The drainage holes weren’t ideal so I added a drainage layer to the pots. A few inches of gravel takes care of that and leaves it well-ventilated and free-draining.
My previous soil mix for the potted trees was OK. It still drained nicely, but it was very low on organics and compost, so it was mostly substrate and relied on my regular fertilising. The roots were nice though - no anerobic dead spots.
I've gone in a very different direction with organic components and a lot of coco fibre. I also added a large spoon of mycorrhiza to the mix. It contains about 50% composted farm manure. It also weighs less than soil which is an important consideration in these large pots.
I have also been using the Canna Bio Vega fertiliser for a while now and it is giving excellent results. Now I am starting to use the Canna soil as part of my mix for the large pot growing.
Getting good results with field grown ( or XXL pots ) involves a lot more than just potting it somewhere with plenty of space for roots and letting it run wild. That will lead to one of those ugly trunk chops with no taper - like so many trident maples that I've seen. I want to create a nice crown and not have to resort to any carving to disguise the faults.
On the lowest branches I have made no reductions in buds and on the apex I leave a single bud. To get some taper I need the lower branches to be very strong. I have added a bit of wire to keep them heading in the right direction - but I still view them all as sacrifice branches.
As beech buds are laid down in the previous summer growing season I will be growing last years foliage this year and may not see the results of the big pots until I get next years growth and the ( hopefully massive ) buds laid down in this years growing season. Each bud should be giving me at least 8 leaves so there will still be a lot of tree at the end of the year.
The large pots start to cost a bit more. Not too bad in the end - about £8 for the pot and another £12 to £15 for gravel and soil.
It seems to me that there is almost as much technique involved in growing good material as there is in refining the Bonsai. Its certainly been very interesting growing these trees so far.
I have recently bought some Scots pine seedlings to add into the collection. For their first few years they will go into normal small pots while I ponder them. This leaves my collection of native trees in good shape. About as many as a hobbyist can handle I think.