Seed Starting Trays Explained: Cell Sizes, Inserts and Humidity Domes
Seed trays look like a commodity, and the thin ones mostly are. But the cell size you choose decides how long a seedling can stay indoors before it gets root-bound, how often you'll water, and whether you'll need to pot up halfway through spring.
This guide explains the standard system most trays follow, how to match cell size to the crop, and when a humidity dome helps or hurts.
- Most trays follow the 1020 system: a roughly 10×20-inch outer flat with a cell insert dropped inside.
- Fewer cells means bigger cells. Bigger cells hold more mix, dry out more slowly and let seedlings stay indoors longer.
- Small, fast crops (lettuce, many herbs) do fine in small cells. Tomatoes, peppers and eggplant usually need bigger cells or a pot-up.
- Squash, cucumbers and melons dislike root disturbance; start them in individual pots shortly before transplanting.
- Use a humidity dome for germination only, then vent and remove it.
- For most people, heavy-duty reusable flats are worth paying more for. Thin flats crack within a season or two.
The 1020 system in plain English
Walk through any garden center and you'll see the same footprint again and again. The industry-standard flat is commonly called a 1020 tray because it measures roughly 10 by 20 inches. Real outside dimensions run a little over that, and they vary slightly by brand, which matters when you stack inserts from one maker in a flat from another.
The pieces:
Flat (with or without holes)
The outer tray. A flat without holes holds water for bottom watering and protects shelves and heat mats.
Cell insert
The grid of individual cells that sits inside the flat. Sold by cell count: 72, 50, 128 and so on.
Humidity dome
A clear lid that holds moisture in during germination. Tall domes leave room for seedlings; some have adjustable vents.
A common and flexible combination is a holeless flat on the bottom, a cell insert in the middle, and a vented dome on top. Some growers also keep a flat with holes to nest inside the holeless one so they can lift the whole set out and drain it.
Cell counts and what they mean
Cell count is the number of cells in an insert that fills a full flat. Because the flat is a fixed size, more cells always means smaller cells. Inserts are also sold as "half" or partial sizes and in deep versions with the same count but more depth.
| Insert | Cell size | Good for | Trade-off |
|---|---|---|---|
| 128 to 200 cells | Very small | Plug production, onions, dense sowings you will pot up soon | Dries fast; seedlings must move on quickly |
| 72 cells | Small | Lettuce, brassicas, many flowers and herbs | The most common size; fine for short stays indoors |
| 50 cells | Medium | Tomatoes and peppers for a shorter indoor period, larger flowers | Fewer plants per tray, but more forgiving watering |
| 32 to 38 cells, or deep cells | Large | Tomatoes, peppers and eggplant kept indoors longer | Uses more mix and shelf space |
| Individual 3–4 inch pots | Largest | Potting up; squash, cucumbers, melons sown late | Takes the most room under lights |
Matching tray to crop
Small cells are fine
Lettuce, kale, cabbage, broccoli and most quick-growing flowers and herbs typically spend a few weeks indoors. A 72-cell insert gives you plenty of plants in a small space, and these seedlings transplant easily.
Go bigger, or plan to pot up
Tomatoes, peppers and eggplant are usually started earlier relative to transplant and grow big roots. Many growers sow them in small cells, then move the strongest seedlings to 3–4 inch pots once they have true leaves. That gives you a chance to pick the best plants and bury tomato stems deeper. If you'd rather not pot up, start them in 50-cell or larger cells.
Start in their own pots
Squash, cucumbers and melons grow fast and are widely described as sensitive to root disturbance. Many gardeners sow them directly outdoors. If you start them inside, use individual pots a few weeks before transplant and avoid pulling them out of crowded cells.
Onions and leeks
These are often sown thickly in an open flat or a few seeds per cell, then trimmed and separated at transplant. Small cells or open flats both work.
If you want to skip plastic cells altogether, soil blocks are a well-established alternative. Our soil block maker guide covers how they compare.
Humidity domes: when to use them
A dome keeps the top of the mix from drying out while seeds germinate. That is useful for small seeds sown on or near the surface, and in dry indoor air. Once seedlings emerge, a closed dome becomes a problem: still, humid air encourages fungal issues like damping-off, and condensation drips onto leaves.
- Sow and water the tray, then put the dome on.
- Check daily for germination and for excessive condensation. Wipe or vent if it's dripping.
- Open the vents as the first seedlings appear.
- Remove the dome once most of the tray is up.
Tall domes are more useful than short ones if you germinate under lights, because seedlings won't press against the lid. Vented domes let you transition gradually. Domes also trap heat, so if you use a heat mat, keep an eye on soil temperature.
Thin vs heavy-duty trays
The cheapest flats are thin enough to flex when you pick them up full of wet mix. They crack, leak and usually last a season or two. Heavy-duty flats made from thicker plastic cost more up front but can last many seasons with care. For a home grower who starts seeds every year, durability usually pays off.
- Rigidity: a full flat should lift from one end without bowing badly.
- Compatibility: check that inserts from the brand you choose fit the flat. "1020" is a nominal size, not a precise standard.
- Drain holes: inserts need them; for at least one bottom flat, you want none.
- Cell shape: square, tapered cells with smooth walls release root balls more easily than flimsy round cells.
- Cleaning: smooth plastic is easier to wash and reuse between seasons.
Bottom watering and cell trays
Cell inserts in a holeless flat make bottom watering easy: pour water into the flat, let the mix wick it up, then pour off what remains after a while. This keeps the surface drier, which helps limit damping-off, and avoids knocking over tiny seedlings. Smaller cells will need water more often than larger ones, especially under lights or on a heat mat.
Seed tray checklist
- Holeless heavy-duty bottom flats, one per insert
- Cell inserts sized for each crop's indoor time
- Tall, vented domes for germination
- 3–4 inch pots for potting up tomatoes and peppers
- Plant labels and a waterproof marker
- Fresh seed-starting mix (not garden soil)
FAQ
What does 1020 mean on a seed tray?
It refers to the common flat size of roughly 10 by 20 inches. Actual outside dimensions are a bit larger and vary by maker, so check that inserts and domes from different brands fit together.
Which cell size is best for tomatoes?
If you plan to pot up, small cells work for germination, followed by 3 to 4 inch pots once seedlings have true leaves. If you want to skip potting up, start them in larger cells such as 50-cell or deep inserts.
Should I reuse seed trays?
Yes, if they're sturdy. Wash out old mix, clean them with soapy water before storing, and let them dry. Many growers also sanitize trays before reuse to reduce disease carryover. Follow the instructions on any cleaning product you use.
Do I need a humidity dome?
Not always, but it helps keep the surface moist during germination, especially for small seeds and in dry indoor air. Remove it once most seedlings are up to keep air moving.
Bottom line
Buy a few heavy-duty holeless flats, a stack of inserts in two or three sizes, and tall vented domes, and you can start nearly anything. Match cell size to how long each crop lives indoors, take domes off once seeds sprout, and plan for potting up the long-season crops. Find more seed-starting guides on the Garden Gear blog.