On Shabbat and Yom Tov, it is biblically forbidden to water a garden. This action falls under the categories of the forbidden work of “sowing”, because it helps the plants grow, and “plowing”, because it softens the soil.
This problem exists whenever one eats or drinks in a garden, near a lawn or other plants, on Shabbat or a holiday. It is an even greater concern when building a sukkah in such a place, where all the festive meals will be eaten.
In practice, it is impossible to be careful not to spill water or other beverages onto the ground, especially with children and guests present in the sukkah. Even if the person spilling the water has no intention to water the plants, this is considered a case of "pesik reisha" (an inevitable outcome). Just like a person who cuts off an animal's head and claims they didn't intend to kill it, the action is considered as if it were intended because the forbidden result is a definite consequence of their actions.
Therefore, the halachic authorities have written that it is best not to eat in a garden at all on Shabbat and Yom Tov, unless one is able to be strictly careful (both themselves and all who are partaking of the meal) to only wash their hands over a basin and not to pour the remnants of their cups onto the ground. One must also be careful with fruit and vegetable seeds, as simply throwing them in the garden can be considered sowing.
If it is necessary to build the sukkah on the grass, one should cover the eating area with a sheet, such as a "florig" cloth (a type of fabric that prevents damage to the grass underneath and is sold in agricultural and gardening supply stores). This ensures that any spilled water does not reach the plants, provided that the ground is not sloped in a way that would cause the water to drain and water the plants at the edge of the sheet.
translated from Sichat HaShevua
