Tray for The Breathing Pot
by Camilo Mora, Ph.D.


Much of the chance for survival of a tree start since you put the seed in a pot.

We have found that the use of breathing bags , which can be made of non-woven material or paper, provide just the extra “edge” that seedlings need to maximize their survival after planting.

However, those bags are difficult to handle at the nursery. While several trays are available commercially, they have several limitations.

This tray has been developed for the purpose of overcoming several shortcomings in the use of seedling bags at the nursery.

You can use the figure to the left for exploring the details of the tray.

Background: After several experiments and trials for mass production of "effective" seedlings, we have found that breathing bags are an optimal solution: 1) the permeable membrane allows for constant oxygenation of the root system, 2) air exposure prunes the roots avoiding all together root bounding, 3) there are no problems with soil drainage, 4) by planting them directly into the ground, we avoid planting shock, and finally, and I like this one, 5) they are very cheap, about $3 cents per bag.

Trees really love growing in these bags.

Example of native Hawaiian trees in "breathable" non-woven bags.

Problem of breathable bags:
There are, however, a few downsides to breathable bags.

If the bags are in contact with each other, the roots will move from bag to bag; so, you really need to keep the bags separated.

The bags are difficult to keep straight up in the nursery and should be prevented of touching the ground, as roots will develop pigtail.

The solution to the problems of breathable bags is a tray. As my colleague Lars Jensen puts it,...a breathable pot "is only as good as the tray it’s in".

There are a few trays available commercially. However, they suffer from a few problems.

First, almost all available trays are very shallow, so you really cannot have deep root trees (a big flaw).

Second, as part of their design, likely to provide stiffness, these trays have many dark spaces or crevices/cavities, which are love by bugs that use them as home.

Finally, such trays cannot be stacked. It may not seem as a major problem but failure to stack the trays results in major costs for shipping, and forget about storage space.

So, we developed our own tray to overcome all the problems above.

Key attributes of the tray (only one cell shown for simplicity).

The tray allows stacking bags up to 12” tall; a huge advantage for root grow. One may want taller bags for deeper roots, but be mindful that you may have to dig the holes later on!. A 12” deep bag was a good compromise.

The tray allows keeping seedlings 1” apart from each other and 1.5” from the ground, which avoid root crossing between bags and maximizes air pruning, avoiding entirely root bounding and pigtail.

The tray does not have cavities, which maximizes air flow and ensures full on oxygenation to the roots, while avoiding crevices; there is no place for bugs to hide on in this tray!.

Finally, using second grade geometry, we utilized a cone shape for the tray, which allows the tray to be stacked while holding perfect cylindrical bags. Stacking one tray on another one adds only 1.5" in height to the volume occupied by the first tray.

Full tray. You can inspect the tray in the image at the top of this page.

Tray development:
We need to start by considering that the tray needs to be stacked. One geometrical figure that can be stacked on top of each other are cones. So lets first create a cone.

Second, you need the cone to hold a cylindrical bag. Hmm, how can you hold a cylinder with a cone? The solution is to create four cones and then extrude a cylinder among them. The edges of the cones can hold the cylinder, and in turn, several copies of the same structure can be stacked.

While functional, you only need the cone edges in contact with the cylinder. So we can make a revolve cut around the cylinder, leaving the cone edges with a few millimeters as bin supports.

Next, you can add the ground isolation, or the component that will elevate the bag from the ground, needed to increase air pruning and avoid pigtail.

Next, you add a top to the tray.

It looks about all we need. So we tested the design by 3-D printing, and discover that the bins are too floppy.

First prototype

Basically, we need to create a lateral support for the bins. First, I created a full contact between neighboring bins...I tried a couple styles...

Second and third prototype

The new cell has lateral contact that provides good stiffness but has too much dark space; prime state for bugs. One can easily foresee a diversity of animals living there. Even with a hole/ventilation in the middle there was still too much dark space.

So I simplified the design and created a basic horizontal bar between neighboring bins.

Fourth prototype

Prototype #4 provided good lateral contact, good stiffness, no crevices, but was not pretty.

Fifth prototype

Prototype #5: still not pretty.

Finally, I selected a simpler lateral contact.

Sixth prototype

Prototype #6 provided lateral contact, good stiffness, no crevices, pretty, but created a conflict for stacking.

Seventh prototype

Prototype #7 provided lateral contact, good stiffness, no crevices, pretty, and no conflict for stacking.

Failed prints.

Of course, the printer failed a few times.

Now, that an optimal cell was selected, we can replicate it to create the tray by creating a grid of 3x3 cells.

First version of the tray.

Exciting, we have a tray that fulfills all the requirements to keep breathable bags at the nursery, it is stackable, easy to handle and transport. So we printed a full version of the tray, it took four days!.

First version of the tray printed.

It should be ok in theory, and it did not look bad. But once we grave it with our hands, we noted that it did not have much torsion stiffness, an actual term!. Basically, if you hold the tray by the top, and turn it, it bends.

So, back to the drawing board to find an structure that will increase torsion stiffness. I tried a lot of things.

Structures tested to increase torsion stiffness.

Attempt 1: Good in theory, prone to torsion in practice.

Attempt 2: Good in theory, prone to torsion in practice.

Attempt 3: Good in theory, prone to torsion in practice.

Attempt 4: Good in theory, prone to torsion in practice.

Attempt 5: Good in theory, prone to torsion in practice.

Attempt 6: Good in theory, prone to torsion in practice.

They all should have worked in theory, but in practice, or at least on the 3D prints, they were prone to torsion. Eventually, two structures filled the bill.

Structures with increased torsion stiffness.

They both were rigid. If you apply 1kg of torque to any of those two structures, in theory they bend only ~1mm. Not bad. They only downside is that any of these designs doubled the weight of the tray to more than 1kg. I guess I would have to live with that.

Developed by Camilo Mora. Thanks to Kyle McDowell for help with the prints and for brainstorming the style to maximize torsion stiffness.

File in Solidworks,