Seed Viability
Part of Seed Saving
Knowing whether your seeds will germinate before you plant them is the difference between a reliable food supply and an empty field. Seed viability testing is a simple, practical skill that lets you assess any seed lot — fresh or years old — and decide whether it is worth planting, how densely to sow, or whether you need to grow out fresh seed before the season is lost.
What Viability Means
A viable seed is one capable of germinating under appropriate conditions. Viability is expressed as a percentage: if 8 out of 10 seeds germinate in a test, viability (germination percentage) is 80%.
Viability degrades over time. It is not a switch (viable or dead) but a gradual decline. As seeds age, the percentage of viable individuals decreases while the rate and uniformity of germination also slow. A 90% germination rate two years ago may be 70% today — still usable but requiring denser sowing to achieve the same plant population.
Why Test Before Planting?
Avoid planting failure: If viability has dropped below 50%, standard sowing rates will produce a sparse, uneven stand. You may not discover the problem until weeks after planting — too late to resow.
Adjust sowing density: Knowing germination percentage allows you to compensate. If a lot tests at 60%, sow at roughly 1.7× the normal rate to achieve the target plant population.
Decide when to grow out: If a critical variety tests below 70% germination, it is time to grow a fresh seed lot before viability falls further and the variety is lost.
Triage a mixed collection: When inheriting or trading a collection of unknown-age seeds, viability testing lets you prioritize which varieties need immediate grow-out and which can wait.
The Standard Germination Test
The standard germination test is simple, requiring only seeds, wet paper towels, and warm conditions.
Materials
- 10 or 20 seeds of the variety to test
- Paper towels, cloth, or other moist germination medium
- Warm location (appropriate for the crop being tested)
- Waterproof plastic bag or container to maintain humidity
- Pencil and paper for records
Procedure
- Moisten a paper towel or cloth with clean water until damp but not dripping
- Place 10 seeds (or 20 for more statistical accuracy) on one half of the towel, spaced apart
- Fold the towel over the seeds
- Label with variety name and date
- Place in a waterproof bag and seal partially (allow some air exchange)
- Place in a warm location at the appropriate germination temperature (see table below)
- Check daily after the expected first germination date; count and remove germinated seeds
- Continue until no new germinations occur for 3 consecutive days, or until the final count date is reached
Temperature Recommendations by Crop
| Crop | Test Temperature (°C) | First Count (days) | Final Count (days) |
|---|---|---|---|
| Tomato | 25–30 | 5 | 14 |
| Pepper | 25–30 | 7 | 14 |
| Beans | 20–25 | 4 | 10 |
| Peas | 15–20 | 5 | 8 |
| Corn | 25 | 4 | 7 |
| Cucumber | 25–30 | 4 | 8 |
| Squash | 25–30 | 4 | 8 |
| Brassicas | 20–25 | 4 | 10 |
| Carrot | 20–25 | 7 | 14 |
| Lettuce | 20 | 4 | 7 |
| Wheat, barley | 20 | 4 | 7 |
| Onion | 20–25 | 6 | 12 |
Counting and Recording
Count a seed as germinated when the radicle (root tip) has emerged to at least 2 mm length. Do not count seeds where the radicle has emerged but appears deformed, discolored, or clearly will not develop into a normal seedling.
Record:
- Date of start
- Number of seeds tested
- Number germinated at first count
- Number germinated at final count
- Final germination percentage
Germination % = (seeds germinated ÷ seeds tested) × 100
Example: 14 germinated out of 20 tested = 14 ÷ 20 × 100 = 70% germination.
Interpreting Results
| Germination % | Interpretation | Action |
|---|---|---|
| 90–100% | Excellent | Sow at standard rate |
| 80–89% | Good | Sow at standard rate or 10% denser |
| 70–79% | Acceptable | Sow 25–40% denser; plan fresh seed lot this season |
| 50–69% | Marginal | Sow at 2× rate; prioritize growing out new seed |
| Below 50% | Poor | Use only if no alternative; plan urgent grow-out |
| Below 25% | Unacceptable | Do not rely on; obtain new seed; grow out emergency lot |
A germination rate that was previously known (e.g., 90% two years ago and now 70%) tells you the rate of decline and how many more years the lot may remain usable.
Expected Viability by Crop Under Good Storage
The following table shows the typical expected germination percentage retention over years of storage in cool, dry conditions (below 12% moisture, 10–15°C). These are averages — actual performance varies with storage quality.
| Crop | 1 yr | 2 yr | 3 yr | 4 yr | 5 yr | 6+ yr |
|---|---|---|---|---|---|---|
| Onion, leek | 80% | 50% | 25% | — | — | — |
| Parsnip | 80% | 50% | 25% | — | — | — |
| Corn | 90% | 80% | 70% | 55% | 40% | Poor |
| Pepper | 90% | 80% | 65% | 50% | 35% | — |
| Parsley | 80% | 65% | 50% | 35% | — | — |
| Carrot | 90% | 75% | 60% | 50% | 35% | Poor |
| Spinach | 85% | 70% | 55% | 40% | — | — |
| Peas | 90% | 85% | 75% | 65% | 55% | 40% |
| Beans | 90% | 85% | 75% | 65% | 55% | 40% |
| Brassicas | 90% | 85% | 80% | 70% | 60% | 50% |
| Tomato | 90% | 85% | 80% | 70% | 60% | 50% |
| Cucumber | 90% | 85% | 80% | 75% | 65% | 55% |
| Squash, melon | 90% | 85% | 80% | 75% | 65% | 55% |
| Wheat, barley | 90% | 85% | 80% | 70% | 60% | 50% |
| Lettuce | 90% | 80% | 70% | 60% | 50% | 35% |
These numbers assume seeds were dried to below 12% moisture at harvest and stored in a cool, stable location. Under better conditions (8% moisture, 5°C), add 50–100% to the expected viability period. Under poor conditions (15% moisture, 25°C ambient), subtract 50–75%.
Vigor Testing
Standard germination tests reveal viability under ideal conditions. Vigor testing reveals performance under stress — which is a better predictor of field germination rates in real conditions.
Cold Test for Corn and Beans
- Conduct the germination test at 10°C instead of the standard 25–30°C
- Count germination at 7 days
- High-vigor lots germinate 70%+ at the cold temperature; low-vigor lots may show only 20–40%
This mimics cold spring soils. A lot that germinates 90% at 25°C but only 40% at 10°C will perform poorly when sown into a cold seedbed.
Accelerated Aging Test
- Place seeds in a high-humidity environment (40°C, 100% relative humidity) for 48–72 hours — this ages seeds rapidly
- Conduct a standard germination test immediately after
- High-vigor lots maintain near-normal germination; low-vigor lots drop sharply
Requires a controlled environment; simpler alternatives are the cold test above or simply observing seedling emergence speed and uniformity in the actual field.
The Cut Test
The cut test provides immediate information about seed viability without waiting for germination.
Procedure:
- Cut 10–20 seeds in half with a sharp blade, through the embryo axis
- Examine the internal tissue:
- Healthy, viable: firm, white or creamy-yellow, moist-looking endosperm/cotyledons, visible embryo
- Dead: shrunken, hollow, brown, or black; discolored or dry; no visible embryo structure
- Calculate percentage of healthy-looking seeds
Cut test results are less precise than germination tests but take less than 10 minutes and are useful for rapid field assessment of unknown seed lots.
The Cut Test Is Good Enough for Old Seed
For very old seed of unknown storage history, the cut test tells you quickly whether there is any point in running a full germination test. If more than 80% of seeds look dead on the cut test, don’t waste time germinating them — get new seed.
Using Viability Data to Plan Sowing
If you know your germination percentage, calculate how many seeds to sow to achieve your target plant population:
Seeds to sow = Target plants × (100 ÷ germination %)
Example: Target 100 tomato plants. Germination test shows 70%. Seeds to sow = 100 × (100 ÷ 70) = 143 seeds. Add 20% extra for transplanting losses = 172 seeds total.
This calculation ensures you achieve your target stand even with imperfect germination, without wasting excessive seed.
Test Every Lot Every Season
Never assume this year’s germination rate matches last year’s test. Run a new viability test each season before planting — especially for any lot more than 3 years old. Conditions vary, and viability can drop faster than expected.
Seed Viability Summary
Germination testing is a simple 7–14 day process using moist paper towel, appropriate temperature, and 10–20 seeds per lot. Results guide sowing density and identify lots needing urgent grow-out. Expected viability ranges from 1–2 years for onions and parsnips to 5–7+ years for cucumbers, squash, and brassicas under good storage. Vigor testing (cold test, accelerated aging) reveals field performance better than standard germination tests. The cut test provides rapid assessment of completely unknown lots. Test every critical seed lot every season before planting.