Ammonia exists in a delicate balance regarding plant life. Whether it kills grass and weeds depends entirely on its concentration and application method. At low, diluted levels, ammonia provides a fundamental building block necessary for plant health and growth. However, when applied in a concentrated form, it transforms into a potent, non-selective chemical agent capable of causing rapid cellular destruction.
Ammonia’s Role as a Plant Nutrient
Nitrogen is a macronutrient required in large quantities for all plant functions, and ammonia is the foundation for nearly all nitrogen-based fertilizers. When ammonia is applied to soil, it quickly reacts with water to form the ammonium ion, which is the primary form of nitrogen absorbed by plant roots. This ammonium is transported within the plant to synthesize amino acids, proteins, and the chlorophyll required for photosynthesis.
In the soil, the ammonium concentration typically remains low, actively promoting robust plant growth. Specialized transport systems in the root membranes facilitate the rapid uptake of this nutrient. This beneficial effect shows that ammonia acts as a growth stimulant when present in carefully managed, low-concentration doses.
How High Concentrations Cause Plant Death
The beneficial role of nitrogen reverses abruptly when ammonia is applied at high concentrations, activating two distinct toxic mechanisms. The first is a chemical burn caused by the strongly alkaline nature of concentrated aqueous ammonia, or ammonium hydroxide. This caustic solution rapidly draws water out of the plant’s cells, leading to severe desiccation and the immediate collapse of foliage tissue.
The second mechanism is nitrogen toxicity, which occurs when the plant’s internal systems are overwhelmed by excess ammonium. To absorb the flood of ammonium ions, the plant’s roots must excrete hydrogen ions, which leads to a sudden drop in the root zone’s pH. This influx disrupts the balance of other essential ions and requires a massive expenditure of energy for detoxification. This process eventually leads to metabolic shutdown, stunted growth, and leaf yellowing.
Comparing Household Ammonia to Herbicide Products
The ammonia sold as a household cleaner is an aqueous solution, typically containing between five and ten percent ammonia. This concentration is highly caustic and unpredictable when used in a garden setting, making it an unsuitable substitute for commercial herbicides. Dedicated, formulated herbicides that utilize ammonium compounds are engineered for specific, predictable effects.
Commercial ammonium-based herbicides are regulated and formulated to break down predictably and target specific mechanisms of plant death. For example, some work by inhibiting enzymes, while others rely on caustic properties to burn tissue. In contrast, household ammonia introduces risks of severe soil pH disruption and environmental runoff that can be highly toxic to aquatic life.
Differential Impact on Grass Versus Weeds
Concentrated ammonia is classified as a non-selective herbicide, meaning it will kill almost any plant tissue it contacts indiscriminately. When applied to a lawn, the chemical burn is not limited to the target weed but will also damage or kill the surrounding turfgrass. The contact action of the caustic solution causes rapid and visible injury to the leaves of any plant.
Broadleaf weeds, which have larger, more exposed leaf surfaces, may appear to die faster because the caustic solution covers more of their top growth. However, grass has a deep and established root system and may recover even if the surface is damaged, unless the application is heavy or repeated. High concentrations of household ammonia will cause significant, non-selective damage to both grass and weeds.