It is a surprising and frustrating experience to notice an acrid, ammonia-like odor in the garden, a smell strongly reminiscent of a cat’s litter box. This phenomenon, which often leads homeowners to suspect stray animals, is actually a common, natural occurrence resulting from the presence of certain landscaping plants. The olfactory confusion stems from volatile organic compounds (VOCs) that are chemically similar to those found in animal waste.
Identifying the Primary Offender
The most frequent source of this ammonia-like scent is the Boxwood shrub, scientifically known as Buxus. Specifically, the widely planted English boxwood, Buxus sempervirens, is the variety most notorious for emitting the unpleasant odor. This evergreen shrub is a staple in formal landscaping, prized for its dense, deep-green leaves and its ability to be tightly clipped into hedges and borders.
The scent is not constant; it often becomes most pronounced on hot, sunny days or immediately following rainfall, as heat and humidity amplify the release of volatile compounds. While tiny, inconspicuous flowers that bloom in late spring are sometimes cited as the main source, the leaves and stems also contain the compounds, which become more noticeable when the foliage is disturbed or pruned.
The Chemical Components of the Odor
The cat urine resemblance is due to the specific chemical structure of the volatile organic compounds the plant releases. The primary compound responsible for this signature odor is trimethylamine (TMA), a nitrogen-containing compound. TMA is a breakdown product of nitrogenous substances found naturally within the plant’s tissues.
This compound also contributes to the scent of decaying fish and the pungent odor associated with concentrated urine. In cats, a sulfur-containing amino acid derivative called felinine breaks down into volatile components, including ammonia and other sulfur compounds, closely mimicking the plant’s TMA emission. The plant’s release of these compounds is thought to be a natural defense mechanism against herbivores. The perception of the odor can vary greatly among individuals, with some people unable to detect it at all.
Alternative Plants That Cause Olfactory Confusion
While the boxwood is the most common culprit, several other popular garden plants can cause similar olfactory confusion, though their odors often present with slightly different notes. The Crown Imperial (Fritillaria imperialis) is a striking spring-blooming bulb, but the entire plant, especially the bulb, releases a powerful, musky scent. This odor is more commonly described as skunky or fox-like rather than pure ammonia, which makes it an effective natural deterrent for rodents and deer.
Another plant, the Valerian (Valeriana officinalis), is an herbaceous perennial whose roots and leaves can carry a strong, earthy scent often described as stale urine or old socks. The foliage of the popular ornamental shrub Lantana (Lantana camara) can also emit a pungent odor when crushed, sometimes perceived as gasoline-like or cat urine.
Practical Steps for Odor Mitigation
Homeowners concerned about the boxwood scent have several options short of removing the plants entirely. One effective strategy is to avoid planting the most odorous variety, Buxus sempervirens, near high-traffic areas like front doors, patios, or open windows. Opting for less pungent cultivars, such as those from the Buxus microphylla (Littleleaf Boxwood) species, can reduce the potential for odor.
Proper cultural practices can also minimize the release of VOCs. Boxwoods should be pruned in late winter or early spring to promote air circulation and reduce plant stress. After trimming, it is important to rake and remove all clippings and debris from the base of the shrub. Allowing dead or decaying plant material to accumulate intensifies the ammonia-like smell as nitrogenous compounds break down on the ground.