If you’re starting seeds in cell trays, thinning is all about one thing:
Leaving the strongest seedling in each cell
When to thin seedlings
Thin once seedlings have:
- First true leaves
- Multiple sprouts in a single cell
- A clearly strongest plant emerging
How to thin in cell trays
In trays, don’t pull seedlings.
Always snip extras at soil level
- Keeps roots of the remaining plant undisturbed
- Prevents accidental damage
- Maintains soil structure in the cell
Seedlings you SHOULD thin in cell trays
- Lettuce
- Bok Choy
- Chard
- Basil
- Kale
- Fennel
- Dill
- Marigold
- Cosmos
- Zinnias
These often germinate multiple seeds per cell or are intentionally over-seeded for success rates.
Some crops should NOT be over-seeded
A lot of growers make this mistake:
Some plants are best planted as one seed per cell from the start, especially:
- Corn
- Beets
- Peppers
- Tomatoes
- Eggplants
- Broccoli/ Cruciferous
- Cucumbers
- Beans
- Peas
- Melons
- Squash/Zucchini
- Sunflowers
- Nasturtium
Root crops (must direct sow)
These are the most important:
- Carrots
- Radishes
- Parsnips
- Turnips
- Rutabaga
Common mistakes
1. Pulling seedlings instead of cutting
This can damage the remaining plant’s roots.
2. Waiting too long
Crowded cells can cause: stretching, weak stems, and too much competition
Key takeaway
In cell trays, thinning is simple:
One strong plant per cell—nothing more, nothing less.
Do it early, do it cleanly, and your trays will grow more uniform and transplant-ready.