A landscaper orders 500 tomato plants. If you sow 500 seeds you will deliver about 380: some seed will not germinate, and some seedlings will not survive pricking out and hardening off. Divide the target by both rates and round up, and you sow enough to fill the order with a few to spare.
500 plants at 85% germination and 90% survival: 500 ÷ 0.765 = 653.6, so sow 654 seeds.
The example
Three crops on the sowing sheet. The pepper seed germinates poorly and loses more at transplant, so it needs far more seed per finished plant.
| A | B | C | D | E | |
|---|---|---|---|---|---|
| 1 | Crop | Target plants | Germ % | Survival % | Seeds to sow |
| 2 | Tomato Celebrity | 500 | 85% | 90% | 654 |
| 3 | Marigold | 1,200 | 92% | 95% | 1,373 |
| 4 | Habanero pepper | 250 | 70% | 85% | 421 |
The formula
One formula, filled down the sowing list:
How it works
Two loss stages multiply, so you divide by both:
C2*D2is the share of seeds that become sellable plants: 85% germination times 90% survival is 76.5%.B2/(C2*D2)scales the target up: 500 ÷ 0.765 is 653.6 seeds.ROUNDUP(...,0)rounds up to a whole seed. 653.6 becomes 654 — you cannot sow six tenths of a seed, and rounding down would leave you one plant short.- Compare crops. The habanero needs 1.68 seeds per finished plant; the marigold needs 1.14.
Most growers add a safety margin on top: ROUNDUP(B2/(C2*D2)*1.05,0) for 5% over. Being ten plants short on a 500-plant order costs more than ten extra seeds.
Try it: interactive demo
Enter the plants you need, the germination percent from the seed packet or your own trials, and your transplant survival percent.
Variations
Packets or plug trays to order
Divide the seed count by seeds per packet (or cells per tray) and round up again.
Plants you will finish from seed on hand
The reverse question. Multiply seeds by both rates and round down.
Pitfalls & errors
The germination rate on the packet was tested under lab conditions on fresh seed. Old seed or a cold greenhouse can cut it by 10–20 points. Use your own trial from last season where you have one.
Track survival separately for direct-sown plugs versus pricked-out seedlings. Pricking out costs more plants, and blending the two rates hides which method is losing them.
Entering the rates as 85 and 90 instead of 85% and 90% divides by 7,650 and tells you to sow 1 seed. Percent cells, or decimals like 0.85, only.
Practice workbook
Frequently asked questions
Should survival include plants culled for quality?
Is there a simpler rule of thumb?
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