The scent of freshly cut grass is sometimes overwhelmed by a powerful, pungent aroma of onion or garlic. This unexpected sensory event occurs when the lawnmower’s blades damage the leaves of certain plants growing within the turf. The resulting odor can linger, and the phenomenon is particularly noticeable in early spring and late fall when these plants are actively growing.
Identifying the Culprit: Wild Onion and Garlic
The source of the potent smell is a group of perennial weeds belonging to the Allium genus, the same plant family that includes cultivated onions, garlic, chives, and leeks. The most common invaders are wild garlic (Allium vineale) and wild onion (Allium canadense). These plants emerge from underground bulbs, often appearing in the lawn during the cooler seasons when turfgrass is dormant or growing slowly.
Wild garlic is typically characterized by thin, round, and hollow leaves that are covered in a waxy coating. Wild onion leaves, in contrast, are generally flatter and solid, emerging directly from the base of the plant. Both species have a grass-like appearance, but they grow faster and taller than the surrounding turf. When the plant tissue is damaged, it releases the volatile compounds responsible for the sharp, familiar scent.
The Chemical Reaction Creating the Odor
The smell is not present in the intact plant but is created instantly by a biochemical reaction triggered by tissue damage, such as cutting. Allium species store non-volatile, sulfur-containing amino acid derivatives, known as S-alk(en)yl-L-cysteine sulfoxides, in their cells. When the cells are ruptured by the lawnmower blade, two components that were previously kept separate come into contact.
The first component is the S-alk(en)yl-L-cysteine sulfoxide precursor, like alliin in garlic. The second component is an enzyme called alliinase, which is released from the cell vacuoles. Alliinase rapidly catalyzes the breakdown of the sulfoxides into unstable sulfenic acids. These sulfenic acids quickly condense with each other to form highly volatile organosulfur compounds, specifically thiosulfinates, which the human nose perceives as the onion or garlic smell.
For example, when wild garlic is cut, alliin is converted by alliinase into allicin, a thiosulfinate compound that gives garlic its characteristic odor. In wild onion, the reaction produces different sulfur compounds resulting in a scent closer to a traditional onion. These volatile sulfur molecules are not stable and continue to break down into other compounds like disulfides and trisulfides, which contribute to the lingering pungency.
Strategies for Controlling Lawn Invaders
Controlling these persistent weeds can be difficult, often requiring a multi-season approach because they reproduce from underground bulbs and bulblets. The most effective non-chemical method involves physically digging out the plants, making sure to remove the entire bulb cluster from the soil to prevent regrowth. Simply pulling the leaves is ineffective, as the foliage typically breaks off, leaving the bulb intact to sprout again.
Cultural control methods focus on maintaining a dense, healthy turf that naturally crowds out weeds. For chemical control, post-emergent, selective broadleaf herbicides containing ingredients like 2,4-D, dicamba, or mecoprop are generally used. The waxy coating on the thin, upright leaves makes it difficult for herbicides to adhere and penetrate. Applying a non-ionic surfactant can help the chemical stick to the waxy surface. Applications are most effective when done in the fall and repeated in late winter or early spring when the plants are actively growing.