This seed starting tray planner tells you how many seeds to sow, how many cells to fill and how many trays to set up for the number of plants you want. Enter your target, the germination rate from the seed packet, how many seeds you plan to sow per cell and how many spare plants you would like, and it does the rest. It saves you from the two classic mistakes: running out of seedlings because some seeds failed, or sowing far more than you can use.
How to use the seed tray planner
- Enter the plants you want in the garden, not the seeds. For example, 24 tomato plants.
- Enter the germination rate. The packet often lists it. If your seed is more than a year or two old, test a sample first: wet paper towel, ten seeds, count how many sprout.
- Choose seeds per cell. Sowing two seeds in each cell is a common way to make sure every cell grows. You snip the weaker seedling later.
- Add spare plants. Ten percent covers losses and weak seedlings. Add more for fussy crops.
- Enter the cells per tray for your trays, such as 32, 50, 72 or 128.
How the numbers work
If each seed germinates independently at your rate, the chance that a cell with two seeds grows at least one plant is one minus the chance that both fail. At 80 percent germination, one seed gives an 80 percent chance, two seeds give 96 percent and three give about 99 percent. The planner divides the plants you want, including spares, by that chance to get the cells to fill, multiplies by seeds per cell for the seed total and divides by cells per tray for the tray count.
Choosing seeds per cell
| Seeds per cell | Best for | Trade-off |
|---|---|---|
| 1 | Fresh, high-germination seed, or costly seed | Some cells will stay empty |
| 2 | Most vegetables and flowers | You must thin to one seedling |
| 3 | Older seed or poor germination | More seed and more thinning |
Tips for healthy seedlings
- Thin with scissors. Snip the extra seedling at soil level instead of pulling it, which can damage the roots of the one you keep.
- Use a seed starting mix with good drainage, and keep it moist but not soggy.
- Give light. Seedlings need strong light as soon as they emerge, or they get tall and weak.
- Label every tray with the crop and sowing date.
- Time it right with the seed starting calculator, and plan how many seeds to buy with the seed quantity calculator.
- Toughen seedlings before planting out using the hardening off schedule calculator.
Limits of this tool
It assumes seeds germinate independently at the stated rate. Real germination changes with temperature, moisture, seed age and the crop, and some seeds, such as certain herbs and perennials, are slow or erratic. Treat the numbers as a planning guide and keep a few extra seeds in reserve.
Frequently asked questions
How many seeds should I plant per cell?
Two is a common choice for most crops. It improves the chance every cell grows a plant, and you thin to the strongest seedling.
What is a good germination rate?
Many vegetable seeds germinate at 80 percent or better when fresh. Lower rates mean older seed or less ideal conditions.
How can I test germination at home?
Place ten seeds on a damp paper towel, seal it in a bag, keep it warm and count the sprouts after a week or two. Multiply the sprouts by ten for the percentage.
Should I start more plants than I need?
A small buffer is wise, since some seedlings fail or grow weak. Ten to twenty percent is typical.
How many trays do I need for 50 plants?
With 72-cell trays and good germination, about one tray, since you would fill roughly 55 cells. Use the planner above for your exact figures.