I grow plum trees from pits using seed starting trays:
Unfortunately, I have a problem where the roots of the seedlings grow in circles around the bottom of the cavities of the trays. The circling roots cause problems when the trees grow to maturity (they strangle each other out).
Instead of the roots growing in circles, I need to train them downwards towards the hole (where they will be air-pruned).
The problem is normally solved by using seed starting trays that have specialized vertical fins that prevent the root from growing in circles:
I would like to retrofit my existing trays (I have several dozen) so that they function like the anti-root circling trays. I think this might be achieved by adding some sort of vertical fin in each cavity that would act as a physical barrier/re-router to the roots (but I'm not sure how to do this).
How can I retrofit my existing trays to prevent root circling?