What stops tree and plant roots invading Subpod and blocking it up?
If you are concerned about large roots invading your Subpod, then you can use a geotextile fabric or landscape cloth underneath your garden bed. This will allow water to flow out, but prevent roots from coming up.
Smaller roots from food and herb plants grown within the garden bed are prevented from filling and blocking the holes in the Subpod as the worm aerator stirs the contents every time food is added, effectively breaking up any small roots that start to invade.
We have found that veggie roots, etc.in the garden bed may migrate towards the nutrients coming out of the Subpod, but we do not necessarily see this as a bad thing. We recently found evidence of some garden beds flourishing during the drought of summer because the roots went towards the Subpod. These gardens weren’t watered, but the plants survived and produced an abundant supply of tomatoes :) Of course we don’t recommend not watering your garden, but it was an interesting find nonetheless.