# Test grow

> A test grow (tester grow) is growing out a sample of a new seed line, often by several independent growers, to measure uniformity, vigour, chemistry and herm rate before release.

- Canonical: https://genetixbay.com/knowledge/glossary/test-grow.html
- Also written: tester grow, tester, testers, test run, grow-out, grow out, beta test grow
- Glossary category: Genetics and breeding · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

A test grow, or tester grow, is a grow-out of a new seed line before it is released or featured. Testers, ideally independent and in different environments, grow the seed and report [germination](https://genetixbay.com/knowledge/glossary/germination.md) rate, vigour, uniformity, flowering time, aroma, yield, lab results and any [intersex plants](https://genetixbay.com/knowledge/glossary/hermaphrodite.md).

## In practice

Tester data is what turns a cross into a line someone can trust. It also shows [genotype](https://genetixbay.com/knowledge/glossary/genotype.md) by environment effects: a line that behaves in one breeder's room may [stretch](https://genetixbay.com/knowledge/glossary/stretch.md) or mould in another's.

Sample size matters, especially for [herm rates](https://genetixbay.com/knowledge/glossary/herm-threshold.md). Ten clean plants cannot distinguish 0% from 20%: a line that herms 1 time in 5 shows no herms in 10 plants about 11% of the time. Demonstrating a herm rate under 2% with reasonable confidence needs about 150 plants. GenetixBay's [Bay Lines](https://genetixbay.com/knowledge/glossary/bay-lines.md), [Seed Club](https://genetixbay.com/knowledge/glossary/seed-club.md) and featuring require a confirmed herm rate under 2% in tester grows.

## Units and formula

Chance of seeing zero herms in n plants when the true rate is p: (1 - p)^n. Rule of three: zero events in n plants gives a 95% upper bound of about 3/n, so 150 clean plants supports a rate under 2%.

## Don't confuse with

- **Progeny test**: testing a parent by its offspring.
- **Pheno hunt**: selecting individuals from seed, not validating a line.

## Myth check

"A 10-plant test grow proves a line is herm-free" is busted; see [Mythbusters](https://genetixbay.com/knowledge/mythbusters.md).

## Related terms

[Seed run](https://genetixbay.com/knowledge/glossary/seed-run.md), [Progeny test](https://genetixbay.com/knowledge/glossary/progeny-test.md), [Uniformity](https://genetixbay.com/knowledge/glossary/uniformity.md), [Genotype by environment interaction](https://genetixbay.com/knowledge/glossary/genotype-by-environment.md), [Pheno hunt](https://genetixbay.com/knowledge/glossary/pheno-hunt.md)

## Read more

- [Seed germination testing and quality control for breeders](https://genetixbay.com/knowledge/breeding/seed-germination-testing-qc.md)
- [Selection and pheno hunting: sample sizes and scoring rubrics](https://genetixbay.com/knowledge/breeding/selection-pheno-hunting.md)

## Sources

1. Falconer DS, Mackay TFC. 1996. Introduction to Quantitative Genetics, 4th edition. Longman, Harlow. ISBN 978-0-582-24302-6