Grow bed/box Medium

[Will.power49]

I am in the process of moving from one property to another. Building a house and business in a small town. With this I am gone from my trees(in the city- new place in the country) 16-24 hours sometimes. A handful of the larger trees I will be repotting into pond baskets or Anderson flats. What I am interested in discussing is. What would you fill a above ground grow bed with? Amend soil at all? fill the bed with something other then dirt? Just dig holes in the yard and place them in? I ask this knowing there are 101 different ways and things, buts hearing the masses out is good sometimes.

I have listened to podcast where Randy Knight was known to place yamadori in grow bed with sawdust. These seems more reasonable when recently at a nursery they had sawdust covering there pots and around them in above ground grow bed. The gentleman said he didn't have issue with it holding to much water, roots worked there way in easy from from Nursery pots and the fungi for the spruce and pine he grew said couldn't hurt. Would beds of spent bonsai soil bulked up with say pumice or lava rock work well? Just Pumice?

Thinking like huge tray for what keep bean and mame size in during the summer. But would be for much longer. This year and Next and least

 

[horibonsai]

I built my own raised bed. The filling ingredients are lightweight aggregate and fine pine bark, which can be purchased in bulk. You can use bark in a pinch, but courser bark than pine fine. I also make my own compost, which ends up in my vegetable garden, and the rest ends up where I grow my pre-bonsai. In most garden mixes, you can find bulk, a mix of bark, sand, and little pea gravel. That will work, and save you time in doing your own mix. I do find the bulk pre-mix stay on the wet side.
Pure pumice is too expensive to ground-grow plants unless you live close to the source. The alternative are lightweight aggregate (expanded shale, or expanded slate), perlite, and vermiculite.

 

[horibonsai]

Sawdust is good but be careful. It needs to cook, so leave it out for a minimum of six months before using it.

 

[Shibui]

Raised beds are bonsai friendly because they have extra drainage that an in ground bed may not have. That means it is not at all critical what is in the raised beds. Normal garden soil should be OK. Having said that I now realise that soils are not all the same so need to qualify. Heavy soils and clay based soils would probably need amending to open them up to allow air and drainage. Any good loam will be Ok as it is. maybe add some aged manure or fertiliser. Sandy or gravel soils would benefit from added organic materials or loamy topsoil.
All raised beds will dry out far quicker than the same soil at ground level. Open soils like most bonsai mixes in raised beds will dry even faster so take that into account when working out how long trees will survive without added water in summer. Just pumice or large amounts of lava rock are likely to need much more water than soil based media.
I expect spent bonsai soil would be good as a raised bed media if you have enough of it. Good loamy soil mixed with spent bonsai mix would be good.

Sawdust is used more as temporary medium or as a mulch. It tends to tie up nitrogen as it starts to decompose and can severely limit plant growth until the trees get roots down to natural soil unless you apply fert regularly. That's likely the reason for the 6 month caveat above. 6 months of breakdown (with added N) gets past the N drawdown so the decaying wood doesn't compete as much with the trees. I would be wary of planting into anything with large amounts of sawdust or wood shavings unless it has been well aged.

 

[Will.power49]

I guess I'm looking at making a giant humidity try. I have a large amount of used bonsai soil, odd amounts of perlite, pumice, compost. Planned on mixing everything together until I get a profile I like. The beds will be off the ground - think big grow box. so the bottom will have like window screen or super fine mesh. Want it to mimic ground growing almost. Also a place for Yamadori to recover in.