One of the thirty-nine Melachot (categories of forbidden activity) that apply on Shabbat is "Koser" ("harvesting"). It is forbidden on Shabbat to detach something from the ground, such as stalks of wheat, or fruits from trees, such as grapes, olives dates and figs. It makes no difference whether the item that is picked is used as food for people or for animals, or even if it is not food at all, such as flowers, leaves and twigs. In all cases, the Torah forbids detaching on Shabbat something that is attached to the ground.
The question thus arises as to whether it is permissible to walk on a lawn on Shabbat. For example, may a person walk about on the lawn in his backyard? When a person walks on grass, there is the possibility that he will inadvertently detach blades of grass from the ground over the course of his walking. Should we thus forbid walking on grass on Shabbat?
Hacham Ovadia Yosef, in his Hazon Ovadia – Shabbat (vol. 4, p. 74), writes that it is entirely permissible to walk on grass on Shabbat, adding that it makes no difference whether the grass is dry or moist. He explains that a person who walks on a lawn has no intention to detach blades of grass, and since it is not definite that grass will be detached through walking, this is a case of "Dabar She’eno Mitkavein," which is permissible. Furthermore, even if grass would definitely be detached as a result of walking, it would still be permissible on Shabbat, because we deal here with a Rabbinic prohibition. Detaching grass with one’s feet is obviously not the standard method of "Koser," and thus it is not forbidden on the level of Torah law; it is forbidden only by force of Rabbinic enactment. A situation of "Pesik Resheh De’lo Niha Leh" – where a person performs a permissible act that will definitely result in a forbidden action, in which the person has no interest – is allowed when dealing with a Rabbinic prohibition. Therefore, Hacham Ovadia writes, it is permissible to walk on grass on Shabbat even if this will definitely result in grass becoming detached from the ground.
Hacham Ovadia adds that for the same reason, it is permissible to wheel a stroller or baby carriage on a lawn on Shabbat. Even if some grass is detached in the process, this is clearly not the standard way of harvesting, and this therefore qualifies as a situation of "Pesik Resheh De’lo Niha Leh" regarding a Rabbinic prohibition, which, as mentioned, is permissible.
The Mishna Berura (commentary by Rav Yisrael Meir Kagan, 1839-1933), in Siman 302 (Se’if Katan 11), writes that if one walked on grass and some pieces of grass became wedged in between his toes, he may not remove them directly by hand on Shabbat. The pieces of grass, according to the Mishna Berura, have the status of Mukseh, and therefore one may not move them with his hand. He must remove them indirectly, such as by running water over his feet.
Hacham Ovadia, however, disputes this ruling. He cites proofs to the fact that substances such as detached blades of grass, which are entirely insignificant and worthless, do not have the status of Mukseh. In his view, then, it is entirely permissible to remove pieces of grass from one’s feet on Shabbat.
Summary: Although it is forbidden to detach any vegetation from the ground on Shabbat, it is permissible to walk or push a carriage on grass, even if this may detach some blades of grass. If one walked on grass and some grass became stuck to his feet, he may remove them from his feet by hand on Shabbat.
