Seed germination is the biological process where a plant embryo, encased within a protective seed, begins to grow into a seedling. When seeds fail to sprout, the cause generally falls into one of three categories: the external environment is inappropriate, the seed is no longer alive, or the seed is biologically programmed not to grow yet. Troubleshooting germination failures requires understanding these distinct categories of roadblocks.
Failure to Meet Environmental Triggers
The most immediate cause of germination failure stems from a mismatch between the seed’s needs and its surroundings. Water is the first requirement, triggering imbibition, where the seed rapidly absorbs moisture to swell and activate internal enzymes. Overwatering is a common mistake because it displaces air from the soil pores, suffocating the embryo by depriving it of the oxygen needed for respiration.
Temperature is a specific trigger, as every species has a narrow range of soil temperature required for optimal germination. If the soil is too cold, the chemical reactions necessary for growth are too slow, or the seed may rot before it can start. Conversely, temperatures that are too high can denature the enzymes within the seed, killing the embryo outright.
Oxygen is required to fuel the high metabolic activity that begins once the seed is hydrated and before the seedling develops leaves for photosynthesis. A heavy, compacted planting medium or waterlogged soil prevents this gas exchange, essentially halting the embryo’s growth. While most seeds are indifferent to light beneath the soil, some tiny seeds, like lettuce, are light-dependent, meaning they require exposure to light to signal that they are near the soil surface and ready to grow.
Issues with Seed Viability
Even with perfect planting conditions, a seed will not germinate if it is no longer biologically viable, meaning the embryo is dead or severely degraded. A seed’s viability naturally decreases over time, even with ideal storage, as its internal food reserves and genetic material break down. Older seeds exhibit a much lower germination rate and may produce weaker seedlings, if they sprout at all.
Improper storage accelerates this decline, particularly exposure to high heat or humidity. Storing seeds in warm, moist conditions can cause them to prematurely initiate germination or encourage fungal growth, rapidly exhausting their energy reserves. Physical damage, such as a cracked seed coat or injury to the embryo during harvesting or handling, can also render a seed non-viable before it is planted.
Biological Dormancy
In some cases, the seed is perfectly healthy and the environmental conditions appear ideal, yet the seed refuses to sprout, a state known as biological dormancy. This is a protective mechanism that prevents the seed from germinating at a time when the resulting seedling would likely not survive. Dormancy is broken only when specific, often prolonged, environmental cues are provided.
One form is physical dormancy, caused by a hard, impermeable seed coat that prevents water uptake and gas exchange. Seeds with this constraint, common in legumes and some woody plants, require mechanical or chemical scarification to weaken the outer layer. Physiological dormancy is driven by internal chemical inhibitors within the embryo itself. These inhibitors must be neutralized by a period of cold and moist conditions, a process known as stratification, which mimics a natural winter cycle.
Pathogens, Pests, and Planting Errors
External biological threats and mechanical mistakes by the gardener represent a final set of common causes for failure. One frequent biological issue is “damping off,” a disease caused by soil-borne fungi and water molds, such as Pythium. These pathogens thrive in overly cool and wet soil and attack the embryo or young seedling stem immediately after germination, causing it to collapse at the soil line.
The quality of the planting medium is also a factor, as dense or compacted soil physically impedes the emergence of the root and shoot. This condition restricts oxygen flow and makes it difficult for the tender radicle to penetrate the substrate. Using non-sterile or contaminated planting mixes can introduce pathogens that kill the seed before it has a chance to sprout.
Planting depth is a frequent error; planting too deep requires the emerging shoot to expend too much of its limited stored energy before reaching the light, leading to exhaustion and failure. Conversely, sowing seeds too shallowly exposes them to rapid drying out or makes them targets for surface-dwelling pests, like insects or rodents, which consume the seed before germination can complete.