Seed longevity varies widely by species. Short-lived seeds like onions, leeks, and parsnips typically last only 1–2 years. Medium-lived seeds like tomatoes, peppers, beans, and carrots can last 3–5 years. Long-lived seeds like cucumbers, melons, lettuce, and squash can remain viable for 5–10+ years when stored properly. The key factors affecting longevity are temperature, humidity, and light exposure.

Store seeds in a cool, dry, and dark environment. Ideal conditions are temperatures between 4–10°C (40–50°F) with humidity below 50%. Use airtight containers (glass jars or sealed bags) and add a silica gel packet to absorb moisture. A refrigerator can work well if the seeds are sealed properly. Avoid storing seeds in garages or sheds where temperature and humidity fluctuate dramatically.

The easiest method is the paper towel germination test: Place 10 seeds on a damp paper towel, fold it gently, put it in a sealed plastic bag, and keep it in a warm spot (around 21°C / 70°F). Check daily and keep the towel moist. After 7–14 days (depending on species), count how many seeds sprouted. If 7 out of 10 sprout, you have roughly 70% viability. Use this rate to decide how many extra seeds to plant.

As seeds age, their germination rate naturally declines. If you need 10 tomato plants and your 4-year-old seeds have an estimated 60% germination rate, planting only 10 seeds would likely yield just 6 plants. To account for reduced viability, you should plant more seeds than your target number — in this case, about 17 seeds (10 ÷ 0.60 ≈ 17). Adding a 20–25% safety margin further accounts for seedlings that may not thrive after germination.

Generally, heirloom and open-pollinated seeds can have slightly shorter shelf lives than modern hybrid varieties, though this varies by species. Hybrid seeds are often bred for vigor and may maintain germination rates better over time. However, proper storage is far more impactful than seed type — well-stored heirloom seeds can outlast poorly stored hybrids. Regardless of seed type, always store them in cool, dry, dark conditions.

Absolutely! Seed packets often have a "packed for" date, not an expiration date. Many seeds remain viable well beyond the printed date — especially long-lived varieties like cucumbers, melons, and lettuce. Instead of discarding old packets, use this estimator to calculate how many to plant, or perform a germination test. Even if only 20% of old seeds sprout, you can compensate by planting more densely and thinning later.