One may not water plants, as it helps them grow and thus constitutes Zore’a. However, one may open irrigation pipes or turn on sprinklers before Shabbat even if they will water plants during Shabbat. Similarly, one may program a computer before Shabbat to activate sprinklers on Shabbat, as on Shabbat no action will be done by a Jew. One who opened irrigation pipes before Shabbat may close them on Shabbat, as this involves no melakha (see above 2:9).
One who eats in a yard must be careful not to wash his hands over plants or their roots (SA 336:3). If the plants are small and their roots are short, the prohibition applies only in the immediate vicinity of the plants, but if the plants are large, the prohibition applies to the entire garden bed around them.
One may pour water on the ground if there are no plants or roots nearby. Even though it is possible that eventually the water will reach roots or that wild flowers will subsequently grow there, this is not prohibited since one does not intend this to happen (Kaf Ha-ĥayim 336:27).
One may not pour water on his own soil if it is suitable for planting. Since this softens the ground, preparing it for planting, it is considered Ĥoresh (MB 336:26; SHT ad loc. 18).