Seed viability refers to the ability of a seed to germinate and produce a healthy plant. The longevity of seeds varies drastically between different plant species and is heavily influenced by storage conditions. While the inherent genetic lifespan determines a seed’s maximum potential, environmental factors dictate how quickly it reaches the end of its useful life.
Seed Viability Based on Plant Type
The natural lifespan of a seed is directly linked to its species. Seeds are broadly categorized into groups based on their inherent genetic longevity, which provides a general timeframe for how long they remain useful for planting.
The shortest-lived seeds typically remain viable for only one to two years before their germination rate drops significantly. This short-lived group includes common garden vegetables such as onions, parsley, parsnips, and sweet corn. Planting fresh seed each year is often recommended for these varieties to ensure a successful harvest.
Many popular garden seeds fall into the medium-lived category, maintaining a good germination percentage for three to five years. This group encompasses common crops like beans, carrots, and brassicas, which include cabbage and broccoli.
Conversely, some seeds possess longevity and are considered long-lived, often remaining viable for five or more years. Seeds from the cucurbit family, such as squash and cucumbers, are known for their durability. Tomatoes also belong to this category.
Environmental Factors that Degrade Seeds
A seed’s inherent lifespan can be dramatically shortened or extended by its storage environment, which controls the speed of cellular degradation. The primary factors that accelerate the loss of viability are moisture, temperature, and light.
Moisture content is the most impactful factor, as high humidity encourages the seed to prematurely break dormancy or promotes fungal and mold growth. Seeds should be dried to a low moisture content, ideally between 5% and 10%, before long-term storage. For every 1% decrease in seed moisture content within a specific range, the lifespan of the seed can effectively double.
Temperature is the second major control, as warmer conditions increase the rate of chemical reactions within the seed, speeding up the aging process. A general rule is that the viability of a seed doubles for every 5.6°C (10°F) decrease in storage temperature.
Exposure to light should also be avoided because it can degrade the stored energy reserves and pigments within the seed embryo. Storing seeds in opaque containers or dark locations helps preserve their quality. By controlling these three external factors—keeping seeds cool, dry, and dark—gardeners can maximize the natural lifespan of their seeds.
Testing the Germination Rate of Old Seeds
For seeds of unknown age or those stored under non-ideal conditions, a simple at-home test can determine their current viability. This diagnostic procedure, often called the paper towel test, provides a clear percentage of seeds still capable of sprouting. It removes the guesswork before planting potentially dead seeds.
The process begins by counting a small, representative sample of seeds, often ten or twenty, and placing them on a moistened paper towel. The towel must be damp, but not dripping, to provide the necessary moisture without cutting off the oxygen supply required for germination. The seeds are then covered, rolled up, and placed into a sealed plastic bag or container to maintain humidity.
The container is kept in a warm location for the typical germination period of the species, usually five to ten days for most vegetables. After this time, the roll is carefully opened, and the number of seeds that have produced a healthy root and shoot is counted. If eight out of ten seeds germinated, the viability rate is 80%.
A germination rate below 40% to 50% is considered unreliable, suggesting it may be better to purchase fresh seeds. If the test shows a moderate rate, such as 60%, the old seeds can still be used. However, gardeners should sow them more densely to compensate for anticipated failures. This simple test provides an actionable estimate of a seed lot’s quality.