I have many contorta, from the whole range of contorta “kingdoms” (from mountain to shore). I have bare rooted several successfully, collected many from the mountains, lava beds and the coast. I’ve worked on a couple from Vancouver Island as well. Most of my contorta are wild-collected but I’ve also transitioned a few nursery-grown seedlings out of a sad state of inappropriate nursery potting. I have also successfully air layered this species, with the air layer being quite large, not just a small shoot. I feel like I can comment with confidence on what the roots of contorta want, relatively confident that this applies across their entire genetic/geographic breadth.
Slip potting is 100% not the way to go with this species, nor is inorganic or dense soil of any kind. Large soil masses (compared to the root system capacity especially) are also villain #2 after organic or compacted or heterogeneous soils.
Pure pumice and/or lava are what they want. The happiest you’ll ever see a shore pine is in an air-breathing mesh pot in pure pumice where the pot isn’t much larger than the root system. They can routinely produce follow-up flushes in such conditions (note: when left alone to run wild with growth, don’t decandle this species) and get their most vigorous growth when the roots are happy. The roots are happy when the soil is homogenous, when it is durable, when it is free of junk and can breathe air and drain quickly. In the PNW I no longer bother hesitating or delaying with the inevitable step: That the soil must be transitioned to this state ASAP with every lodgepole or shore pine that I get, even if it means a lean year and heat mats under the pot. It’s ALWAYS worth it to completely transition the soil to a super squeaky clean air-breathing volcanic setup. Visit places like white river west sno park on mount hood and you will see it in the quality of the needles and growth there. Coast-wise, collect a shore pine from dense often-wet, sometimes salty sand, then rehabilitate it in pumice back home and you completely transform the tree.
@Scorpius is on point with the aeration tips and control of water, and timing of repot. Lodgepole and shore both respond very well to sitting on seedling heat mats if you want to really rev up root growth for a few cold weeks, the results of that can be astounding (I have bare rooted a few lower value yamadori to prove this out, with success). I have not sheltered these trees against any heat extremes, PNW heat dome 2021 was no problem, but NE winters I’m uncertain about so tread with care. The last note I’d share is that they live up to their latin name with regards to wiring. It’s exceptionally difficult to kill a branch from extreme bending as long as you support the bend with correct wiring technique and as long as the tree is in the “good” root setup (air breathing) as described above. Wait to do daring things until after transitioning the roots and getting a first post-transition vigorous feedback growth that says the tree’s recovered.
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