Seed viability refers to the ability of a seed’s embryonic plant to germinate and grow into a healthy, self-sustaining seedling. Many old seeds can still grow, but their viability is time-dependent and constantly declining. A seed is a living organism in a state of suspended animation, engaged in slow metabolic processes that consume its stored energy reserves. This aging process, known as seed deterioration, eventually leads to the complete loss of the capacity to germinate. The challenge is distinguishing between true dormancy and the irreversible cellular decline caused by age.
Factors That Determine Seed Longevity
A seed’s potential lifespan is determined by its inherent genetic makeup and the storage environment. Plant species are categorized into two groups based on storage behavior. Orthodox seeds, including most grains and vegetable seeds, can be dried to a low moisture content (typically below 10%) without damage, remaining viable for many years. Recalcitrant seeds, such as those from avocados and mangoes, cannot tolerate desiccation and must maintain a high moisture level, limiting their lifespan to only a few weeks or months.
The primary external enemies of seed longevity are high temperature, high moisture content, and exposure to oxygen. These conditions accelerate cellular deterioration by increasing metabolic activity within the seed’s embryo. Deterioration involves the accumulation of reactive oxygen species (ROS), which initiate lipid peroxidation. This chemical reaction damages the fatty acid components of cell membranes, causing them to lose integrity.
As deterioration progresses, ROS also damage the seed’s nucleic acids, including the DNA and RNA. In a dry, well-stored orthodox seed, the cytoplasm enters a protective “glassy state” that halts cellular damage. Increased temperature or moisture disrupts this stable state, activating inefficient mitochondrial respiration that produces more ROS and leads to physiological collapse. Even under ideal storage, seeds lose vigor before they lose viability, often producing weak seedlings.
Simple Methods for Testing Viability
Before dedicating garden space to old seeds, the simple home germination test, often called the paper towel method, can accurately determine their viability percentage. Select a sample of ten seeds from the packet and place them on a damp paper towel that is moist but not dripping wet. Roll or fold the towel, place it inside a sealed plastic bag or container, and label it with the seed type and date.
Incubate the sealed bag in a warm location, ideally between 70 and 85 degrees Fahrenheit, and check the seeds every few days for two weeks. Count the number of sprouted seeds to calculate the germination rate. If seven out of ten seeds sprout, the viability is 70%, which is a good enough rate to plant by sowing more densely to compensate for anticipated failures.
A common alternative, the float test, involves dropping seeds into water and discarding those that float, but this method is highly unreliable. A floating seed merely indicates that it is less dense, possibly due to air pockets, a lightweight structure, or being too dry. The float test only measures density, not the actual health or viability of the embryo, and should not be used as a definitive test.
Maximizing Germination of Older Seeds
When a viability test confirms that old seeds still possess life, they require specialized care to maximize successful germination. Older seeds have lower stored energy reserves and reduced vigor, struggling more than fresh seeds to push through soil and resist disease. Pre-treatments are beneficial to give them a head start.
Pre-Treatment and Planting Techniques
- Soak the seeds in a diluted solution of hydrogen peroxide (H₂O₂) for about 30 minutes. This sanitizes the seed surface against mold and provides an extra boost of oxygen to stimulate initial metabolic processes.
- Physical scarification, which involves lightly scratching or nicking the hard seed coat, can help aged seeds that have developed a tougher exterior.
- Use a sterile, fine-textured starting medium, such as a peat-based mix or sandier soil, which offers less resistance than heavy garden soil.
- Sow aged seeds more shallowly than the standard recommendation. Planting them at a depth of two to three times their diameter, or even surface-sowing very small seeds, reduces the energy needed for the seedling to reach the light.
- Maintain a stable, warm temperature, often slightly higher than for fresh seeds, using a heat mat if necessary. Warmth supports the weakened metabolic processes within the aging embryo.