Seed germination is the biological process where a seed embryo emerges from its protective coat and begins to grow into a seedling. When a planted seed fails to sprout, the cause is usually attributable to the seed’s environment or its internal condition. Understanding these common pitfalls allows a gardener to diagnose the problem and find a clear solution.
Environmental Missteps
The most frequent reason seeds fail to germinate lies in controllable physical factors, such as moisture, temperature, and placement. Seeds require consistent moisture to initiate imbibition, the process where the seed absorbs water and activates its internal metabolic machinery. Too little water prevents the seed from softening its coat or starting biochemical processes, leading to desiccation and failure.
Conversely, saturation is detrimental because the seed needs oxygen for respiration to fuel early growth. Excess water fills the soil’s air pockets, creating hypoxic conditions that suffocate the developing embryo and cause it to rot. Achieving the correct balance of a consistently damp, but not sodden, growing medium is necessary for success.
Temperature is another environmental factor, as every plant species has an optimal temperature range for germination. Planting too early into cold soil stalls the process, leaving the seed vulnerable to pathogens. Temperatures that exceed the maximum threshold can kill the embryo outright; for example, warm-season crops like tomatoes require soil temperatures between 75°F and 85°F to sprout reliably.
The physical placement of the seed also governs its energy management, which is why planting depth matters. A seed planted too deeply exhausts its stored energy reserves before the shoot can push through the soil surface and reach sunlight. Planting too shallowly risks drying out quickly or failing to germinate if the species requires darkness. Certain small seeds, known as photoblastic seeds, require light to trigger germination, ensuring they are near the surface where they can quickly begin photosynthesis.
Seed Viability and Storage
Sometimes, failure to sprout relates to the internal health of the seed itself, not the planting environment. Seed viability, the measure of a seed’s ability to germinate and produce a healthy seedling, degrades over time, even under ideal conditions. This deterioration is a natural aging process involving cumulative damage to cellular components.
The rate of degradation is accelerated by poor storage conditions, primarily high temperature and high relative humidity. Storing seeds in a warm, humid environment depletes the embryo’s energy stores and weakens protective structures. For the best longevity, seeds should be stored in a cool environment, ideally between 35°F and 40°F, and kept dry with a relative humidity below 40%.
Gardeners can assess the remaining viability of older seed packets using a simple, at-home germination test, such as the paper towel method. This involves placing a small sample of seeds on a damp paper towel, rolling it up, and keeping it warm for the typical germination period. If less than 70% of the tested seeds sprout, the remaining batch should be sown more thickly than recommended to compensate for the high failure rate.
Dormancy Requirements
In some cases, a healthy seed will refuse to sprout because it is locked in a state of dormancy. Dormancy is a natural, protective mechanism that prevents germination until specific environmental cues are met. This state is distinct from a dead seed and is typically categorized into two types: physiological and physical.
Physiological dormancy is caused by chemical inhibitors within the seed, which require a period of cold and moisture to break down, a process known as stratification. This simulates the natural winter experienced by temperate-climate plants. Seeds often require this cold-moist treatment for 30 to 90 days at temperatures between 33°F and 50°F; without it, the embryo remains chemically blocked.
Physical dormancy is caused by a hard, water-impermeable seed coat, which prevents the uptake of water necessary for imbibition. To overcome this, a process called scarification is necessary, which mimics natural abrasion from weathering or animal digestion. Gardeners can manually scarify seeds by nicking the coat with a file, rubbing them with sandpaper, or soaking them briefly in hot water to weaken the barrier.
Pathogens and Pests
A final category of germination failure involves biological threats that destroy the seed or seedling before it can establish itself. The most common disease is “damping off,” a condition caused by various soil-borne fungi and fungus-like organisms. This disease can strike in two ways: pre-emergence, where the seed rots before sprouting, or post-emergence, where the young seedling collapses at the soil line.
Damping off thrives in conditions that stress the seedling, such as overwatering, which creates a wet environment for fungal spores to infect roots. Poor air circulation and cool soil temperatures also increase the risk, as they keep the soil surface moist and the seedling vulnerable for longer. Prevention relies on using sterile seed-starting mix, ensuring good drainage, and providing strong air movement around young plants.
Beyond microbial threats, various pests actively consume planted seeds or newly emerged sprouts. Soil-dwelling insects like wireworms and seedcorn maggots bore into the seed or emerging root, causing failure before the sprout breaks the soil surface. Other pests, such as cutworms, attack the tender stem of the seedling right at the soil line, often severing the plant overnight. Planting into warm soil and promoting rapid germination shortens the vulnerable period when seeds and sprouts are most susceptible.