Testing Old Seed Viability

Why Test Seed Viability

Seeds are alive — they contain a dormant plant embryo sustained by stored energy reserves. Over time, those reserves deplete, the embryo's cellular structures degrade, and the seed loses the ability to germinate. How quickly this happens depends on the species, the storage conditions (temperature, humidity), and the initial seed quality. A seed packet purchased last year may germinate at 95%. The same packet three years later might germinate at 60%. Five years later, maybe 20%. Without testing, you are gambling — and a failed planting discovered in June is a season-ending loss for that crop.

Testing eliminates the gamble. A five-minute setup tells you exactly what percentage of the seed is viable, so you can adjust planting density accordingly or replace seed that has declined below useful germination rates. The test costs nothing (you already have paper towels and plastic bags) and saves both the seed cost and the growing time lost to a planting failure.

The Paper Towel Germination Test

Step 1: Count out exactly 10 seeds from the packet. Ten is the ideal test size — it makes percentage calculation trivial (each seed that germinates equals 10%) and uses minimal seed from your supply.

Step 2: Moisten a paper towel until damp but not dripping. Wring out excess water — the towel should be uniformly moist without any standing water. Fold the towel in half and lay the 10 seeds on one half, spaced at least an inch apart so roots do not tangle.

Step 3: Fold the other half of the towel over the seeds. Slide the folded towel into a plastic zip-closure bag and seal, leaving a small air pocket inside. Label the bag with the variety name and the date.

Step 4: Place in a warm location (70–80°F is ideal for most vegetables). The top of a refrigerator, a shelf near a water heater, or any consistently warm spot works. Avoid direct sunlight — the bag can overheat and cook the seeds.

Step 5: Check daily starting on day 5. Open the bag, unfold the towel, and count any seed that has produced a visible root (radicle) or shoot. A seed is "germinated" once the root emerges — it does not need to produce leaves. Re-moisten the towel if it has dried, refold, reseal, and return to the warm spot.

Step 6: After the full germination period (7–14 days depending on species), count the total germinated seeds and calculate the percentage. Seven out of 10 = 70%. Four out of 10 = 40%.

Interpreting Results

80–100% germination: Excellent — plant at normal spacing and density. The seed is as good as new.

60–79% germination: Good — plant at slightly increased density (25–50% more seed than normal) and thin to the strongest seedlings. The seed is usable with minor compensation.

40–59% germination: Marginal — plant at double normal density. Expect patchy stands that need replanting in gaps. Consider whether the variety is worth the extra seed and effort.

Below 40% germination: Poor — replace the seed. The cost of new seed is less than the time and space wasted trying to establish a crop from mostly dead seed. Exception: rare or irreplaceable varieties are worth attempting at any germination rate.

Viability Expectations by Species

Some species are genetically programmed for short seed life, while others persist for years. Knowing the expected range helps you prioritize which packets to test and which to replace without testing.

Short-lived (1–2 years): Onion, leek, parsnip, parsley, chive. Test these annually if older than one year. Buy fresh each season if convenient — the reliability gain is worth the small cost.

Medium-lived (3–4 years): Pepper, bean, pea, carrot, corn, spinach, lettuce. Test if older than two years. These species store well under good conditions but decline noticeably by year four.

Long-lived (4–6+ years): Tomato, cucumber, melon, squash, brassicas (cabbage, broccoli, kale), radish. These species maintain viability remarkably well in cool, dry storage. Properly stored tomato seed germinating at high rates after 8 to 10 years is not uncommon.

Improving Germination of Old Seed

If a germination test reveals marginal viability (40–60%), several techniques can improve the success rate of old seed in the garden. Pre-soaking in room-temperature water for 12 to 24 hours before planting hydrates the seed coat and initiates the germination process before the seed enters the potentially hostile soil environment. This is particularly effective for hard-seeded species (beans, peas, corn, okra) where the seed coat may have hardened during storage.

Scarification (lightly nicking or sanding the seed coat) helps water penetrate seeds with extremely hard coats. Use fine sandpaper or a nail file to barely scratch the surface — you want to thin the coat, not damage the embryo inside. This technique is standard practice for morning glory, sweet pea, and other hard-coated ornamental seeds, and helps marginal-viability seed of these species germinate more reliably.

Bottom heat from a seedling heat mat (70–80°F soil temperature) accelerates germination for warm-season crops whose viability has declined. Old tomato and pepper seed that germinates slowly at room temperature often shows dramatically improved performance on a heat mat — the consistent warmth provides the optimal conditions the aging embryo needs to push through the weakened seed coat.

Record Keeping for Seed Inventory

Record germination test results on the seed packet itself — a simple notation like "7/10 germ, July 2026" tells you everything you need to know when planning next season. For large seed collections, maintain a simple spreadsheet or list with variety, purchase/harvest year, last germination test date, and result. This data accumulates into a valuable reference that guides purchasing decisions: you learn which varieties maintain viability in your storage conditions and which decline quickly, which seed companies provide consistently high-germination seed, and when specific varieties in your collection need testing or replacement.