Seed Trays – Cell Counts, Drainage and Free Alternatives
Two decisions: how many cells, and whether you buy a tray at all. Everything else about trays is drainage and hygiene.
TL;DR
- 72 cells fits a windowsill; 36 or 50 cells if seedlings stay in the tray longer.
- Drainage holes are non-negotiable. Water must leave.
- Free options work for a month, not a season — match the container to how long the plant stays.
Cell count, honestly
A tray's job is to hold mix and let water out. Beyond that, cell count is a space decision. Seventy-two cells in a standard 10 by 20 inch tray means about 2 square inches per plant — enough for a tomato for six weeks, not enough for a squash for three. Thirty-six and fifty cell trays give each plant twice the root volume and half the plant count.
The practical rule: count how many plants you will put in the ground, add a fifth for losses, and buy the smallest number of trays that holds that. Two 72-cell trays cost roughly what one 36-cell tray does and give you four times the plants; the constraint is shelf space under the light, not money.
Free and cheap alternatives that actually work
| Container | Works for | Watch out for |
|---|---|---|
| Cardboard egg carton | A week of germination | Collapses when wet, no real volume |
| Yogurt or takeaway cups | Four to six weeks of growth | Needs a drilled drain hole; clear ones grow algae |
| Newspaper pots | A month, then plant the whole pot | Wicks water away, needs frequent bottom watering |
| Toilet roll tubes | Peas, beans, sweetcorn | Too narrow and too tall for most other crops |
| Plastic fruit punnets | Salad leaves and herbs | Holes in the base, and the lid must come off at sprouting |
The pattern is that free containers trade water management for cost. Cardboard and newspaper wick moisture away from the roots, so you water more often; cups and punnets hold water but need drainage cut by hand. If you are starting a lot of plants, a purchased tray with a base and dome is simply less work — the soil blocker is the third option, and the one that skips containers entirely.
Reusing trays without last year's problems
Scrub the mix residue out, soak ten minutes in roughly one part bleach to nine parts water, rinse and dry in the sun. Damping off and root rot both overwinter happily in a dirty tray, and it is the cheapest thing you can do to protect a season. Trays crack after two or three seasons of bleaching — that is normal, and the reason a stack of spares is worth keeping.
Once the tray is ready, the seed starting basics guide covers what goes in it and how deep, and the calendar tells you the week to start.
Tools for this job
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72-cell tray with dome and base
The default: fits a windowsill, drains properly, dome holds humidity until sprouting.
Deep-root seedling trays
Taller cells for crops that resent transplanting, like cucurbits.
Self-watering seed starter kit
A reservoir underneath keeps a week of neglect from killing the tray.
Related guides
Frequently asked questions
What cell count should I buy?
A 72-cell tray holds the most plants per square foot and suits a windowsill or one shelf of lights. Larger cells (36 or 50) give bigger seedlings more root room but fewer plants. Most beginners are better served by two 72-cell trays than one 36-cell tray.
Do seed trays need drainage holes?
Yes, and water must be able to leave. A tray without holes collects water at the bottom, which is how roots rot and how damping off starts. If you are reusing a container, put holes in the base before anything else.
Can I use egg cartons or yogurt cups?
Yes, with two caveats. Cardboard egg cartons hold almost no water and break down as soon as they are wet, so they are good for a week of germination and not for six weeks of growth. Yogurt cups work well if you drill a drain hole; a clear plastic lid over the top makes a usable humidity dome.
Do newspaper pots work?
They do, and they are free — a folded strip around a jar, tucked at the base, holds a seedling for a month. The trade-off is that the paper wicks water from the mix, so they need more frequent bottom watering and they start to collapse if kept soaking.
How do I clean trays between seasons?
Scrub off all mix residue, then soak in a mild bleach solution (about one part household bleach to nine parts water) for ten minutes, rinse, and dry in the sun. Skipping this is how a fungal problem from last year shows up in this year seedlings.