Whether necessary or not I always have about one half inch of substrate soil placed on the tile below the roots. I always determined it was the best growth environment for the roots because the substrate does hold some moisture for the smallest of root growths. And the roots I develop work well with substrate soil. I usually also have a substrate soil covering of at least one to two inches of substrate soil on top of the roots.
I learned once that I should not have roots try to grow into a hollow space…roots just don’t do that naturally…that’s why I poke a chopstick all around the substrate when repotting. If hollow spots were okay I would skip the whole chop sticking process. So, even when using a tile, I make sure the roots have contact and coverage by substrate and soil components. Roots by themselves leave hollow spots.
I have not used tiles in a box. There is no reason not to use a tile in a box though. I would treat it the same way as I do for ground planting. My tile use planting has just been all in the ground. I move from the ground to a box as my next step. In the box I can wire-hold roots into the box from the bottom. And, just like on a tile, I place some substrate soil on the bottom of the box below the roots for the same reasons. When I box plant a tree I’m not necessarily filling the entire depth of the box with substrate soil. I use just enough for the tree to develop with moisture available. This usually means about one half to one inch of substrate soil below the roots and then about one into to two inches above the roots so the roots are covered. On trees with more delicate roots, or difficult in any way to wire to the box bottom roots, I always use medium size rocks for top of the final substrate soil layer as anchors. A tree might have four or five rocks. And, no, I’ve never had dry spots below the rocks. And the rocks actually look naturally attractive.
I don’t secure roots to a tile. Well, not in the literal sense. Once I’ve covered the roots in ground growing I use and reuse large rocks to anchor the tree and therefore the roots to the tile and ground. The rocks provide a good uneven growth surface below the rock and above the roots. There are no dry spots. If I have a stubborn root though I will use small stones to accomplish placement before further covering with substrate soil. After that I plan on natural growth. When I tile-grow roots I know I going to cut off most of the root length anyway. I’m usually root cutting at about two to three inches from the trunk. I really don’t care what path the roots took beyond that circumference. A root tangled mess beyond the needed circumference is not something I worry about.
