Grass has a serrated (toothed) edge on the blades. So when you rub yourskin against it, the grass makes tiny cuts. A more common reason for grassitching is because there are tiny hairs on the grass and these hairs secretea fluid that is irritating to the skin.

Grass blades do not have a smooth surface - they are covered in many very tiny bristles known as 'trichomes'.You may be able to see these on a blade of grass with a good magnifying glass. Grass (and many other plants) uses trichomes to discourage insects and other animals from feeding on them - it can be difficult for small herbivorous insects to manuever around the trichomes. Also, in some species they trap a layer of air next to the surface of the plant, which reduces evaporation on windy or dry days. But the downside to us is that it make grass itchy when we touch it.

Grass is stiff and contains tiny shards of glass called phytoliths. It can cause tiny cuts on skin, that result in itching.