If a sink’s drainpipe empties onto soil where plants grow, it may not be used on Shabbat by anyone who has an interest in the plants being watered. It goes without saying that one may not use this sink on Shabbat if it was intentionally set up to water the plants.
Nevertheless, many poskim maintain that one who does not care about watering the plants may use such a sink on Shabbat – for example, if the plants do not belong to him and he has no interest in their growth. In a time of need, one may rely on these poskim. To be sure, one who pours water directly onto plants violates a prohibition, even if he does not intend to water them, because he is helping them grow. In contrast, here the water is poured indirectly and is therefore a case of grama and permitted (SSK 12:19). If the water from the sink reaches plants that have already been adequately watered, whether through heavy rains or water that drained from the sink before Shabbat, then even one who is interested in the plants growing may use the sink on Shabbat, since he is not helping them at all.
Rain sometimes falls on Sukkot, and in order to prevent one’s sukka from getting wet one might extend a sliding roof over it. When the rain stops, he will want to retract the roof, but he knows that if he does so, water that accumulated on the rooftop will spill onto the nearby plants. May he retract the roof anyway on Shabbat and Yom Tov? It depends: If the rain was hard enough and long enough to saturate the ground, one may retract the roof, because the extra water serves no purpose. However, if there was only a little rain, the roof may not be retracted, because this will water the plants and violate Zore’a.