Question: Is there any issue with having a picnic in a nearby park on Shabbos within the eruv?
Answer: The Shulchan Aruch (OC 336:3) writes that one may walk normally on wet or dry grass on Shabbos as one isn’t intending to rip up the grass. One who eats in the garden must be careful not to wash their hands there as they will inevitably be watering the grass which is an issue of zorea, planting (Mishna Berura 336:26). The Rema adds that as it is so difficult not to spill any water while eating, it is commendable not to bring drinks out to the garden.
The Mishna Berura (336:27) writes that some argue that there is less of a concern in someone else’s garden, as one isn’t bothered if the plants grow there or not (peski reshei delo niche lei). Nonetheless, the Aruch Hashulchan (OC 336:27) writes that it is difficult to say that people aren’t bothered at all, and therefore urges people to be most vigilant, advising people not to hold meals there. Likewise, the Kaf Hachaim (OC 336:25) points out that even the lenient authorities would concede that one is bothered about a close friend’s garden.
Seemingly, therefore, all poskim would agree that as people want their parks to look good, any water that spills on the grass would be advantageous (and therefore an issue of pesik reshei denicha lei).In conclusion, one eating outdoors should ideally not bring any drinks onto the grass unless they have put a cloth down first.