Germination is the process where a seed sprouts into a fledgling plant, representing the beginning of a new life cycle. When a seed fails to germinate, it is a common and frustrating occurrence. The reasons for failure are broadly divided into two categories: internal limitations within the seed itself and external conditions in the surrounding environment. Understanding whether the seed is non-viable or if the conditions are simply not right helps transform a gardener’s approach from guesswork to precision. Success depends on a balance between the seed’s inherent quality and the convergence of external factors.
Issues Related to Seed Quality and Health
A seed that never sprouts has often lost its viability. The most common cause of non-viability is the passage of time, as seed longevity varies significantly by species. For example, onion seeds may deteriorate within a year, while lettuce seeds can remain viable for five years or more under ideal conditions. As seeds age, their DNA, proteins, and cell membranes are compromised, leading to a progressive decline in germination speed and eventual death.
Improper storage conditions significantly accelerate this aging process. Storing seeds in environments with high temperatures or high humidity rapidly reduces viability because moisture and heat promote destructive chemical changes. Physical damage, such as cracks in the seed coat, also makes seeds vulnerable to pathogens and prevents proper germination. A seed may also never have been viable if it was harvested before reaching full maturity or if genetic issues occurred during the parent plant’s development.
Failure to Meet Essential Environmental Needs
Even a perfectly healthy seed will fail if the environment does not provide the precise cues needed for growth. Four environmental factors—water, temperature, oxygen, and light—must be met for germination to proceed. The initial step is imbibition, the rapid uptake of water that activates metabolic enzymes. Too little moisture prevents activation, leaving the seed dormant. Excessive water can also be detrimental, potentially causing imbibition damage and cell membrane disruption.
Temperature is a primary trigger, controlling the rate of chemical reactions necessary for the embryo to grow. Every species has an optimal soil temperature range, typically between 68 and 77 degrees Fahrenheit for many warm-season vegetables, where germination is fastest. Temperatures outside this range, whether too cold or too hot, slow or stop the enzymatic activity required to break down food reserves. Too-cool temperatures can also increase the seed’s susceptibility to diseases.
Oxygen is necessary for cellular respiration, which provides energy for the growing embryo. Waterlogged or heavily compacted soil excludes air, drastically reducing the oxygen concentration available to the seed. While initial water absorption is not oxygen-dependent, subsequent metabolic activity and seedling growth require a sufficient supply. Species vary in their sensitivity to low oxygen; for example, carrot and parsley are more sensitive than broccoli or cabbage seeds.
Light is the fourth factor, and its requirement depends entirely on the plant species. Seeds requiring light to germinate are positive photoblastic; they use a photoreceptor to sense burial depth. If planted too deeply, the absence of light prevents sprouting. Conversely, negative photoblastic seeds are inhibited by light and must be buried to germinate successfully.
Internal Biological Dormancy
A seed may be perfectly viable yet still refuse to germinate, a condition known as dormancy. This is an evolutionary adaptation preventing sprouting during unfavorable conditions, ensuring the seed waits for the correct seasonal cues before committing to growth. Dormancy manifests in two primary forms: physical and physiological.
Physical Dormancy
Physical dormancy is caused by a seed coat that is too hard or impermeable to water, blocking the imbibition necessary to start germination. This is common in many legume families. In nature, the hard coat is typically broken down by freezing/thawing, microbial action, or passage through an animal’s digestive tract. Gardeners replicate this through scarification, a process that mechanically or chemically scratches the seed coat, allowing water to penetrate.
Physiological Dormancy
Physiological dormancy is a complex internal block caused by chemical inhibitors within the embryo, often a plant hormone like abscisic acid. These inhibitors prevent the embryo from growing even when water and temperature are adequate. This form of dormancy is typically broken by stratification, a process of cold, moist conditions that mimics a winter season. Placing seeds in a moist medium in a refrigerator allows the cold to break down the chemical inhibitors, signaling that it is safe to grow.
Destruction by Pathogens and Pests
Fungal Pathogens
Even after a seed is ready to sprout, it can be destroyed by biological threats in the soil. Fungal pathogens are a common culprit, causing a condition known as “damping off.” This disease, often caused by fungi like Pythium or Fusarium, attacks the seed or emerging seedling, causing the stem to rot at the soil line. Damping off is particularly problematic in overly wet or cool soils, which favor fungal growth over rapid seedling development.
Pests and Predation
Predation by insects and small animals represents another source of germination failure. Wireworms and seedcorn beetles are common insect pests that feed directly on the planted seed or the delicate embryo. Wireworms, the larvae of click beetles, can hollow out the seed completely. Rodents, such as mice or squirrels, and birds may also dig up and consume seeds, especially larger ones, before germination is completed.