Enrichment · Cultural Studies · Gardening

Count the sprouts,
then sow exactly
enough.

Old or saved seed rarely comes up at the rate on the packet. Test a few in a damp paper towel, type the count, and the calculator gives the real rate and how many cells and seeds to sow so the tray fills.

Work out the rateWhat you get

The germination calculator

The worked example is made up: 50 two-year-old saved tomato seeds, 24 plants wanted. Nothing you type leaves this browser.

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What you get

Enrichment · Cultural StudiesFree

What you finish with
The germination rate with a 95% interval, and for the plants you want the cells and seeds to sow at one, two or three seeds per cell and the likely empty cells. It prints as a summary and downloads as a spreadsheet.
Who it is for
Home and school gardeners, seed savers, and students running germination experiments.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Cultural Studies space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Roll 20 to 100 seeds in a damp paper towel, bag it and keep it warm.
  2. After the days on the packet, count the sprouts.
  3. Type the seeds tested and sprouted.
  4. Add the plants you want, survival and seeds per cell.
  5. Sow the cells shown and print the summary.

Work out the rate

How the numbers work

A proportion, its uncertainty and a little probability.

The worked example: saved tomato seedRate82%41 ÷ 50Two seeds a cell96.8%1 − 0.18² come up× 90% survive87.1%plants per cell24 plants ÷ that28 cells56 seedsThe worked example: saved tomato seedRate82%41 ÷ 50Two seeds a cell96.8%1 − 0.18² come up× 90% survive87.1%plants per cell24 plants ÷ that28 cells56 seeds
Two seeds in a cell fail together only if both fail, so the chance of a gap falls from 18% to about 3%. That is why gardeners double-sow old seed.

The test and its range

The rate is simply the share that sprouted. A test of 20 or 50 seeds is a sample, so the page gives a 95% Wilson interval, which behaves well even for small tests and rates near 0 or 100%. Testing more seeds narrows it. Seed sold in the United States must meet federal germination standards under the Federal Seed Act, so a packet’s rate is a fair start for fresh seed; saved and older seed needs its own test.

Seeds per cell

If each seed sprouts with the same chance and independently, a cell sown with two seeds stays empty only when both fail, so the empty share is the failure rate squared. That is why two or three seeds per cell, thinned to the strongest, fills a tray from weak seed. Survival covers seedlings lost to damping off or transplanting.

Sources

  1. Wilson, E. B. (1927). Probable inference, the law of succession, and statistical inference. Journal of the American Statistical Association, 22(158), 209–212.
  2. Association of Official Seed Analysts. AOSA Rules for Testing Seeds: germination testing (analyzeseeds.com).
  3. U.S. Department of Agriculture, Agricultural Marketing Service. Federal Seed Act and regulations, 7 CFR part 201 (ams.usda.gov).
  4. University of Minnesota Extension. Starting seeds indoors: testing old seed (extension.umn.edu).

The worked example is made up. Results depend on warmth and moisture during the test. Your figures stay in this browser. Support: team@caveatacademic.com.