The Torah (Shemot 20) states: “Remember the day of Shabbat to sanctify it. Six days you shall work and perform all of your labor and the seventh day shall be a Shabbat for Hashem, your G-d etc. For in six days Hashem created the Heavens and the earth and on He rested on the seventh day. Thus, Hashem blessed the seventh day and sanctified it.” The Torah warns us about the commandment of Shabbat observance twelve times, for it is the basis of our belief in Hashem’s creation of the world. Our Sages teach us that one who observes the Shabbat is tantamount to having fulfilled the entire Torah and one who desecrates the Shabbat is tantamount to having denied the entire Torah. The reason for this is, as we have mentioned above, because Shabbat observance is the basis of our belief.
Any works forbidden to be performed on Shabbat are referred to as “primary works” and each of these works have “subcategories.” For instance, one of the “primary works” on Shabbat is building , such as erecting a building, while its subcategory is cheese making (turning milk into cheese), for joining several pieces or components together to form one larger object is similar to building and is forbidden on Shabbat (as we have explained regarding the issue of making ice cubes on Shabbat).
There are thirty-nine primary forbidden works on Shabbat which are all enumerated in the Mishnah (Shabbat 73a).
The first of these forbidden works is that of planting; included in this forbidden work is any action one performs with the intention of causing something to grow or blossom, such as planting on Shabbat, pruning trees on Shabbat so that they grow nicely, grafting trees together, and the like. The Gemara (Mo’ed Katan 2b) states that one who water plants or trees on Shabbat is likewise liable for the prohibition of planting on Shabbat, for water causes the seeds and trees to grow. Based on this, one should point out to those eating in gardens or orchards on Shabbat not to wash their hands over grass or plants as well as not to pour any beverage over plants, for by doing so one is watering the plants and this constitutes the forbidden work of planting on Shabbat. Although this may not be one’s intention, it is nevertheless forbidden to cause plants to grow.
Maran Ha’Bet Yosef writes in the name of the Sefer Ha’Terumah that it is proper to avoid eating on Shabbat in a place where weeds or plants are attached to the ground, for it is difficult to make sure one does not spill water on them, thus causing one to transgress the prohibition of planting on Shabbat. Clearly, the same applies to eating on top of grass which is quite common nowadays.