Question: Is it halachically permissible to turn on the hose which waters one’s garden before the onset of Shabbat when one knows that the water will continue to run on Shabbat? Additionally, is it permissible to turn off the running hose on Shabbat itself?
Answer: We have previously explained that included in the forbidden work of planting on Shabbat is anything that aids in a plant’s growth such as planting, watering, fertilizing, and the like. It is therefore understood that the root of our question lies in the forbidden work of planting on Shabbat.
Lighting Candles and Watering on Erev Shabbat
Clearly, the Torah only prohibited performing forbidden work on Shabbat; however, a forbidden work that was started before the onset of Shabbat is permitted. We therefore customarily light Shabbat candles on Erev Shabbat based on the edict of our Sages in order for these candles to provide light for us on Shabbat itself. It is obvious that that this does not pose any sort of prohibition since the actual lighting is not done on Shabbat itself. (The Karraites who denied the validity of the Oral Torah distorted the meaning of the verse, “You shall not ignite a fire in any of your dwellings on the day of Shabbat” to mean that a Jew may not have light in his house on Shabbat as the verse states, “The wicked shall be silenced in the darkness.” Rav Sa’adia Gaon battled against them fiercely, for a large number of people were swayed by them. This is evident from the fact that the chapter in Shulchan Aruch which deals with lighting the Shabbat candles is Chapter 263 which is the numerical value of רס"ג and is an acronym for Rav Sa’adia Gaon.) Based on this we can infer that it is halachically permissible to turn on one’s sprinkler before the onset of Shabbat in order for it to continue to water the garden/lawn on Shabbat itself, for there is no forbidden work taking place on Shabbat itself. (There are, nevertheless, some works that may not be performed on Erev Shabbat, as we shall discuss soon.)
Turning Off the Hose on Shabbat
Similarly, it seems that one may be lenient and turn off the hose on Shabbat, for there is no prohibition to stop watering on Shabbat. Thus, this is also permissible.
After ruling that it is permissible to turn on sprinklers before the onset of Shabbat, Hagaon Harav Eliezer Yehuda Waldenberg zt”l quotes an important halachic ruling in his Responsa Tzitz Eliezer in the name of Hagaon Chazon Ish that if one has several pipes or hoses that one uses to water one’s garden, when turning them off on Shabbat, one should not turn them off one by one; rather, one should close the main pipe which supplies water to all the hoses and only then may one turn off the various hoses one by one. His reason for this is because at the time that one shuts off one hose, this causes the water pressure in the other hoses to increase; if so, one is causing an increased watering of a part of the garden on Shabbat and this constitutes the forbidden work of watering on Shabbat.
Maran Rabbeinu Ovadia Yosef zt”l rejects this ruling completely and writes that there is no prohibition to turn off the hoses one by one, for at the time one is shutting off the hoses, the garden has already been sufficiently watered and it makes sense to say that the garden will not benefit from the excess water which will result from the increased water pressure. If so, this does not constitute the forbidden work of watering on Shabbat, for the Torah only prohibits the kind of watering that will benefit plant growth. However, if one waters a garden that has already been watered sufficiently, this is not prohibited.
Thus, regarding a garden upon which much rain has fallen in a way that additional water will not benefit the plants, it is no longer prohibited to water such a garden on Shabbat. He proceeds to list other reasons to defend his view. Therefore, there is no need to follow this stringency and according to the letter of the law, as long as the garden is already sufficiently watered, one may shut off all of the hoses that are watering it on Shabbat as one would on weekdays and this does not constitute any prohibition.
Summary: One may turn on a sprinkler before the onset of Shabbat so that the water will continue to water the garden/lawn on Shabbat itself. When the watering is finished, one may turn off the sprinklers on Shabbat as usual.