How to Choose Plug Tray Cell Counts: From 512-Cell Seeding to 50-Cell Finishing

Start With the Crop Stage, Not the Tray Catalogue

Cell count is the single most consequential decision in a plug program, and it is usually made backwards: growers pick a tray they already own and force the crop to fit it. The better approach is to work from the production stage outward. Every young plant passes through up to four physically different environments — high-density seeding, germination and early growth, growing-on, and finishing or grafting — and each stage rewards a different cell geometry. A tray that is perfect for the first ten days of a tomato’s life is wrong for the following twenty.

This guide walks through the standard cell-count bands used in commercial propagation, what each band does well, and the trade-offs in cell volume, depth, bench space and machine compatibility that should drive your choice.

The Four Cell-Count Bands

512–288 cells: precision seeding

At the top of the density range, a standard-format tray carries 288 to 512 cells, each typically holding only a few millilitres of substrate. These trays exist for one reason: bench space is the most expensive real estate in a propagation greenhouse, and a seedling needs very little room in its first days.

High-cell trays are standard for crops that are transplanted young and small — many bedding plants, herbs and leafy crops — and for the first stage of multi-stage vegetable programs. They demand precision in return: vacuum or needle seeders must place seed accurately into very small cells, media filling must be uniform, and watering has to be fine and frequent because small cells dry out quickly. Cell depth is shallow, so root development is intentionally limited; the plant is expected to move on before it becomes root-bound.

200–128 cells: germination and early growth

This is the workhorse band for much of commercial young-plant production. Cells are large enough to hold a more forgiving volume of substrate — commonly in the range of roughly 10–25 cc depending on tray format and depth — while density remains high enough for efficient germination rooms and early bench space.

Many vegetable and ornamental programs seed directly into 128 or 200 cell trays and step plants up later, or transplant out to the field or a finishing pot from this size. It is also a common sowing size ahead of grafting in some solanaceous programs, where seedlings are raised to a graftable stem diameter before the grafting stage itself.

105–72 cells: growing-on and spacing

Once plants are growing actively, light interception becomes the limiting factor, not substrate. Trays in the 72–105 cell band give each plant more leaf room and a larger root volume — typically enough to hold a plant through several additional weeks without the growth checks that come from tight, root-bound cells.

This band is widely used for growing-on ornamentals, for larger vegetable transplants, and as the destination tray when high-cell seedlings are spaced up. Depth options matter here: deeper cells in this band support stronger root systems for crops that will be held longer or shipped further.

50–21 cells: finishing, grafting and holding

Large-cell trays — 50, 40, 36, 32, 28, 24 and 21 cells in standard formats — are where young plants finish. Cell volumes are substantially larger, commonly several tens of cc and upward, and deeper profiles are typical. This is the band for:

Grafting programs. Grafted tomato, pepper, cucumber and eggplant plants are commonly healed and finished in large-cell trays where stem calibre, root volume and plant-to-plant spacing support recovery and hardening.
Spacing and finishing. Plants stepped up from high-cell trays finish here before dispatch, with room to develop the canopy the customer is paying for.
Holding and hardening. Larger substrate volume buffers water and nutrition, which makes plants more forgiving during hardening-off and transport.

This finishing band is the range ModernAgriPro currently supplies: our plug tray program covers 21–50 cell trays for spacing, grafting, finishing and holding stages, alongside flat and shuttle trays for logistics. We are extending our range toward higher cell counts; if you run 72–512 cell seeding trays today, send us your current specification and we will confirm tooling options and timing.

Cell Volume and Depth: The Trade-Offs Behind the Count

Two trays with the same cell count can behave very differently. Three variables decide how a cell actually performs:

Cell volume determines how much water and nutrient the root zone buffers. Small volumes are efficient but unforgiving — irrigation timing becomes critical, and salinity builds faster. Large volumes forgive scheduling mistakes and support longer crop times, at the cost of bench space and substrate per plant.

Cell depth shapes the root system. Deeper cells typically produce a longer, better-structured root plug that pulls cleanly and transplants with less check; shallower cells suit short cycles and crops that move young. Depth also affects air pruning and drainage: a deeper, tapered cell generally dries more evenly from top to bottom.

Cell shape and spacing influence both roots and canopy. Square cells use tray area more efficiently than round cells at the same count and tend to guide roots downward rather than around; plant-to-plant spacing sets how long a canopy can develop before neighbours compete for light.

A practical rule: choose the smallest cell that reliably carries the plant to its next stage without a growth check. Anything larger wastes bench space; anything smaller costs uniformity — and uniformity is what your customers actually buy.

Machine Compatibility: The Tray Is Part of the Line

In automated propagation, the tray is not a container; it is a machine component. Before committing to a cell count, check the full chain:

Seeding lines. Drum and needle seeders are set up for specific tray formats and cell layouts. A new tray must match your seeder’s plate or drum configuration and present cells in the exact grid the machine expects.
Media filling and dibbling. Filling lines and dibble boards are format-specific; cell top dimensions must match the dibbler, and tray rigidity must survive the filler without flexing.
Transplanters. If plugs will be machine-transplanted, plug dimensions and tray pitch must match the transplanter’s grippers or ejector pins. This is where nominal-but-different trays from different suppliers cause the most expensive surprises.
De-nesting and handling. Automatic de-nesters rely on consistent tray rims and stacking features. Trays that nest too tightly or vary in rim geometry jam lines.

The consistent theme is dimensional discipline: outer dimensions, cell pitch and rim geometry must hold from batch to batch. Whatever cell count you choose, require a documented specification from the supplier and verify samples on your own line before committing a season to a tray — which is exactly how we structure trials with propagators (see the process on For Propagators).

A Simple Selection Workflow

List each crop and the stage the tray must serve: seed, germinate, grow on, graft, finish, or ship.
Set the target days-in-tray; longer holding means more volume and depth.
Choose the smallest cell count band that carries the plant through that stage without restriction.
Confirm format compatibility with your seeder, filler and transplanter.
Trial one crop cycle and judge the result on uniformity at dispatch, not on germination alone.

FAQ

What cell count should I use for grafted tomato plants?

Grafting itself is done on young seedlings, but healed and finished grafted plants are commonly held in large-cell trays — typically in the 21–50 cell range — where root volume and spacing support recovery, hardening and uniform dispatch.

Is a higher cell count always cheaper per plant?

Not necessarily. High-cell trays reduce substrate and bench-space cost per plant, but they add a transplanting step, tighter irrigation management and more risk of growth checks. Total cost per dispatched plant is the figure that matters.

Can I seed vegetables directly into a 50-cell tray?

Yes, and some programs do, especially where field transplant size matters more than bench efficiency. The trade-off is greenhouse space: you will use several times more bench area per thousand plants than a high-cell seeding stage.

How do I know if a new tray will work on my seeding line?

Match the tray’s documented outer dimensions, cell layout and rim geometry against your seeder and filler setup, then run physical samples on the line. Paper compatibility is not enough; a one-crop trial is the standard qualification step.

What cell counts does ModernAgriPro supply?

Our current plug tray range covers 21–50 cells for spacing, grafting and finishing, plus flat and shuttle trays for young-plant logistics. Higher cell counts are in development — send us your current tray specification and we will confirm options.