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I grow plum trees from pits using seed starting trays:

enter image description here


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).

enter image description here


The problem is normally solved by using seed starting trays that have specialized vertical fins that prevent the root from growing in circles:

enter image description here

Question:

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?

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That first picture is a very healthy little start. Roots this tiny can circle and be just fine. Why is air root pruning a good thing?

That little start is now ready to be up potted; a 3" or 4" pot with potting soil. I want to get my starts to that point start up potting and to delay up potting any longer by "training" roots to grow to the hole to get air pruned...makes no sense.

I've transplanted thousands and thousands of potted plants; from tiny little pot 'fillers' to massive balled and burlapped trees. Encircling roots are only a problem for maturing trees, woody perennials. There were always lots of over grown plants in pots from the nursery. What we did was take a knife, a shovel edge and CHOP that root ball a bit. When planting any starts I always mess up the roots a bit. Rip and tear...gently sorta? Never lost a single plant, I am serious. Plants are tough. I had to teach my crews how to properly transplant...these guys were never gentle to begin with. No time to be gentle. We are talking about transplanting potted plants from the nursery into the landscape.

I am trying to help you relax about root circling. Especially in starter pots and trays. Absolutely a non issue. Honest. It is a good thing for the roots to be filling that little bit of soil, not being air pruned out of the hole. I have never heard about this air pruning. Please let me know more about why you think this method might work.

Transplant that plant and all others at that stage into a 3" pot with potting soil only. Time for a little 5-5-5 fertilizer btw as well.

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Retrofitting trays would probably be tedious. It seems like it could be possible using a glue and some thin pieces of plastic.

I have an alternate proposal that seems worth considering: soil blocks and soil blockers to make them. Soil blocks are something I just started using this year as an amateur home seedling grower. However, they have been in use for several years if not decades among commercial growers. Soil blocks air-prune and prevent root-bound plants because the whole section is surrounded by air, not just in one spot.

Instead of using cells, you create soil blocks and place those blocks into your tray. The first block can be very small, just a half inch cube, allowing a very dense alignment of seeds over a heating mat during the germination period. Once the seeds are germinated, there is a progression of 2 inch cubes and finally a 4 inch cube if you want to keep them indoors.

Image of seedling in small soil block being up-potted into larger and larger blocks

The most famous maker of these tools is Ladbrooke Soil Blockers and a picture of their tools is below for demonstration of the concept. However, the idea can be achieved with home-made soil-block making jigs.

enter image description here

This was my first year working with them and I love them. It took a few times and watching some videos to really get the hang of the process. I was skeptical that the blocks would hold their shape during indoor watering, but they really did stay together just fine.

If you sell these trees commercially then a final step would be to place them into the container you will sell shortly before selling.

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If i've understood the nature of the "fin" you want, you could glue a length of angle stock into each opening. ABS plastic is a black material that cuts and glues easily.

angle stock

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