The third makes the most sense because its more stable, however stacking blocks directly on each other isnt very stable and more prone to falling over.
Stacking them as you have them pictured isnt very stable either. Cement blocks are cheap so build a better structure for the safety of your trees.
I would try to design a stack that staggers the blocks to make it more stable, even if it means doubling width using 2 rows of blocks to stagger them in a cube to build height.
This all true, I’ve done block work before but I was thinking this is a low stress application. That’s why I even considered the first two, I was thinking toespace like cabinets lol. But judging by your reaction maybe I am wrong? I was going to mortar them once I settled on the design, I thought that would be plenty.
This is just free cycle stuff right now and am my basing design of off some ideas I saw online for benches and planters. With a 16 foot bench and 8 blocks per section, doubling the columns to tie them together crosswise I would have to almost double my block count. $50 for more blocks is at least one new tree!
How does putting a post inside the lower blocks sound as a solution? It would take away some of my lower shelf options but would add stability. I also have this idea but I don’t like it as much as it also at least doubles my block count but it will be rock solid stable.
If blocks are mortared or glued and set on flat surface (not on top of the rock, that was to layout ideas is all) i thought it should be pretty stable. What if I added another block to the third pic above so the bottom row would have a 90° out one side in front and another in back? Plus posts somewhere to prevent tipping even if hit fairly hard.
Next thought will require moms input, because the house I’m fixing up will be her retirement house. But what about pouring a footer and mortaring to it and/or filling the bottom row with cement to tie it all in? I think I’ve got some quickcrete in the shed. Maybe footer, one extra block so bottom row has two rotated to widen stance, posts and concrete inside the bottom row! Lol It’s more labor but it’s free! That seems a bit overkill, I’ll just try and get mom to look at it, she’s an architect. Long term it will be hers and I’ll build mine at my new house when I’m done with this one.
Hell maybe I should just do a big 3 shape kind of like you are it looks like. cost goes way the hell up but a very straightforward design.