Picnic

Question

If you are having a picnic in the park on Shabbat can you do netilat yadayim over the grass or is it considered watering the grass which would be prohibited? I was thinking that it should be permitted because it is a pessik reshe d'la niha leh. In other words, your intention is to wash you hands not to water the grass which occurs simply as a side effect. Is that right?