Seedling Tray Cell Design: Drainage, Taper, and Plug Removal

A seedling tray cell has to drain water, hold growing media in place, and release an intact plug. Wall taper helps the plug come out in one direction. Drain openings balance water and air against media loss. Wall thickness and ribs keep the tray rigid enough to handle. These features interact, so changing one can shift the others.

What Is a Seedling Tray Cell?

A tray is a molded sheet of connected containers, and a cell is one of them. The dimensions below describe its geometry.

• Cell count: how many cells fit on a tray, which sets spacing
• Individual cell volume: how much media each seedling has
• Depth: top to bottom, which affects drainage behavior
• Opening: the top dimension, which sets spacing and the growing area
• Bottom geometry: flat, tapered to a point, or with a raised rail

Two trays with one cell count can differ, as the count tells you nothing about depth or shape. The U.S. Forest Service’s Tropical Nursery Manual chapter on containers treats volume, density, depth, and shape as separate variables. A product designer can read those same variables as geometry, and that is the view taken below, with crop questions left to growers.

How Cell Taper Affects Plug Removal

The Forest Service notes that most containers are tapered from top to bottom. From a mold designer’s view, that taper is the same idea as draft on any molded part.

• Straight walls: the plug has to slide along its full length, so friction lasts all the way out
• Tapered walls: the plug can lose contact with the wall soon after it starts moving
• Mechanical interference: a rough interior or a wall with a bulge can lock the plug in place
• Root plug adhesion: roots cling to walls and through openings, adding grip that geometry has to overcome
• Ejection direction: the tray’s taper means the plug should move toward the wider top, which is the direction a push-up pin provides

A firm plug helps. The Forest Service’s Nursery Manual for Native Plants describes a firm plug as one in which roots bind the medium just enough to facilitate extraction without the medium falling off. Geometry alone cannot rescue a plug that has not formed.

Drainage Hole Geometry

• Number of openings: more openings give water additional exits but reduce base area
• Opening size: the Forest Service says a hole must be large enough to promote good drainage and encourage air pruning, while also small enough to prevent excessive loss of growing medium during filling
• Position: a center bottom hole can be blocked when a tray sits on a solid surface, while side-low openings may stay clear but can weaken the wall
• Drainage and media retention: the same manual says the correct drain size is related to the medium, as sandy mixes are more likely to fall out of large holes, whereas fibrous organic media tend to hold together
• Blockage risk: anything beneath the tray, or fine particles washing down, can reduce drainage

That manual’s point is useful to the tray designer: one drain size cannot be best for every medium, so a tray intended for a specific market should be checked against the media used there. Depth interacts here too. The manual explains that a saturated zone forms at the bottom of a container and that the medium controls the depth of this layer, so a shallower cell holds a larger share of its medium in that zone.

Wall Thickness and Cell Rigidity

Thin walls save material and cut cycle time, but they cost stiffness. The cell wall has to resist push-up pins and hand pressure locally, and the sheet as a whole has to survive being lifted. Ribs between cells or along the rim can stiffen the sheet without adding thickness everywhere. A tray that flexes when lifted can stress its cells or shift its plugs, and a full tray of wet media may be picked up by one edge.

As a rough illustration with assumed values, a 128-cell tray with 25 cm³ cells holds 3.2 liters of media. At an assumed wet density of 0.8 kg per liter, that adds about 2.6 kg (5.6 lb) to the weight of the tray. Actual figures depend on the medium and its moisture.

Cell Shape and Corner Geometry

Round cells have no corners to trap roots, but they leave unused space between cells. Square cells go the other way, and the Forest Service says square containers maximize the growing space used in a greenhouse. A tapered polygon sits between the two, with flat faces and some of the space efficiency. Whatever the outline, a sharp internal corner is hard to mold cleanly and adds stress under flexing, while a radius helps release in the tool and in use.

Ribs and root behavior go beyond tray geometry and into how a container manages root growth, the subject of how air pruning and root guiding differ in forestry containers.

Plug Removal Methods

• Manual extraction: a grower pulls or pinches. Tall rigid cells and tight plugs raise the force, and that force can reach the stem
• Push-up extraction: a pin or dibble pushes through the drain opening. The hole size and the bottom strength need to suit that pin
• Automated transplanter compatibility: machines expect consistent cell pitch, tray stiffness, and plug firmness. A tray that flexes or varies in pitch can misalign with the fingers or pins
• Substrate cohesion: a crumbly medium can fall apart during removal no matter how good the tray is

Removal can damage the tray as well as the plug. US patent 11,357,174, for a seed starting tray, says in its background that trays formed from a thin, rigid material are prone to being damaged or broken, especially when removing the plug. That is an inventor’s statement of the problem, written to justify a design, so treat it as a prompt for testing.

Check the removal method against the design early. A drain hole sized for a particular medium may be too small for a transplanter pin, and that trade-off is better resolved before the tool is cut.

Material and Reuse Considerations

The polymer grade sets stiffness and impact resistance at thin wall sections, and a nursery tests both. The Forest Service states that intense heat and ultraviolet rays in nurseries can cause some plastics to become brittle, although many container plastics now contain UV inhibitors. Heat also softens plastics, so a tray that feels rigid in the morning may flex in a hot greenhouse.

Reuse raises the stakes on all of that. The same manual notes that some containers are designed for one use, while others can be reused for 10 or more crop rotations. Each rotation adds washing and sanitizing, so a reusable tray’s walls, ribs, and drains all have to last that many cycles.

Comparing Three Cell Geometries

The comparisons here are illustrative. They show how geometry trades off, and they are not agronomic recommendations.

Geometry	Mechanical advantage	Limitation
Shallow, wide	Stiff and easy to fill; short plug slides out a short distance	Larger share of media in the saturated bottom layer, per the manual's reasoning
Deep, tapered	Taper breaks wall contact as the plug starts moving	Needs a stronger pin path; a long plug may be less stable if roots are sparse
Deep, narrow base	Steep wall angle eases release	Small drain, small base contact; tip can break if the plug is pushed hard

Each design feature has a job, and each job can be checked.

Design feature	Mechanical purpose	Nursery implication	Validation method
Top opening	Sets cell pitch	Spacing between plants	Measure pitch across the tray
Wall taper	Plug release	Extraction force	Pull or push force with rooted plugs
Drain opening	Water exit, pin access	Media loss, drainage	Fill and irrigate; measure media loss
Wall thickness	Cell stiffness	Tray life	Flex and cycle test
Rib layout	Tray stiffness	Handling when wet	Lift a full wet tray by one edge

Any claim about plant growth should come from research on that crop and that medium, so tray geometry is stated here only in mechanical terms.

Engineering Review Before Tooling

Check these on the CAD model and with the mold builder.

Check sheet: tick each point as you work through it

• Draft: every cell wall has to release from the core, which on thin walls and tall cells is a real constraint
• Wall uniformity: uneven walls can warp a large thin sheet
• Drain openings: how they are formed in the tool, and whether shutoffs will wear
• Rib design: ribs can add stiffness, but can also leave sink on the other side
• Part release: a large, thin tray needs even ejection
• Tray stacking: empty trays need to separate. Cell taper can help with nesting, while it can also cause sticking

Run a physical sample with a real medium and a real plug before finalizing. A geometry that looks good empty can behave differently once roots have grown through it.

Seedling Tray FAQ

Why can two seedling trays with the same cell count behave differently during plug removal?

Count says nothing about depth, taper, surface finish, or drain size. A steeper taper breaks wall contact sooner, while a rougher interior holds on longer. Hole size decides whether a push-up pin fits at all. Material and stiffness change how the tray reacts to force, so identical counts can produce very different handling.

Can a larger drainage opening create mechanical or media-retention problems?

Yes. The Forest Service warns that holes should not be so large that the medium falls out during filling, particularly with sandy mixes. Mechanically, a larger opening removes material from the base, which can weaken it under a push-up pin. Size it for the medium and removal method you expect, then test both.

What to Lock Before the Mold Is Cut

Two decisions are expensive to change once steel is cut: how plugs come out, and whether the tray is reusable. The removal method decides drain size, bottom strength, and pitch. Reuse decides the wall, the grade, and the UV package. A tray drawn before those two are settled may have to be redesigned around them later, when a transplanter pin does not fit or a wall turns brittle in its second season. Medium class, taper, and depth can then be chosen within those limits, with draft checked at the same time. Ribs belong where a wet-tray lift test shows flex. None of this replaces a sample with real roots, since a geometry that looks good empty can still hold on to a rooted plug.