Grass causes human skin to itch because it is stiff and has saw-like edges that cause small cuts, according to the University of California, Santa Barbara. Tiny hairs on the grass can also cause itching.
Some people have serious allergic reactions to certain grasses, according to the University of California Cooperative Extension. Additionally, grasses are home to a variety of mites and biting insects. The extension advises treating welts caused by bug bites with an over-the-counter lotion or a paste made from baking soda, which can neutralize acid from bites. WebMD notes that bug bites and allergic reactions should not be treated with alcohol or witch hazel, which can both further irritate skin.