What Is a Seed Starter Tray? (Cell Gauge Specs)
A seed starter tray holds growing medium in separate cells so seeds can sprout and develop before transplanting. “Cell gauge” can mean two different things: cell size and count, or the thickness of the plastic. Compare the cell opening, depth, volume, tray material, thickness, drainage, and care instructions; a cell count alone cannot tell you whether a tray fits your crop or will last.
Choosing a tray can feel like choosing a box from a stack of look-alikes: the label may show the same number of cells, while the space inside and the plastic strength differ. I compare trays by separating planting capacity from material thickness. That makes product claims easier to check and helps match a tray to the crop, bench, and number of uses you expect.
What a Seed Starter Tray Is and What “Cell Gauge” Means
A seed starter tray is a shallow container divided into individual planting cells. Each cell holds growing medium for one or more seeds, depending on the crop and sowing plan. “Cell gauge” is not a clear specification by itself: ask whether it means cell dimensions or plastic thickness, then check both.
Cell count tells you how many spaces the tray has, not how much medium each holds. Two trays labeled “50-cell” may have different openings, depths, shapes, and volumes. They may not fit the same inserts or suit the same growing period.
Plastic thickness is often reported in mils. One mil equals 0.001 inch, or 25.4 micrometres. So, for unit conversion, 10 mil equals 0.254 millimetres and 20 mil equals 0.508 millimetres. These numbers do not, on their own, guarantee a certain service life.
Ask where thickness was measured. Forming a flat plastic sheet into cells can make the walls and corners thinner than other areas. For a useful comparison, seek the stated thickness, measurement location, and tolerance. If those details are not supplied, treat thickness claims as incomplete rather than assuming all trays with the same label are equal.
Tray Material, Cell Geometry & Gauge Selection
Tray selection starts with fit and crop needs, then moves to material and thickness. Record the outside footprint, cell count, opening, depth, and stated volume. Check the plastic type, drainage, and care limits too. These details describe capacity and likely handling needs more clearly than a cell-count label alone.
A common propagation format called “1020” is about 10 × 20 inches (254 × 508 millimetres). This is an approximate format, not a promise of exact fit. Rim shape, nesting features, and product design can change outside dimensions, so measure the tray and bench or bottom tray before buying.
| What to check | What to record | Why it matters |
|---|---|---|
| Footprint | Outside length and width | Confirms fit on a bench or in a bottom tray |
| Cell layout | Cell count and arrangement | Shows capacity, but not cell size |
| Cell geometry | Top opening and depth | Helps compare room for roots and medium |
| Cell volume | Manufacturer’s stated volume, if available | Supports a more direct capacity comparison |
| Plastic thickness | Value, unit, location, and tolerance | Makes “heavy duty” claims easier to assess |
| Material | Polymer named by the maker | Helps you check care and temperature instructions |
| Drainage | Hole count and placement | Shows how excess water can leave each cell |
Polypropylene, polystyrene, and PET are among the plastics used for trays. The material name alone does not give a safe washing temperature or sanitation method. Product design and manufacturing can also matter. Check the maker’s stated service-temperature range and cleaning instructions for the exact tray.
Cell volume is especially useful when available from the maker. A tapered cell does not have the same capacity as a straight-sided cell with the same top opening and depth. If you compare dimensions, note the shape as well. For a simple geometric estimate, multiply length × width × depth; this will overstate capacity when the cell tapers or has rounded corners.
If volume is not listed, ask the manufacturer rather than relying on cell count. You can also compare trays by filling them with a known volume of medium, but that is a practical estimate, not a published product specification. Keep cell dimensions and plastic thickness as separate measurements in your notes.
Filling, Sowing & Watering Technique
Correct use begins with a tray that fits its support and drains freely. Use moist, suitably fine seed-starting medium, fill cells without firm packing, and sow at the depth recommended for the crop. Water gently, then monitor moisture so the medium stays evenly damp without leaving roots persistently waterlogged.
Before filling, inspect the base and confirm that drainage holes are open. A solid bottom tray can catch runoff if it is compatible with the cell tray and bench setup. Avoid leaving standing water in the bottom tray for long periods unless your watering method and crop care plan call for it.
Fill each cell evenly, but do not press the mix into a hard plug. Compacted medium can limit air spaces and make watering less even. Leave enough room to water without spilling medium over the rim. Sow seed according to the crop’s instructions, since seed size and sowing depth vary.
Water with a gentle flow so seeds and medium stay in place. Check moisture by looking at the medium and following the crop’s needs; the tray’s cell count does not tell you how often to water. If cells dry at different rates, review cell size, airflow, and placement, rather than assuming the tray itself is defective.
Cleaning, Sanitizing & Reuse Care
Reusable trays need prompt cleaning after use. Remove growing medium, wash with detergent, rinse, and sanitize only with a product and method approved for that tray. Follow the product label and tray maker’s directions. Dry the tray fully before stacking it, and discard it if cracks, brittleness, warping, or poor drainage make reuse unreliable.
There is no universal maximum temperature for seed trays. Hot water, steam, dishwashers, and chemical cleaners can deform or weaken plastic unless the specific tray and method are rated for them. Do not infer heat tolerance from a polymer name alone. A warped base may prevent cells from draining or make the tray difficult to carry.
Avoid abrasive pads and unapproved solvents or strong cleaners. Scratches can make plastic harder to clean, while some chemicals can damage or craze it. Use a sanitizer only at the concentration and contact time on its label, and check that the tray material is compatible. Rinse if the label or maker directs you to do so.
After cleaning, inspect the cells, base, and rim. Look for blocked holes, cracks, brittle areas, and a base that no longer sits flat. Let the tray air-dry fully before nesting or storing it, so moisture is not trapped between stacked trays.
Transplant Readiness & Bench Presentation
A tray is ready for transplant use when its cells hold medium securely, drain as intended, and suit the crop’s planned time in the tray. The footprint should sit steadily on its support, and the cells should be easy to handle without bending or spilling. Check these points before sowing, not after the tray is full.
A smaller cell may be a practical choice when you plan to transplant seedlings soon. A larger or deeper cell may hold more medium, but the right size depends on the crop and how long plants will remain there. There is no single cell volume that fits every seedling.
Check whether the filled tray is manageable to lift. Wet medium adds weight, and a flexible tray may bow when carried. A compatible support tray or carrying method can help, but confirm dimensions and fit rather than assuming every “1020” product nests with every other one.
Worked Tray-Selection Examples
These examples show how to apply the specifications, not universal crop recommendations. I use the same sequence for each setup: confirm the footprint, compare cell geometry and volume, then check material, thickness, drainage, and care limits. That order helps prevent a misleading cell-count label from driving the purchase.
Example 1: A bench with limited space. A grower has a shelf described as fitting 1020 trays. Before ordering, they measure its usable area and compare it with the tray’s outside dimensions. They then check the cell opening, depth, and volume against the crop’s expected time before transplanting. The 1020 name alone is not enough to confirm fit.
Example 2: A tray intended for repeated use. A homeowner compares two products with the same cell count. One lists material and thickness; the other only says “heavy duty.” The first gives more information for checking care instructions and comparing construction. They still confirm thickness measurement location, drainage, temperature limits, and sanitation directions before choosing.
Example 3: A tray with unclear specifications. A buyer finds a “50-cell” listing with no cell dimensions or volume. They ask the seller for the opening, depth, material, and thickness details. If those cannot be confirmed, they avoid assuming the tray matches a known 50-cell product and select one with clearer specifications.
Conclusion: A Practical Buying Checklist
A useful tray specification joins planting capacity to construction details. Before buying, check footprint, cell count, opening, depth, volume, plastic type, thickness, drainage, and care limits. Once the tray arrives, inspect it before filling. These steps help you compare products without treating a marketing label as a technical standard.
- Measure the tray’s outside dimensions and the support it must fit.
- Record cell count, opening, depth, and stated volume.
- Ask what “gauge” means and where thickness was measured.
- Confirm the material, temperature range, and sanitation directions.
- Check drainage holes and inspect for cracks or warping.
Seed Starter Tray FAQ
These answers clarify the terms that most often cause confusion when comparing propagation trays. Use the manufacturer’s specifications for the exact product, especially for plastic thickness, temperature limits, and cleaning. Cell count, material name, or a “heavy-duty” label cannot replace those details.
What is a seed starter tray?
It is a tray divided into cells that hold growing medium and seeds until seedlings are ready to transplant.
What does cell gauge mean?
The phrase is ambiguous. It may refer to cell dimensions or plastic thickness, so ask which measurement the seller means.
How many mils are in a millimetre?
One millimetre is about 39.37 mils. One mil is 0.001 inch, or 25.4 micrometres.
Does a 50-cell tray have a standard cell size?
No. Cell count does not define opening, depth, shape, or volume. Compare those specifications for each product.
What does 1020 tray mean?
It usually describes an approximate 10 × 20-inch format. Check outside dimensions because products can vary.
Which plastic is best for a seed tray?
There is no universal best choice. Compare the specific tray’s material, thickness, intended reuse, and maker’s care instructions.
Can I sanitize a seed tray with boiling water or steam?
Only if the maker rates that exact tray for that method. Otherwise, heat may warp or damage it.
How do I know if a tray is reusable?
Check for cracks, brittleness, warping, blocked drainage, or damage that makes handling or cleaning unreliable.
Does a larger cell always grow a better seedling?
No. Cell size should match the crop and planned time before transplanting. More volume is not automatically better for every setup.
What should I do if a tray listing gives only cell count?
Ask for cell dimensions, volume, material, thickness, drainage details, and care limits. If those remain unknown, do not assume it matches another tray with the same count.
(This article was written by one of our staff writers, Julian R. Hayes. Visit our Meet the Team page.)
