needles as the new buds pushed, but is doing great now. Not the same time of year, but a much more severe repotting.
The rock rubble drainage layer in the outer pot is about 1.5” deep. Here’s a photo for reference to see what I’m working with. I’m worried all of these exposed roots will dessicate, and/or have a hard time making it through winter if not buried in soil. I’m thinking at this stage it’s best to cut the pot off carefully, and replant in a slightly larger pot without disturbing the rootball?
That is the approach I would take except that I would very slightly loosen the edges of the root ball so your new soil can integrate a bit with the old and you don’t end up with a horizontally perched water table.
I would use a pot just high enough and large enough to accommodate the exposed roots and then do any root work during the appropriate time of the year. Slip potting it can potentially do more damage. You do have some nice new white root tips growing out from the pot indicating good health. The media that I suggest using to cover those expose roots would be a medium grade pumice.
This is helpful, thank you! Can I ask on the benefit of using pumice in this case vs. a similar organic mix it’s in now, or even an akadama-pumice-lava rock mix with some peat or other organic material added in to sort of match the current soil?
You want a media that retains water but at the same time has a high porosity for oxygen exchange. The danger of adding media right now is that you can disrupt the oxygen/water balance of the current media the tree is in. Pumice is good at retaining water and at the same time highly porous which allows for great oxygen exchange. It is used as starting media for cuttings and seedlings because it promotes great root growth due to the water capacity and oxygen exchange.
I concur with the use of pumice to both hold moisture and give good oxygen exchange. I use it will screened bark for trees in development and is what I would use for this slip pot. The screened bark will help with nutrient availability and help better match your current organic mix. Straight pumice is great for trees that have been collected that do not have a nicely defined root ball.
I’ve ordered a couple of nursery pot options that hopefully will work. If not, would a wooden box be an okay substitute until I can properly repot in the spring? It would be easy to build a wood box just slightly larger than its current pot with some extra cedar lumbar I have on hand.
I use wood boxes all of the time. I actually prefer them to most pots since the roots circle less, I can build custom shapes (shallower and wider), and I can get really good drainage. Go for it.
