The mystery of a persistent, unpleasant ammonia-like odor in a garden, often described as stale cat urine, is a common source of homeowner frustration. Many people initially suspect a neighborhood feline is repeatedly marking its territory on their plants or property. This distinctive and unwelcome scent, which seems to intensify on warm days or when the foliage is disturbed, is frequently mistaken for actual animal spray. To solve this garden puzzle, it is necessary to look beyond animal behavior and identify the specific botanical source of the malodor.
Identifying the Main Culprit Bush
The plant most commonly identified as the source of this peculiar garden odor is the Boxwood, specifically the English Boxwood (Buxus sempervirens). This dense, evergreen shrub is a staple in formal and traditional landscaping, prized for its small, glossy oval leaves and ability to be pruned into tight hedges or intricate topiary shapes. The variety is a slow-growing, long-lived plant that can reach heights of five to ten feet, providing excellent structure and screening in a garden setting.
The characteristic scent is not constant but is usually most noticeable under specific environmental conditions. Heat and humidity can cause the odor to become more pronounced as the plant’s volatile compounds are released more rapidly into the air. Similarly, the smell often intensifies immediately after the plant has been pruned, which releases the compounds from cut leaves and stems.
Even without human disturbance, certain natural phases of the plant can trigger the smell, such as when the tiny, inconspicuous flowers bloom in the late spring. While the scent can be described as resinous by some, the overwhelming consensus is the comparison to cat urine. Gardeners often plant the shrub near entryways, only to discover its undesirable aromatic property once the weather warms up.
Understanding the Chemical Source of the Scent
The chemical explanation for the Boxwood’s unfortunate aroma lies in the production of specific volatile organic compounds (VOCs) within the plant tissue. The primary molecule responsible for the cat-urine similarity is believed to be trimethylamine (TMA). TMA is a nitrogenous compound, meaning it contains nitrogen, and it naturally results from the metabolic breakdown of nitrogen-containing substances within the plant.
This molecule is also found in the urine of various mammals, including cats, where it contributes to the familiar pungent odor, particularly as the urine ages and breaks down. The release of TMA from the plant’s leaves and stems mimics this animal waste smell due to the presence of the same compound.
When the sun warms the Boxwood’s foliage, or when moisture is present, the plant releases these volatile, nitrogen-containing molecules into the surrounding air. This mechanism of releasing nitrogen compounds is a natural process for the plant, but the human nose interprets the specific chemical structure of trimethylamine as a strong, unpleasant ammoniacal scent. The intensity of the smell is a direct result of the concentration of these VOCs in the immediate environment of the shrub.
Other Plants and Environmental Factors That Mimic the Odor
While Boxwood is the primary culprit, other plants can occasionally produce a similar, pungent odor that confuses gardeners. Certain species of Viburnum, a common flowering shrub, can emit a strong scent on rainy or damp days that some observers liken to cat urine. The perennial herb Valerian (Valeriana officinalis), known for its medicinal uses, has roots and leaves that release a repelling odor that is often described as resembling feline waste.
Some weeds and ornamental bulbs also possess this undesirable aromatic quality. For example, the foliage of the Crown Imperial (Fritillaria imperialis) releases a sulfuric scent that is sometimes misinterpreted as the ammoniacal smell of urine.
Furthermore, environmental factors unrelated to plant chemistry can also be the source of the garden’s malodor. Newly applied wood mulch, especially if it is decomposing rapidly, can release an ammonia-like smell as its high nitrogen content breaks down. Finally, it is important to rule out actual animal markings, as high nitrogen content in the soil or an unobserved neighborhood cat can always be the true source of the unpleasant scent.