So I’ve been growing this Olive and many others for decades (from seed). In the past few years, I moved them from small bonsai pots to large nursery pots to thicken the trunks. Progress seems very slow.
Thanks,
Steve
Pumice and some coconut coir allowing roots to grow into the ground seemed to work great for this guy! Great video.
I wonder how large that pine would have been if he grew in premium gardening soil.
Interesting video. The best thickening in the video was when the roots escaped into the ground, but it resulted in something that has to be cut back very hard and redeveloped.
For the olives, I would suggest the Anderson flat on the ground, with a 50/50 mix of organic (I like sieved bark) and aggregate, along with a year or two of free growth. Sliding a shovel under the Anderson flat to cut the escape roots after the 2nd year is apt to be a good idea. On concrete the growth will be less, but the wider pot, organic mix, and free growth will be better than in the smaller pot and a lave-pumice-akadama mix. Remember that we use akadama to scale the roots and refine the branches.
Thanks Marty. I’m challenged with space in my backyard. Not a lot of open ground that gets sun - the sunniest spots are concrete.
For the flats, the 5” walls are what I assume folks use for this stage?
Also, is your reference to aggregate as simple as it sounds - like a construction type aggregate of stone? Or would pumice be better?
I would use pumice, lava, or pearlite, as the aggregate with pumice being the first choice. The combination with bark is basically a nursery mix that keeps a little more structure. The 5" tall 16" wide (13 cm by 40 cm) Anderson flat would be my suggestion. I build similar wood boxes with mesh bottoms for this purpose. I also agree that the olives are going to want lots of sun.
And now on my quest for bulk pumice.
Use the search icon and type pumice. There have been a few threads with links to various suppliers. Some landscaping supply companies have it in bulk, the ones that have various rock, sand, bark, decomposed granite, etc.
Clearly a series of mirrors is in order 
Where I live pumice is fairly non-existent. This is the cheapest source for me. The description is a tad misleading. It’s more like 4gal, but whatever. If you’re on the west coast, especially the PNW, you can find pumice fairly readily…or so I’m told.
It doesn’t sound like you have the space, but putting a tree in a grow bag into the ground works as well. That’s assuming you don’t care about branch development. I use this strat when my goal is purely to grow the trunk. However, with my “field growing” conifer I find that I’m able to at least setup the primary branching structure.
Thank you for sharing this interesting video 
Pedro Morales was in Dallas recently and shared this method, so I will share it here. Some things of note, Pedro is from Puerto Rico and thus works almost exclusively with topicals and very few plants not native to the island. After years of fighting to have Pines and other non native bonsai subjects he too has embraced the path of least resistance and grown to love the native material and thus eliminate(to a great degree) the bonsai shuffle and the fight we all all have with species that do not thrive in our local climate.
Pedro said he grows his own yamadori in the backyard now, the searching is much easier this way. He has 40 some odd plants growing by this method and finds that in 2-3 years he gets trunks that are the size of 5-6 year plants he was growing in ground before. Old tires have been the answer. He places landscape fabric on the ground then stacks two/2 tires on top of each other. He fills with a 50/50 mix of sand and organic material, no sifting, no fuss, nothing special. It can be growing mix, landscape mix, manure, compost, he does not care and regular sand. He is able to manipulate them and branch select and prune while growing and when time to “collect” he takes a saw and saws between the 2 tires.
He believes that the black tires is the key. They warm up in the day and provides additional warmth at night. Roots like warmth and the tires make a sort of heated root bed. The sand holds additional water and also provides a buffer for excess heat, but allows for easy root growth.