Roundup is one of the most widely used herbicides globally, with glyphosate as its active ingredient. This non-selective, systemic weed killer targets nearly all plant life it contacts. A common question is whether it can also destroy seeds waiting in the soil. The short answer is that glyphosate generally does not kill fully dormant seeds, a difference rooted deeply in plant biochemistry.
The Specific Mechanism of Glyphosate
Glyphosate works by interfering with a metabolic process found exclusively in plants and certain microorganisms. As a systemic herbicide, it must be absorbed by the green parts of the plant, such as leaves and stems, and then translocated to the growing points. Its primary mode of action is the inhibition of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase).
The enzyme is part of the shikimate pathway, a process plants use to synthesize three essential aromatic amino acids: phenylalanine, tyrosine, and tryptophan. These amino acids are fundamental building blocks for proteins and hormones necessary for plant growth. By blocking EPSP synthase, glyphosate starves the plant of these components, leading to a shutdown of its physiological functions. The absence of this pathway in mammals makes glyphosate highly selective for plants.
Why Dormant Seeds Remain Unharmed
Glyphosate does not kill a dormant seed because the seed is in a state of metabolic rest. A fully mature, dry seed has minimal internal activity, meaning its shikimate pathway is not operational. Without an active shikimate pathway, the target enzyme, EPSP synthase, is not functioning, rendering the herbicide ineffective.
Furthermore, glyphosate is absorbed through foliage, not roots, and does not readily penetrate the hard outer layer of a dormant seed. Even if the chemical reached the inner embryo, the lack of active metabolic processes prevents interference with the seed’s stored reserves. Therefore, a seed that has not yet begun germination remains physiologically immune to the herbicide’s effects.
Vulnerability During Germination and Emergence
A seed’s immunity ends the moment it transitions from dormancy to active germination. This transition begins with imbibition (the uptake of water), which signals the embryo to activate its internal metabolic machinery. As the seed’s internal processes resume, the shikimate pathway is awakened to prepare for growth.
If a seed absorbs glyphosate during the early stages of water uptake, the herbicide can be translocated into the developing embryo. The seed may still pass a standard germination test using stored energy reserves, but the seedling will be stunted and abnormal once it relies on its own active metabolism. The most significant damage occurs when the young plant’s first green tissues, such as the cotyledons or true leaves, emerge from the soil. These tissues absorb the chemical, allowing glyphosate to circulate and inhibit the shikimate pathway, leading to the seedling’s demise. This makes actively growing seedlings far more susceptible than a resting seed.
How Soil Residue Affects Future Planting
Although glyphosate is foliarly absorbed, its fate in the soil is relevant to future planting. The chemical binds tightly to soil particles, especially those rich in clay and organic matter, preventing easy absorption by plant roots. This tight binding results in low residual activity compared to other herbicides.
Soil microbes break down glyphosate into inactive compounds, with a half-life ranging from a few days to several months, depending on soil conditions. While the residue is unlikely to directly kill a newly planted seed, high concentrations can cause indirect harm. Glyphosate is a known chelator, meaning it can bind with essential micronutrients like manganese and zinc, making them less available for the developing seedling. This nutrient tie-up, along with possible negative effects on beneficial soil microbes that also use the shikimate pathway, can indirectly impact the vigor of the resulting crop.